30 seconds of interviews
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Interviews sind entmutigend und können selbst den erfahrensten Experten unter Druck vergessen. Überprüfen Sie und lernen Sie, welche Fragen häufig in Interviews auftreten, die von der Community, die sie beantwortet hat, kuratiert und auf alles vorbereitet werden, was sie fragen werden. Indem Sie Erfahrungen und Beispiele in der Praxis zusammenbringen, können Sie von nervös bis zur nächsten großen Gelegenheit vorbereitet sein.
30 Sekunden der Interviews ist eine Community -Anstrengung. Leisten Sie also gerne einen Beitrag auf jede erdenkliche Weise. Jeder Beitrag hilft!
Haben Sie eine hervorragende Idee oder kennen Sie einige coole Fragen, die nicht auf der Liste stehen? Lesen Sie die Beitragsrichtlinien und senden Sie eine Pull -Anfrage.
Schließen Sie sich unserem Gitter -Kanal an, um die Entwicklung des Projekts zu unterstützen.
batches , die die maximale Anzahl ganzer Chargen zurückgibt, die aus einem Rezept gekocht werden können.bind , die der Function.prototype.bind äquivalent entspricht.Prototype.bind.setState ?children -Requisite?== und === ?0.1 + 0.2 === 0.3 ?map() und forEach() ?# mit Ausnahme der letzten vier (4) Zeichen maskiert.this Kontext in React -Komponentenklassen richtig haben?null und undefined ?pipe , die eine Funktionszusammensetzung von links nach rechts ausführt, indem Sie eine Funktion zurückgeben, die ein Argument akzeptiert.i++ und dem Präfix ++i -Inkrement -Operatoren?this Keyword und wie funktioniert es?'use strict' und was sind einige der wichtigsten Vorteile für die Verwendung?var , let , const und ohne Keyword -Anweisungen?setState ?children -Requisite?className anstelle class wie in HTML?this Kontext in React -Komponentenklassen richtig haben?alt -Attributs auf Bildern?defer und async -Attribute für ein <script> -Tag?<header> Elemente enthalten? Was ist mit <footer> Elementen?<header> , <article> , <section> , <footer>localStorage und sessionStorage .rel="noopener" verwendet?em und rem -Einheiten?col-{n} / 12 -Verhältnis des Containers aufnimmt.@media -Eigenschaften nennen?Eine staatenlose Komponente ist eine Komponente, deren Verhalten nicht von ihrem Zustand abhängt. Staatenlose Komponenten können entweder funktionelle oder Klassenkomponenten sein. Staatenlose funktionelle Komponenten sind leichter aufrechtzuerhalten und zu testen, da sie garantiert die gleiche Ausgabe erzeugen, die dieselben Requisiten haben. Staatenlose funktionelle Komponenten sollten bevorzugt werden, wenn keine Lebenszyklushaken verwendet werden müssen.
this insgesamt.
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== und === ? Triple Equals ( === ) prüft die strenge Gleichheit, was bedeutet, dass sowohl der Typ als auch der Wert gleich sein müssen. Double Equals ( == ) führt dagegen zuerst den Typ -Zwang aus, so dass beide Operanden vom gleichen Typ sind und dann einen strengen Vergleich anwenden.
== unintuitive Ergebnisse erzielen kann.
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Ein Element ist ein einfaches JavaScript -Objekt, das einen DOM -Knoten oder eine Komponente darstellt. Elemente sind rein und nie mutiert und billig zu kreieren.
Eine Komponente ist eine Funktion oder Klasse. Komponenten können Zustand haben und Requisiten als Eingabe aufnehmen und einen Elementbaum als Ausgang zurückgeben (obwohl sie generische Container oder Wrapper darstellen können und nicht unbedingt DOM ausgeben müssen). Komponenten können Nebenwirkungen in Lebenszyklusmethoden initiieren (z. B. AJAX -Anforderungen, DOM -Mutationen, Vernetzung mit Bibliotheken von Drittanbietern) und können teuer zu erstellen.
const Component = ( ) => "Hello"
const componentElement = < Component />
const domNodeElement = < div />
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Eine staatliche Komponente ist eine Komponente, deren Verhalten von ihrem Zustand abhängt. Dies bedeutet, dass zwei separate Instanzen der Komponente, wenn die gleichen Requisiten angegeben sind, im Gegensatz zu reinen Funktionskomponenten nicht unbedingt denselben Ausgang erzeugen.
// Stateful class component
class App extends Component {
constructor ( props ) {
super ( props )
this . state = { count : 0 }
}
render ( ) {
// ...
}
}
// Stateful function component
function App ( ) {
const [ count , setCount ] = useState ( 0 )
return // ...
} useState() initialisiert.
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Ein Promise ist in einem dieser Staaten:
Ein ausstehendes Versprechen kann entweder mit einem Wert erfüllt oder mit einem Grund (Fehler) abgelehnt werden. Wenn eine dieser Optionen auftritt, werden die zugehörigen Handler mit der dann aufgerufenen Methode eines Versprechens in die Warteschlange gestellt.
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i++ und dem Präfix ++i -Inkrement -Operatoren?Beide erhöhen den variablen Wert um 1. Der Unterschied besteht darin, dass sie bewertet werden.
Der Postfix -Inkrementoperator bewertet den Wert , bevor er inkrementiert wurde.
let i = 0
i ++ // 0
// i === 1Der Präfix -Inkrement -Operator bewertet den Wert , nachdem er inkrementiert wurde.
let i = 0
++ i // 1
// i === 1
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batches , die die maximale Anzahl ganzer Chargen zurückgibt, die aus einem Rezept gekocht werden können. /**
It accepts two objects as arguments: the first object is the recipe
for the food, while the second object is the available ingredients.
Each ingredient's value is a number representing how many units there are.
`batches(recipe, available)`
*/
// 0 batches can be made
batches (
{ milk : 100 , butter : 50 , flour : 5 } ,
{ milk : 132 , butter : 48 , flour : 51 }
)
batches (
{ milk : 100 , flour : 4 , sugar : 10 , butter : 5 } ,
{ milk : 1288 , flour : 9 , sugar : 95 }
)
// 1 batch can be made
batches (
{ milk : 100 , butter : 50 , cheese : 10 } ,
{ milk : 198 , butter : 52 , cheese : 10 }
)
// 2 batches can be made
batches (
{ milk : 2 , sugar : 40 , butter : 20 } ,
{ milk : 5 , sugar : 120 , butter : 500 }
)Wir müssen alle Zutaten des Rezepts zur Verfügung haben und in Mengen, die mehr als oder gleich der Anzahl der erforderlichen Einheiten sind. Wenn nur einer der Zutaten nicht verfügbar oder niedriger als benötigt ist, können wir keine einzelne Charge erstellen.
Verwenden Sie Object.keys() , um die Zutaten des Rezepts als Array zurückzugeben, und verwenden Sie dann Array.prototype.map() um jeden Zutaten zu dem Verhältnis der verfügbaren Einheiten auf die vom Rezept erforderliche Menge abzubilden. Wenn einer der vom Rezept erforderlichen Zutaten überhaupt nicht verfügbar ist, wird das Verhältnis zu NaN bewertet, sodass der logische oder den Bediener in diesem Fall auf 0 zurückfällt.
Verwenden Sie den Spread ... Operator, um das Array aller Zutatenverhältnisse in Math.min() zu versetzen. Übergeben dieses gesamte Ergebnis in Math.floor() runden die maximale Anzahl ganzer Chargen zurück.
const batches = ( recipe , available ) =>
Math . floor (
Math . min ( ... Object . keys ( recipe ) . map ( k => available [ k ] / recipe [ k ] || 0 ) )
)
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typeof typeof 0 Es bewertet "string" .
typeof 0 bewertet die Zeichenfolge "number" und typeof "number" bewertet "string" .
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Unter Verwendung des Objektspread ... können die aufzählbaren Eigenschaften des Objekts in das neue Objekt kopiert werden. Dies erzeugt einen flachen Klon des Objekts.
const obj = { a : 1 , b : 2 }
const shallowClone = { ... obj }Mit dieser Technik werden Prototypen ignoriert. Darüber hinaus werden verschachtelte Objekte nicht geklont, sondern ihre Referenzen werden kopiert, so dass verschachtelte Objekte immer noch dieselben Objekte wie das Original bezeichnen. Tiefklonierung ist viel komplexer, um jede Art von Objekt (Datum, Regexp, Funktion, Set usw.) effektiv zu klonen, die innerhalb des Objekts verschachtelt werden können.
Andere Alternativen sind:
JSON.parse(JSON.stringify(obj)) kann verwendet werden, um ein einfaches Objekt tief zu klonen, aber es ist CPU-intensiv und akzeptiert nur gültige JSON (daher streift es Funktionen und erlaubt keine kreisförmigen Referenzen).Object.assign({}, obj) ist eine andere Alternative.Object.keys(obj).reduce((acc, key) => (acc[key] = obj[key], acc), {}) ist eine weitere ausführlichere Alternative, die das Konzept in größerer Tiefe zeigt.
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Synchron bedeutet, dass jede Operation auf die Vorgänge warten muss.
Asynchrones Mittel kann ein Vorgang erfolgen, während noch ein weiterer Vorgang verarbeitet wird.
In JavaScript ist der gesamte Code aufgrund der einzigen Thread-Natur synchron. Asynchrone Operationen, die nicht Teil des Programms (wie XMLHttpRequest oder setTimeout ) außerhalb des Haupt -Threads verarbeitet werden, werden jedoch von nativem Code (Browser -APIs) gesteuert, aber der Teil des Programms wird weiterhin synchron ausgeführt.
alert blockieren den Haupt Thread, damit keine Benutzereingabe registriert ist, bis der Benutzer ihn schließt.
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Obwohl zwei verschiedene Objekte die gleichen Eigenschaften mit gleichem Werten haben können, gelten sie im Vergleich von == oder === nicht gleich. Dies liegt daran, dass sie durch ihre Referenz (Ort im Speicher) verglichen werden, im Gegensatz zu primitiven Werten, die nach Wert verglichen werden.
Um zu testen, ob zwei Objekte in der Struktur gleich sind, ist eine Helferfunktion erforderlich. Es wird durch die eigenen Eigenschaften jedes Objekts iteriert, um zu testen, ob sie die gleichen Werte haben, einschließlich verschachtelter Objekte. Optional können die Prototypen der Objekte auch auf Äquivalenz getestet werden, indem er als 3. Argument true .
Hinweis: Diese Technik versucht nicht, die Äquivalenz anderer Datenstrukturen als einfache Objekte, Arrays, Funktionen, Daten und primitive Werte zu testen.
function isDeepEqual ( obj1 , obj2 , testPrototypes = false ) {
if ( obj1 === obj2 ) {
return true
}
if ( typeof obj1 === "function" && typeof obj2 === "function" ) {
return obj1 . toString ( ) === obj2 . toString ( )
}
if ( obj1 instanceof Date && obj2 instanceof Date ) {
return obj1 . getTime ( ) === obj2 . getTime ( )
}
if (
Object . prototype . toString . call ( obj1 ) !==
Object . prototype . toString . call ( obj2 ) ||
typeof obj1 !== "object"
) {
return false
}
const prototypesAreEqual = testPrototypes
? isDeepEqual (
Object . getPrototypeOf ( obj1 ) ,
Object . getPrototypeOf ( obj2 ) ,
true
)
: true
const obj1Props = Object . getOwnPropertyNames ( obj1 )
const obj2Props = Object . getOwnPropertyNames ( obj2 )
return (
obj1Props . length === obj2Props . length &&
prototypesAreEqual &&
obj1Props . every ( prop => isDeepEqual ( obj1 [ prop ] , obj2 [ prop ] ) )
)
}
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XSS bezieht sich auf die clientseitige Code-Injektion, bei der der Angreifer böswillige Skripte in eine legitime Website oder Webanwendung injiziert. Dies wird häufig erreicht, wenn die Anwendung die Benutzereingabe nicht bestätigt und frei dynamischen HTML -Inhalten frei injiziert.
Ein Kommentarsystem ist beispielsweise gefährdet, wenn es keine Benutzereingaben validiert oder entkommt. Wenn der Kommentar unabgeordnetes HTML enthält, kann der Kommentar die Website, die andere Benutzer gegen ihr Wissen ausführen, ein <script> -Tag injizieren.
textContent anstelle von innerHTML den Browser daran, die Zeichenfolge über den HTML -Parser auszuführen, das Skripte darin ausführen würde.
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Cross-Origin-Ressourcenfreigabe oder CORS ist ein Mechanismus, der zusätzliche HTTP-Header verwendet, um eine Browser-Berechtigung zu erteilen, um auf Ressourcen auf Ressourcen auf einen Server zuzugreifen, der sich als die Website der Website unterscheidet.
Ein Beispiel für eine Cross-Origin-Anfrage ist eine Webanwendung, die von http://mydomain.com bedient wird, die AJAX verwendet, um eine Anfrage für http://yourdomain.com zu stellen.
Aus Sicherheitsgründen beschränken Browser die von JavaScript initiierten HTTP-Anfragen. XMLHttpRequest und fetch folgen der gleichorientierten Richtlinie, was bedeutet, dass eine Webanwendung, die diese APIs verwendet, nur HTTP-Ressourcen aus demselben Ursprung anfordern kann, auf die die Anwendung zugegriffen wurde, sofern die Antwort aus dem anderen Ursprung die richtigen CORS-Header enthält.
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Das DOM (Dokumentobjektmodell) ist eine plattformübergreifende API, die HTML- und XML-Dokumente als Baumstruktur behandelt, die aus Knoten besteht. Diese Knoten (wie Elemente und Textknoten) sind Objekte, die programmatisch manipuliert werden können, und alle sichtbaren Änderungen, die vorgenommen wurden, werden live im Dokument reflektiert. In einem Browser steht JavaScript diese API zur Verfügung, in der DOM -Knoten manipuliert werden können, um ihre Stile, Inhalte und ihre Platzierung im Dokument zu ändern oder mit Ereignishörern zu interagieren.
<head> mit einem defer oder in einem DOMContentLoaded -Ereignishörer platziert werden. Skripte, die DOM -Knoten manipulieren, sollten ausgeführt werden, nachdem das DOM konstruiert wurde, um Fehler zu vermeiden.document.getElementById() und document.querySelector() sind gemeinsame Funktionen für die Auswahl von Dom -Knoten.innerHTML -Eigenschaft auf einen neuen Wert einstellen, wird die Zeichenfolge über den HTML -Parser ausgeführt. Dies bietet eine einfache Möglichkeit, dynamische HTML -Inhalte an einen Knoten anzuhängen.
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bind , die der Function.prototype.bind äquivalent entspricht.Prototype.bind. function example ( ) {
console . log ( this )
}
const boundExample = bind ( example , { a : true } )
boundExample . call ( { b : true } ) // logs { a: true } Geben Sie eine Funktion zurück, die eine willkürliche Anzahl von Argumenten akzeptiert, indem Sie sie mit dem Rest ... Operator sammeln. Geben Sie aus dieser Funktion das Ergebnis des Aufrufens des fn mit Function.prototype.apply zurück, um den Kontext und das Argumentationsargumente auf die Funktion anzuwenden.
const bind = ( fn , context ) => ( ... args ) => fn . apply ( context , args )
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var , let , const und ohne Keyword -Anweisungen? Wenn vor einer variablen Zuordnung kein Schlüsselwort vorhanden ist, werden entweder eine globale Variable zugewiesen, wenn man nicht existiert, oder eine bereits deklarierte Variable neu zuordnet. Wenn die Variable noch nicht deklariert wurde, wird die Variable als Eigenschaft des globalen Objekts ( window in Browsern) zugewiesen, wenn die Variable noch nicht deklariert wurde. Im strengen Modus wird ein Fehler erfasst, um zu verhindern, dass unerwünschte globale Variablen erstellt werden.
var war die Standardanweisung, um eine Variable bis zum ES2015 zu deklarieren. Es schafft eine funktionsübergreifende Variable, die neu zugewiesen und neu eingestuft werden kann. Aufgrund des Mangels an Blockabläufen kann dies jedoch zu Problemen führen, wenn die Variable in einer Schleife wiederverwendet wird, die einen asynchronen Rückruf enthält, da die Variable weiterhin außerhalb des Blockbereichs existiert.
Im Folgenden ist die Schleife bereits abgeschlossen und die i -Variable 10 ist 10 setTimeout sodass alle zehn Rückrufe auf dieselbe Variable im Funktionsbereich verweisen.
for ( var i = 0 ; i < 10 ; i ++ ) {
setTimeout ( ( ) => {
// logs `10` ten times
console . log ( i )
} )
}
/* Solutions with `var` */
for ( var i = 0 ; i < 10 ; i ++ ) {
// Passed as an argument will use the value as-is in
// that point in time
setTimeout ( console . log , 0 , i )
}
for ( var i = 0 ; i < 10 ; i ++ ) {
// Create a new function scope that will use the value
// as-is in that point in time
; ( i => {
setTimeout ( ( ) => {
console . log ( i )
} )
} ) ( i )
} let wurde in ES2015 eingeführt und ist die neue bevorzugte Methode, um Variablen zu deklarieren, die später neu zugewiesen werden. Der Versuch, eine Variable erneut neu anzuklagen, macht einen Fehler. Es ist blockiert, so dass die Verwendung in einer Schleife es auf die Iteration hält.
for ( let i = 0 ; i < 10 ; i ++ ) {
setTimeout ( ( ) => {
// logs 0, 1, 2, 3, ...
console . log ( i )
} )
} const wurde in ES2015 eingeführt und ist die neue bevorzugte Standard -Methode, um alle Variablen zu deklarieren, wenn sie nicht später neu zugewiesen werden, selbst für Objekte, die mutiert werden (solange sich der Verweis auf das Objekt nicht ändert). Es ist blockiert und kann nicht neu zugewiesen werden.
const myObject = { }
myObject . prop = "hello!" // No error
myObject = "hello" // Error let und const gibt es jedoch ein Konzept, das als Temporal Dead Zone (TDZ) bezeichnet wird. Während die Erklärungen immer noch gehoben sind, gibt es einen Zeitraum zwischen dem Eintritt in den Geltungsbereich und dem Deklarieren, wo sie nicht zugegriffen werden können.var und wie let es lösen kann, sowie eine Lösung, die var hält.var sollte nach Möglichkeit vermieden werden und bevorzugen const als Standard -Deklarationsanweisung für alle Variablen, es sei denn, sie werden später neu zugewiesen, verwenden Sie dann let , wenn ja. let vs const
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Die Ereignisdelegation ist eine Technik der Delegation von Ereignissen an einen einzigen gemeinsamen Vorfahren. Aufgrund des Ereignisses sprudeln die Ereignisse den Dom -Baum auf, indem er alle Handler auf jedes Vorfahren bis zur Wurzel, die es möglicherweise hört, auf jedes Vorfahren ausführt.
DOM -Ereignisse bieten nützliche Informationen zu dem Element, das das Ereignis über Event.target initiiert hat. Dies ermöglicht das übergeordnete Element, das Verhalten zu bewältigen, als würde das Zielelement das Ereignis und nicht alle Kinder des Elternteils oder des Elternteils selbst anhören.
Dies bietet zwei Hauptvorteile:
Anstatt:
document . querySelectorAll ( "button" ) . forEach ( button => {
button . addEventListener ( "click" , handleButtonClick )
} )Die Ereignisdelegation umfasst die Verwendung einer Bedingung, um sicherzustellen, dass das untergeordnete Ziel unserem gewünschten Element entspricht:
document . addEventListener ( "click" , e => {
if ( e . target . closest ( "button" ) ) {
handleButtonClick ( )
}
} )
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setState ? Die Rückruffunktion wird aufgerufen, wenn setState fertig ist und die Komponente gerendert wird. Da setState asynchron ist, wird die Rückruffunktion für jede Postaktion verwendet.
setState ( { name : "sudheer" } , ( ) => {
console . log ( "The name has updated and component re-rendered" )
} ) setState -Fertigstellung aufgerufen und für jede Postaktion verwendet.setState
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In JavaScript gibt es zwei syntaktische Hauptkategorien: Ausdrücke und Aussagen. Ein dritter ist zusammen zusammen und wird als Expressionsaussage bezeichnet. Sie werden grob zusammengefasst als:
Eine allgemeine Faustregel:
Wenn Sie es drucken oder einer Variablen zuweisen können, ist dies ein Ausdruck. Wenn Sie nicht können, ist es eine Aussage.
let x = 0
function declaration ( ) { }
if ( true ) {
}Aussagen erscheinen als Anweisungen, die etwas tun, aber keine Werte erzeugen.
// Assign `x` to the absolute value of `y`.
var x
if ( y >= 0 ) {
x = y
} else {
x = - y
} Der einzige Ausdruck im obigen Code ist y >= 0 der einen Wert erzeugt, entweder true oder false . Ein Wert wird nicht von anderen Teilen des Codes erzeugt.
Ausdrücke erzeugen einen Wert. Sie können an Funktionen weitergegeben werden, da der Dolmetscher sie durch den Wert ersetzt, den sie lösen.
5 + 5 // => 10
lastCharacter ( "input" ) // => "t"
true === true // => true Es gibt eine äquivalente Version der zuvor verwendeten Aussagen als Ausdruck mit dem bedingten Operator:
// Assign `x` as the absolute value of `y`.
var x = y >= 0 ? y : - y Dies ist sowohl ein Ausdruck als auch eine Aussage, da wir eine Variable x (Anweisung) als Bewertung (Ausdruck) deklarieren.
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Ein Wert ist entweder wahr oder falsy, je nachdem, wie er in einem booleschen Kontext bewertet wird. Falsy bedeutet falsch und wahrheitsgemäß, bedeutet wahr. Im Wesentlichen sind es Werte, die bei der Durchführung bestimmter Operationen zu true oder false gezwungen werden.
Es gibt 6 falsy -Werte in JavaScript. Sie sind:
falseundefinednull"" (leere Zeichenfolge)NaN0 (sowohl +0 als auch -0 )Jeder andere Wert wird als Wahrheit angesehen.
Die Wahrheit eines Wertes kann untersucht werden, indem sie in die Boolean Funktion weitergegeben wird.
Boolean ( "" ) // false
Boolean ( [ ] ) // true Es gibt eine Verknüpfung dafür, die das logische Nichts unter Verwendung des Logical ! Operator. Verwenden ! Einmal wird ein Wert in sein umgekehrtes booleanes Äquivalent konvertieren (dh nicht falsch ist wahr) und ! Noch einmal wandelt sich um und wandelt den Wert effektiv in einen Booleschen um.
! ! "" // false
! ! [ ] // true
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Initialisieren Sie eine leere Länge n . Verwenden Sie Array.prototype.reduce() , um Werte in das Array mit der Summe der letzten beiden Werte mit Ausnahme der ersten beiden Werte hinzuzufügen.
const fibonacci = n =>
[ ... Array ( n ) ] . reduce (
( acc , val , i ) => acc . concat ( i > 1 ? acc [ i - 1 ] + acc [ i - 2 ] : i ) ,
[ ]
)
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0.1 + 0.2 === 0.3 ? Es bewertet false , weil JavaScript den IEEE 754-Standard für Mathematik verwendet und 64-Bit-schwimmende Zahlen verwendet. Dies verursacht Präzisionsfehler bei Dezimalberechnungen, kurz gesagt, aufgrund von Computern, die in Base 2 arbeiten, während Dezimaler Basis 10 ist.
0.1 + 0.2 // 0.30000000000000004Eine Lösung für dieses Problem wäre die Verwendung einer Funktion, die feststellt, ob zwei Zahlen ungefähr gleich sind, indem ein Fehlerrand (Epsilon) Wert definiert wird, dass die Differenz zwischen zwei Werten kleiner als.
const approxEqual = ( n1 , n2 , epsilon = 0.0001 ) => Math . abs ( n1 - n2 ) < epsilon
approxEqual ( 0.1 + 0.2 , 0.3 ) // true
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map() und forEach() ? Beide Methoden iterieren durch die Elemente eines Arrays. map() ordnet jedes Element auf ein neues Element zu, indem Sie die Rückruffunktion in jedem Element aufrufen und ein neues Array zurückgeben. Auf der anderen Seite ruft forEach() die Rückruffunktion für jedes Element auf, gibt jedoch kein neues Array zurück. forEach() wird im Allgemeinen verwendet, wenn eine Nebenwirkung auf jede Iteration verursacht wird, während map() eine häufige funktionelle Programmierungstechnik ist.
forEach() , wenn Sie über ein Array iterieren müssen und Mutationen an die Elemente verursachen müssen, ohne Werte zurückzugeben, um ein neues Array zu erzeugen.map() ist die richtige Wahl, um Daten unveränderlich zu halten, wenn jeder Wert des ursprünglichen Arrays einem neuen Array zugeordnet ist.
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Die Kurzschlussbewertung umfasst logische Operationen, die von links nach rechts bewertet und früh anhalten.
true || false In der obigen Stichprobe unter Verwendung von Logical oder JavaScript wird nicht das zweite Operand false untersucht, da der Ausdruck unabhängig davon zu true bewertet wird. Dies wird als Kurzschlussbewertung bezeichnet.
Dies funktioniert auch für logisch und.
false && trueDies bedeutet, dass Sie einen Ausdruck haben können, der einen Fehler macht, wenn Sie bewertet werden, und dies wird keine Probleme verursachen.
true || nonexistentFunction ( )
false && nonexistentFunction ( )Dies gilt für mehrere Operationen aufgrund der Bewertung von links nach rechts.
true || nonexistentFunction ( ) || window . nothing . wouldThrowError
true || window . nothing . wouldThrowError
trueEin häufiger Anwendungsfall für dieses Verhalten ist die Festlegung von Standardwerten. Wenn der erste Operand falsy ist, wird der zweite Operand bewertet.
const options = { }
const setting = options . setting || "default"
setting // "default"Ein weiterer häufiger Anwendungsfall ist nur die Bewertung eines Ausdrucks, wenn der erste Operand wahr ist.
// Instead of:
addEventListener ( "click" , e => {
if ( e . target . closest ( "button" ) ) {
handleButtonClick ( e )
}
} )
// You can take advantage of short-circuit evaluation:
addEventListener (
"click" ,
e => e . target . closest ( "button" ) && handleButtonClick ( e )
) Wenn im obigen Fall e.target kein Element enthält, das mit dem Selektor "button" übereinstimmt, wird die Funktion nicht aufgerufen. Dies liegt daran, dass der erste Operand falsy sein wird und der zweite Operand nicht bewertet wird.
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Manchmal. Aufgrund der automatischen Semikolon -Einfügung von JavaScript platziert der Dolmetscher Semikolons nach den meisten Aussagen. Dies bedeutet, dass Semikolons in den meisten Fällen weggelassen werden können.
Es gibt jedoch einige Fälle, in denen sie erforderlich sind. Sie sind zu Beginn eines Blocks nicht erforderlich, aber wenn sie einer Linie folgen und:
[ const previousLine = 3
; [ 1 , 2 , previousLine ] . map ( n => n * 2 )( const previousLine = 3
; ( function ( ) {
// ...
} ) ( ) In den obigen Fällen fügt der Dolmetscher nach 3 kein Semikolon ein, und daher wird der 3 als Versuchszugriff auf Objekteigenschaften angezeigt oder als Funktion aufgerufen, was Fehler auslöst.
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var foo = 1
var foobar = function ( ) {
console . log ( foo )
var foo = 2
}
foobar ( ) Aufgrund des Heizens wird die lokale variable foo vor der console.log -Methode aufgerufen. Dies bedeutet, dass die lokale Variable foo als Argument an console.log() anstelle des globalen, der außerhalb der Funktion deklariert wurde, übergeben wird. Da der Wert jedoch nicht mit der variablen Deklaration gehisst wird, wird die Ausgabe undefined , nicht 2 .
strict Modus
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Heben ist ein JavaScript -Mechanismus, bei dem während der Kompilierungsphase Variable und Funktionserklärungen in den Speicher gebracht werden. Dies bedeutet, dass, egal wo Funktionen und Variablen deklariert werden, sie auf die Spitze ihres Geltungsbereichs verschoben werden, unabhängig davon, ob ihr Geltungsbereich global oder lokal ist.
Der Wert wird jedoch nicht mit der Erklärung gehoben.
Der folgende Ausschnitt:
console . log ( hoist )
var hoist = "value"entspricht:
var hoist
console . log ( hoist )
hoist = "value" Daher gibt die Protokollierung hoist und undefined auf die Konsole und nicht "value" aus.
Hoisting also allows you to invoke a function declaration before it appears to be declared in a program.
myFunction ( ) // No error; logs "hello"
function myFunction ( ) {
console . log ( "hello" )
}But be wary of function expressions that are assigned to a variable:
myFunction ( ) // Error: `myFunction` is not a function
var myFunction = function ( ) {
console . log ( "hello" )
}
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In HTML, the attribute name is in all lowercase and is given a string invoking a function defined somewhere:
< button onclick =" handleClick() " > </ button >In React, the attribute name is camelCase and are passed the function reference inside curly braces:
< button onClick = { handleClick } /> In HTML, false can be returned to prevent default behavior, whereas in React preventDefault has to be called explicitly.
< a href =" # " onclick =" console.log('The link was clicked.'); return false " /> function handleClick ( e ) {
e . preventDefault ( )
console . log ( "The link was clicked." )
}
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This technique is very common in JavaScript libraries. It creates a closure around the entire contents of the file which creates a private namespace and thereby helps avoid potential name clashes between different JavaScript modules and libraries. The function is immediately invoked so that the namespace (library name) is assigned the return value of the function.
const myLibrary = ( function ( ) {
var privateVariable = 2
return {
publicMethod : ( ) => privateVariable
}
} ) ( )
privateVariable // ReferenceError
myLibrary . publicMethod ( ) // 2
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function greet ( ) {
return
{
message : "hello"
}
} Because of JavaScript's automatic semicolon insertion (ASI), the compiler places a semicolon after return keyword and therefore it returns undefined without an error being thrown.
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Since a JSX element tree is one large expression, you cannot embed statements inside. Conditional expressions act as a replacement for statements to use inside the tree.
For example, this won't work:
function App ( { messages , isVisible } ) {
return (
< div >
if (messages.length > 0 ) {
< h2 > You have { messages . length } unread messages. </ h2 >
} else {
< h2 > You have no unread messages. </ h2 >
}
if (isVisible) {
< p > I am visible. </ p >
}
</ div >
)
} Logical AND && and the ternary ? : operator replace the if / else statements.
function App ( { messages , isVisible } ) {
return (
< div >
{ messages . length > 0 ? (
< h2 > You have { messages . length } unread messages. </ h2 >
) : (
< h2 > You have no unread messages. </ h2 >
) }
{ isVisible && < p > I am visible. </ p > }
</ div >
)
}
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Keys are a special string attribute that helps React identify which items have been changed, added or removed. They are used when rendering array elements to give them a stable identity. Each element's key must be unique (eg IDs from the data or indexes as a last resort).
const todoItems = todos . map ( todo => < li key = { todo . id } > { todo . text } </ li > )<li> tag. <li> element, if you extract list items as components.
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Lexical scoping refers to when the location of a function's definition determines which variables you have access to. On the other hand, dynamic scoping uses the location of the function's invocation to determine which variables are available.
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# except for the last four (4) characters. mask ( "123456789" ) // "#####6789"There are many ways to solve this problem, this is just one one of them.
Using String.prototype.slice() we can grab the last 4 characters of the string by passing -4 as an argument. Then, using String.prototype.padStart() , we can pad the string to the original length with the repeated mask character.
const mask = ( str , maskChar = "#" ) =>
str . slice ( - 4 ) . padStart ( str . length , maskChar )
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const a = [ 1 , 2 , 3 ]
const b = [ 1 , 2 , 3 ]
const c = "1,2,3"
console . log ( a == c )
console . log ( a == b ) The first console.log outputs true because JavaScript's compiler performs type conversion and therefore it compares to strings by their value. On the other hand, the second console.log outputs false because Arrays are Objects and Objects are compared by reference.
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In the classical inheritance paradigm, object instances inherit their properties and functions from a class, which acts as a blueprint for the object. Object instances are typically created using a constructor and the new keyword.
In the prototypal inheritance paradigm, object instances inherit directly from other objects and are typically created using factory functions or Object.create() .
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MIME is an acronym for Multi-purpose Internet Mail Extensions . It is used as a standard way of classifying file types over the Internet.
MIME type actually has two parts: a type and a subtype that are separated by a slash (/). For example, the MIME type for Microsoft Word files is application/msword (ie, type is application and the subtype is msword).
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The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value. An example can be the following snippet, which after 100ms prints out the result string to the standard output. Also, note the catch, which can be used for error handling. Promise s are chainable.
new Promise ( ( resolve , reject ) => {
setTimeout ( ( ) => {
resolve ( "result" )
} , 100 )
} )
. then ( console . log )
. catch ( console . error ) Promise s!
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The latest ECMAScript standard defines seven data types, six of them being primitive: Boolean , Null , Undefined , Number , String , Symbol and one non-primitive data type: Object .
Symbol data typeArray , Date and function are all of type object
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fs . readFile ( filePath , function ( err , data ) {
if ( err ) {
// handle the error, the return is important here
// so execution stops here
return console . log ( err )
}
// use the data object
console . log ( data )
} )Zu den Vorteilen gehören:
As you can see from below example, the callback is called with null as its first argument if there is no error. However, if there is an error, you create an Error object, which then becomes the callback's only parameter. The callback function allows a user to easily know whether or not an error occurred.
This practice is also called the Node.js error convention , and this kind of callback implementations are called error-first callbacks .
var isTrue = function ( value , callback ) {
if ( value === true ) {
callback ( null , "Value was true." )
} else {
callback ( new Error ( "Value is not true!" ) )
}
}
var callback = function ( error , retval ) {
if ( error ) {
console . log ( error )
return
}
console . log ( retval )
}
isTrue ( false , callback )
isTrue ( true , callback )
/*
{ stack: [Getter/Setter],
arguments: undefined,
type: undefined,
message: 'Value is not true!' }
Value was true.
*/
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Callbacks are functions passed as an argument to another function to be executed once an event has occurred or a certain task is complete, often used in asynchronous code. Callback functions are invoked later by a piece of code but can be declared on initialization without being invoked.
As an example, event listeners are asynchronous callbacks that are only executed when a specific event occurs.
function onClick ( ) {
console . log ( "The user clicked on the page." )
}
document . addEventListener ( "click" , onClick ) However, callbacks can also be synchronous. The following map function takes a callback function that is invoked synchronously for each iteration of the loop (array element).
const map = ( arr , callback ) => {
const result = [ ]
for ( let i = 0 ; i < arr . length ; i ++ ) {
result . push ( callback ( arr [ i ] , i ) )
}
return result
}
map ( [ 1 , 2 , 3 , 4 , 5 ] , n => n * 2 ) // [2, 4, 6, 8, 10]
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null and undefined ? In JavaScript, two values discretely represent nothing - undefined and null . The concrete difference between them is that null is explicit, while undefined is implicit. When a property does not exist or a variable has not been given a value, the value is undefined . null is set as the value to explicitly indicate “no value”. In essence, undefined is used when the nothing is not known, and null is used when the nothing is known.
typeof undefined evaluates to "undefined" .typeof null evaluates "object" . However, it is still a primitive value and this is considered an implementation bug in JavaScript.undefined == null evaluates to true .
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Often used to store one occurrence of data.
const person = {
name : "John" ,
age : 50 ,
birthday ( ) {
this . age ++
}
}
person . birthday ( ) // person.age === 51 Often used when you need to create multiple instances of an object, each with their own data that other instances of the class cannot affect. The new operator must be used before invoking the constructor or the global object will be mutated.
function Person ( name , age ) {
this . name = name
this . age = age
}
Person . prototype . birthday = function ( ) {
this . age ++
}
const person1 = new Person ( "John" , 50 )
const person2 = new Person ( "Sally" , 20 )
person1 . birthday ( ) // person1.age === 51
person2 . birthday ( ) // person2.age === 21 Creates a new object similar to a constructor, but can store private data using a closure. There is also no need to use new before invoking the function or the this keyword. Factory functions usually discard the idea of prototypes and keep all properties and methods as own properties of the object.
const createPerson = ( name , age ) => {
const birthday = ( ) => person . age ++
const person = { name , age , birthday }
return person
}
const person = createPerson ( "John" , 50 )
person . birthday ( ) // person.age === 51 Object.create()Sets the prototype of the newly created object.
const personProto = {
birthday ( ) {
this . age ++
}
}
const person = Object . create ( personProto )
person . age = 50
person . birthday ( ) // person.age === 51 A second argument can also be supplied to Object.create() which acts as a descriptor for the new properties to be defined.
Object . create ( personProto , {
age : {
value : 50 ,
writable : true ,
enumerable : true
}
} )
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Parameters are the variable names of the function definition, while arguments are the values given to a function when it is invoked.
function myFunction ( parameter1 , parameter2 ) {
console . log ( arguments [ 0 ] ) // "argument1"
}
myFunction ( "argument1" , "argument2" ) arguments is an array-like object containing information about the arguments supplied to an invoked function.myFunction.length describes the arity of a function (how many parameters it has, regardless of how many arguments it is supplied).
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JavaScript always passes by value. However, with objects, the value is a reference to the object.
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You can use an arrow function to wrap around an event handler and pass arguments, which is equivalent to calling bind :
< button onClick = { ( ) => this . handleClick ( id ) } />
< button onClick = { this . handleClick . bind ( this , id ) } / >
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Fragments allow a React component to return multiple elements without a wrapper, by grouping the children without adding extra elements to the DOM. Fragments offer better performance, lower memory usage, a cleaner DOM and can help in dealing with certain CSS mechanisms (eg tables, Flexbox and Grid).
render ( ) {
return (
< React . Fragment >
< ChildA />
< ChildB />
< ChildC />
</ React . Fragment >
) ;
}
// Short syntax supported by Babel 7
render ( ) {
return (
< >
< ChildA />
< ChildB />
< ChildC />
</ >
) ;
}
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pipe that performs left-to-right function composition by returning a function that accepts one argument. const square = v => v * v
const double = v => v * 2
const addOne = v => v + 1
const res = pipe ( square , double , addOne )
res ( 3 ) // 19; addOne(double(square(3))) Gather all supplied arguments using the rest operator ... and return a unary function that uses Array.prototype.reduce() to run the value through the series of functions before returning the final value.
const pipe = ( ... fns ) => x => fns . reduce ( ( v , fn ) => fn ( v ) , x )
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The event loop handles all async callbacks. Callbacks are queued in a loop, while other code runs, and will run one by one when the response for each one has been received.
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NaN (Not-a-Number) is the only value not equal to itself when comparing with any of the comparison operators. NaN is often the result of meaningless math computations, so two NaN values make no sense to be considered equal.
isNaN() and Number.isNaN()const isNaN = x => x !== xNaN
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The two terms can be contrasted as:
In JavaScript, objects are mutable while primitive values are immutable. This means operations performed on objects can change the original reference in some way, while operations performed on a primitive value cannot change the original value.
All String.prototype methods do not have an effect on the original string and return a new string. On the other hand, while some methods of Array.prototype do not mutate the original array reference and produce a fresh array, some cause mutations.
const myString = "hello!"
myString . replace ( "!" , "" ) // returns a new string, cannot mutate the original value
const originalArray = [ 1 , 2 , 3 ]
originalArray . push ( 4 ) // mutates originalArray, now [1, 2, 3, 4]
originalArray . concat ( 4 ) // returns a new array, does not mutate the original
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Big O notation is used in Computer Science to describe the time complexity of an algorithm. The best algorithms will execute the fastest and have the simplest complexity.
Algorithms don't always perform the same and may vary based on the data they are supplied. While in some cases they will execute quickly, in other cases they will execute slowly, even with the same number of elements to deal with.
In these examples, the base time is 1 element = 1ms .
arr [ arr . length - 1 ]1msConstant time complexity. No matter how many elements the array has, it will theoretically take (excluding real-world variation) the same amount of time to execute.
arr . filter ( fn )1000msLinear time complexity. The execution time will increase linearly with the number of elements the array has. If the array has 1000 elements and the function takes 1ms to execute, 7000 elements will take 7ms to execute. This is because the function must iterate through all elements of the array before returning a result.
arr . some ( fn )1ms <= x <= 1000msThe execution time varies depending on the data supplied to the function, it may return very early or very late. The best case here is O(1) and the worst case is O(N).
arr . sort ( fn )10000ms Browsers usually implement the quicksort algorithm for the sort() method and the average time complexity of quicksort is O(NlgN). This is very efficient for large collections.
for ( let i = 0 ; i < arr . length ; i ++ ) {
for ( let j = 0 ; j < arr . length ; j ++ ) {
// ...
}
}1000000msThe execution time rises quadratically with the number of elements. Usually the result of nesting loops.
const permutations = arr => {
if ( arr . length <= 2 ) return arr . length === 2 ? [ arr , [ arr [ 1 ] , arr [ 0 ] ] ] : arr
return arr . reduce (
( acc , item , i ) =>
acc . concat (
permutations ( [ ... arr . slice ( 0 , i ) , ... arr . slice ( i + 1 ) ] ) . map ( val => [
item ,
... val
] )
) ,
[ ]
)
}Infinity (practically) msThe execution time rises extremely fast with even just 1 addition to the array.
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A pure function is a function that satisfies these two conditions:
Pure functions can mutate local data within the function as long as it satisfies the two conditions above.
const a = ( x , y ) => x + y
const b = ( arr , value ) => arr . concat ( value )
const c = arr => [ ... arr ] . sort ( ( a , b ) => a - b ) const a = ( x , y ) => x + y + Math . random ( )
const b = ( arr , value ) => ( arr . push ( value ) , arr )
const c = arr => arr . sort ( ( a , b ) => a - b ) setInnerHTML ).
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Recursion is the repeated application of a process. In JavaScript, recursion involves functions that call themselves repeatedly until they reach a base condition. The base condition breaks out of the recursion loop because otherwise the function would call itself indefinitely. Recursion is very useful when working with data structures that contain nesting where the number of levels deep is unknown.
For example, you may have a thread of comments returned from a database that exist in a flat array but need to be nested for display in the UI. Each comment is either a top-level comment (no parent) or is a reply to a parent comment. Comments can be a reply of a reply of a reply... we have no knowledge beforehand the number of levels deep a comment may be. This is where recursion can help.
const nest = ( items , id = null , link = "parent_id" ) =>
items
. filter ( item => item [ link ] === id )
. map ( item => ( { ... item , children : nest ( items , item . id ) } ) )
const comments = [
{ id : 1 , parent_id : null , text : "First reply to post." } ,
{ id : 2 , parent_id : 1 , text : "First reply to comment #1." } ,
{ id : 3 , parent_id : 1 , text : "Second reply to comment #1." } ,
{ id : 4 , parent_id : 3 , text : "First reply to comment #3." } ,
{ id : 5 , parent_id : 4 , text : "First reply to comment #4." } ,
{ id : 6 , parent_id : null , text : "Second reply to post." }
]
nest ( comments )
/*
[
{ id: 1, parent_id: null, text: "First reply to post.", children: [...] },
{ id: 6, parent_id: null, text: "Second reply to post.", children: [] }
]
*/ In the above example, the base condition is met if filter() returns an empty array. The chained map() won't invoke the callback function which contains the recursive call, thereby breaking the loop.
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Memoization is the process of caching the output of function calls so that subsequent calls are faster. Calling the function again with the same input will return the cached output without needing to do the calculation again.
A basic implementation in JavaScript looks like this:
const memoize = fn => {
const cache = new Map ( )
return value => {
const cachedResult = cache . get ( value )
if ( cachedResult !== undefined ) return cachedResult
const result = fn ( value )
cache . set ( value , result )
return result
}
}
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Refs provide a way to access DOM nodes or React elements created in the render method. Refs should be used sparringly, but there are some good use cases for refs, such as:
Refs are created using React.createRef() method and attached to React elements via the ref attribute. In order to use refs throughout the component, assign the ref to the instance property within the constructor:
class MyComponent extends React . Component {
constructor ( props ) {
super ( props )
this . myRef = React . createRef ( )
}
render ( ) {
return < div ref = { this . myRef } />
}
}Refs can also be used in functional components with the help of closures.
React.createRef() and attach to elements via the ref attribute.
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These two types of programming can roughly be summarized as:
A common example of declarative programming is CSS. The developer specifies CSS properties that describe what something should look like rather than how to achieve it. The "how" is abstracted away by the browser.
On the other hand, imperative programming involves the steps required to achieve something. In JavaScript, the differences can be contrasted like so:
const numbers = [ 1 , 2 , 3 , 4 , 5 ]
const numbersDoubled = [ ]
for ( let i = 0 ; i < numbers . length ; i ++ ) {
numbersDoubled [ i ] = numbers [ i ] * 2
}We manually loop over the numbers of the array and assign the new index as the number doubled.
const numbers = [ 1 , 2 , 3 , 4 , 5 ]
const numbersDoubled = numbers . map ( n => n * 2 )We declare that the new array is mapped to a new one where each value is doubled.
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Functional programming is a paradigm in which programs are built in a declarative manner using pure functions that avoid shared state and mutable data. Functions that always return the same value for the same input and don't produce side effects are the pillar of functional programming. Many programmers consider this to be the best approach to software development as it reduces bugs and cognitive load.
.map , .reduce etc.)
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Portal are the recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.
ReactDOM . createPortal ( child , container ) The first argument ( child ) is any renderable React child, such as an element, string, or fragment. The second argument ( container ) is a DOM element.
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Event-driven programming is a paradigm that involves building applications that send and receive events. When the program emits events, the program responds by running any callback functions that are registered to that event and context, passing in associated data to the function. With this pattern, events can be emitted into the wild without throwing errors even if no functions are subscribed to it.
A common example of this is the pattern of elements listening to DOM events such as click and mouseenter , where a callback function is run when the event occurs.
document . addEventListener ( "click" , function ( event ) {
// This callback function is run when the user
// clicks on the document.
} )Without the context of the DOM, the pattern may look like this:
const hub = createEventHub ( )
hub . on ( "message" , function ( data ) {
console . log ( ` ${ data . username } said ${ data . text } ` )
} )
hub . emit ( "message" , {
username : "John" ,
text : "Hello?"
} ) With this implementation, on is the way to subscribe to an event, while emit is the way to publish the event.
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Context provides a way to pass data through the component tree without having to pass props down manually at every level. For example, authenticated user, locale preference, UI theme need to be accessed in the application by many components.
const { Provider , Consumer } = React . createContext ( defaultValue )
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Static methods belong to a class and don't act on instances, while instance methods belong to the class prototype which is inherited by all instances of the class and acts on them.
Array . isArray // static method of Array
Array . prototype . push // instance method of Array In this case, the Array.isArray method does not make sense as an instance method of arrays because we already know the value is an array when working with it.
Instance methods could technically work as static methods, but provide terser syntax:
const arr = [ 1 , 2 , 3 ]
arr . push ( 4 )
Array . push ( arr , 4 )
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A closure is a function defined inside another function and has access to its lexical scope even when it is executing outside its lexical scope. The closure has access to variables in three scopes:
In JavaScript, all functions are closures because they have access to the outer scope, but most functions don't utilise the usefulness of closures: the persistence of state. Closures are also sometimes called stateful functions because of this.
In addition, closures are the only way to store private data that can't be accessed from the outside in JavaScript. They are the key to the UMD (Universal Module Definition) pattern, which is frequently used in libraries that only expose a public API but keep the implementation details private, preventing name collisions with other libraries or the user's own code.
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this keyword and how does it work? The this keyword is an object that represents the context of an executing function. Regular functions can have their this value changed with the methods call() , apply() and bind() . Arrow functions implicitly bind this so that it refers to the context of its lexical environment, regardless of whether or not its context is set explicitly with call() .
Here are some common examples of how this works:
this refers to the object itself inside regular functions if the object precedes the invocation of the function.
Properties set as this do not refer to the object.
var myObject = {
property : this ,
regularFunction : function ( ) {
return this
} ,
arrowFunction : ( ) => {
return this
} ,
iife : ( function ( ) {
return this
} ) ( )
}
myObject . regularFunction ( ) // myObject
myObject [ "regularFunction" ] ( ) // my Object
myObject . property // NOT myObject; lexical `this`
myObject . arrowFunction ( ) // NOT myObject; lexical `this`
myObject . iife // NOT myObject; lexical `this`
const regularFunction = myObject . regularFunction
regularFunction ( ) // NOT myObject; lexical `this` this refers to the element listening to the event.
document . body . addEventListener ( "click" , function ( ) {
console . log ( this ) // document.body
} ) this refers to the newly created object.
class Example {
constructor ( ) {
console . log ( this ) // myExample
}
}
const myExample = new Example ( ) With call() and apply() , this refers to the object passed as the first argument.
var myFunction = function ( ) {
return this
}
myFunction . call ( { customThis : true } ) // { customThis: true } this Because this can change depending on the scope, it can have unexpected values when using regular functions.
var obj = {
arr : [ 1 , 2 , 3 ] ,
doubleArr ( ) {
return this . arr . map ( function ( value ) {
// this is now this.arr
return this . double ( value )
} )
} ,
double ( ) {
return value * 2
}
}
obj . doubleArr ( ) // Uncaught TypeError: this.double is not a function this is the global object ( window in browsers), while in strict mode global this is undefined .Function.prototype.call and Function.prototype.apply set the this context of an executing function as the first argument, with call accepting a variadic number of arguments thereafter, and apply accepting an array as the second argument which are fed to the function in a variadic manner.Function.prototype.bind returns a new function that enforces the this context as the first argument which cannot be changed by other functions.this context to be changed based on how it is called, you must use the function keyword. Use arrow functions when you want this to be the surrounding (lexical) context. this on MDN
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children prop? children is part of the props object passed to components that allows components to be passed as data to other components, providing the ability to compose components cleanly. There are a number of methods available in the React API to work with this prop, such as React.Children.map , React.Children.forEach , React.Children.count , React.Children.only and React.Children.toArray . A simple usage example of the children prop is as follows:
function GenericBox ( { children } ) {
return < div className = "container" > { children } </ div >
}
function App ( ) {
return (
< GenericBox >
< span > Hello </ span > < span > World </ span >
</ GenericBox >
)
}
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Callback refs are preferred over the findDOMNode() API, due to the fact that findDOMNode() prevents certain improvements in React in the future.
// Legacy approach using findDOMNode()
class MyComponent extends Component {
componentDidMount ( ) {
findDOMNode ( this ) . scrollIntoView ( )
}
render ( ) {
return < div />
}
}
// Recommended approach using callback refs
class MyComponent extends Component {
componentDidMount ( ) {
this . node . scrollIntoView ( )
}
render ( ) {
return < div ref = { node => ( this . node = node ) } />
}
} findDOMNode() .
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The main purpose is to avoid manipulating the DOM directly and keep the state of an application in sync with the UI easily. Additionally, they provide the ability to create components that can be reused when they have similar functionality with minor differences, avoiding duplication which would require multiple changes whenever the structure of a component which is reused in multiple places needs to be updated.
When working with DOM manipulation libraries like jQuery, the data of an application is generally kept in the DOM itself, often as class names or data attributes. Manipulating the DOM to update the UI involves many extra steps and can introduce subtle bugs over time. Keeping the state separate and letting a framework handle the UI updates when the state changes reduces cognitive load. Saying you want the UI to look a certain way when the state is a certain value is the declarative way of creating an application, instead of the imperative way of manually updating the UI to reflect the new state.
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'use strict' do and what are some of the key benefits to using it? Including 'use strict' at the beginning of your JavaScript source file enables strict mode, which enforces more strict parsing and error handling of JavaScript code. It is considered a good practice and offers a lot of benefits, such as:
eval() and arguments .this coercion, throwing an error when this references a value of null or undefined .delete .
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getData ( function ( a ) {
getMoreData ( a , function ( b ) {
getMoreData ( b , function ( c ) {
getMoreData ( c , function ( d ) {
getMoreData ( d , function ( e ) {
// ...
} )
} )
} )
} )
} ) Refactoring the functions to return promises and using async/await is usually the best option. Instead of supplying the functions with callbacks that cause deep nesting, they return a promise that can be await ed and will be resolved once the data has arrived, allowing the next line of code to be evaluated in a sync-like fashion.
The above code can be restructured like so:
async function asyncAwaitVersion ( ) {
const a = await getData ( )
const b = await getMoreData ( a )
const c = await getMoreData ( b )
const d = await getMoreData ( c )
const e = await getMoreData ( d )
// ...
}There are lots of ways to solve the issue of callback hells:
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The virtual DOM (VDOM) is a representation of the real DOM in the form of plain JavaScript objects. These objects have properties to describe the real DOM nodes they represent: the node name, its attributes, and child nodes.
< div class =" counter " >
< h1 > 0 </ h1 >
< button > - </ button >
< button > + </ button >
</ div >The above markup's virtual DOM representation might look like this:
{
nodeName : "div" ,
attributes : { class : "counter" } ,
children : [
{
nodeName : "h1" ,
attributes : { } ,
children : [ 0 ]
} ,
{
nodeName : "button" ,
attributes : { } ,
children : [ "-" ]
} ,
{
nodeName : "button" ,
attributes : { } ,
children : [ "+" ]
}
]
}The library/framework uses the virtual DOM as a means to improve performance. When the state of an application changes, the real DOM needs to be updated to reflect it. However, changing real DOM nodes is costly compared to recalculating the virtual DOM. The previous virtual DOM can be compared to the new virtual DOM very quickly in comparison.
Once the changes between the old VDOM and new VDOM have been calculated by the diffing engine of the framework, the real DOM can be patched efficiently in the least time possible to match the new state of the application.
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this context in React component classes? In JavaScript classes, the methods are not bound by default. This means that their this context can be changed (in the case of an event handler, to the element that is listening to the event) and will not refer to the component instance. To solve this, Function.prototype.bind() can be used to enforce the this context as the component instance.
constructor ( props ) {
super ( props ) ;
this . handleClick = this . handleClick . bind ( this ) ;
}
handleClick ( ) {
// Perform some logic
}bind approach can be verbose and requires defining a constructor , so the new public class fields syntax is generally preferred: handleClick = ( ) => {
console . log ( 'this is:' , this ) ;
}
render ( ) {
return (
< button onClick = { this . handleClick } >
Click me
</ button >
) ;
}this (referring to the component instance) is preserved: < button onClick = { e => this . handleClick ( e ) } > Click me </ button > Note that extra re-rendering can occur using this technique because a new function reference is created on render, which gets passed down to child components and breaks shouldComponentUpdate / PureComponent shallow equality checks to prevent unnecessary re-renders. In cases where performance is important, it is preferred to go with bind in the constructor, or the public class fields syntax approach, because the function reference remains constant.
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A stateless component is a component whose behavior does not depend on its state. Stateless components can be either functional or class components. Stateless functional components are easier to maintain and test since they are guaranteed to produce the same output given the same props. Stateless functional components should be preferred when lifecycle hooks don't need to be used.
this keyword altogether.
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A stateful component is a component whose behavior depends on its state. This means that two separate instances of the component if given the same props will not necessarily render the same output, unlike pure function components.
// Stateful class component
class App extends Component {
constructor ( props ) {
super ( props )
this . state = { count : 0 }
}
render ( ) {
// ...
}
}
// Stateful function component
function App ( ) {
const [ count , setCount ] = useState ( 0 )
return // ...
} useState() .
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Comments must be wrapped inside curly braces {} and use the /* */ syntax.
const tree = (
< div >
{ /* Comment */ }
< p > Text </ p >
</ div >
)
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An element is a plain JavaScript object that represents a DOM node or component. Elements are pure and never mutated, and are cheap to create.
A component is a function or class. Components can have state and take props as input and return an element tree as output (although they can represent generic containers or wrappers and don't necessarily have to emit DOM). Components can initiate side effects in lifecycle methods (eg AJAX requests, DOM mutations, interfacing with 3rd party libraries) and may be expensive to create.
const Component = ( ) => "Hello"
const componentElement = < Component />
const domNodeElement = < div />
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When several components need to share the same data, then it is recommended to lift the shared state up to their closest common ancestor. For example, if two child components share the same data, it is recommended to move the shared state to parent instead of maintaining the local state in both child components.
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className instead of class like in HTML? React's philosophy in the beginning was to align with the browser DOM API rather than HTML, since that more closely represents how elements are created. Setting a class on an element meant using the className API:
const element = document . createElement ( "div" )
element . className = "hello"Additionally, before ES5, reserved words could not be used in objects:
const element = {
attributes : {
class : "hello"
}
}In IE8, this will throw an error.
In modern environments, destructuring will throw an error if trying to assign to a variable:
const { class } = this . props // Error
const { className } = this . props // All good
const { class : className } = this . props // All good, but cumbersome! However, class can be used as a prop without problems, as seen in other libraries like Preact. React currently allows you to use class , but will throw a warning and convert it to className under the hood. There is currently an open thread (as of January 2019) discussing changing className to class to reduce confusion.
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You can use an arrow function to wrap around an event handler and pass arguments, which is equivalent to calling bind :
< button onClick = { ( ) => this . handleClick ( id ) } />
< button onClick = { this . handleClick . bind ( this , id ) } / >
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setState ? The callback function is invoked when setState has finished and the component gets rendered. Since setState is asynchronous, the callback function is used for any post action.
setState ( { name : "sudheer" } , ( ) => {
console . log ( "The name has updated and component re-rendered" )
} ) setState finishes and is used for any post action.setState
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There are four different phases of component's lifecycle:
Initialization : In this phase, the component prepares setting up the initial state and default props.
Mounting : The react component is ready to mount to the DOM. This phase covers the getDerivedStateFromProps and componentDidMount lifecycle methods.
Updating : In this phase, the component gets updated in two ways, sending the new props and updating the state. This phase covers the getDerivedStateFromProps , shouldComponentUpdate , getSnapshotBeforeUpdate and componentDidUpdate lifecycle methods.
Unmounting : In this last phase, the component is not needed and gets unmounted from the browser DOM. This phase includes the componentWillUnmount lifecycle method.
Error Handling : In this phase, the component is called whenever there's an error during rendering, in a lifecycle method, or in the constructor for any child component. This phase includes the componentDidCatch lifecycle method.
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In HTML, the attribute name is in all lowercase and is given a string invoking a function defined somewhere:
< button onclick =" handleClick() " > </ button >In React, the attribute name is camelCase and are passed the function reference inside curly braces:
< button onClick = { handleClick } /> In HTML, false can be returned to prevent default behavior, whereas in React preventDefault has to be called explicitly.
< a href =" # " onclick =" console.log('The link was clicked.'); return false " /> function handleClick ( e ) {
e . preventDefault ( )
console . log ( "The link was clicked." )
}
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Since a JSX element tree is one large expression, you cannot embed statements inside. Conditional expressions act as a replacement for statements to use inside the tree.
For example, this won't work:
function App ( { messages , isVisible } ) {
return (
< div >
if (messages.length > 0 ) {
< h2 > You have { messages . length } unread messages. </ h2 >
} else {
< h2 > You have no unread messages. </ h2 >
}
if (isVisible) {
< p > I am visible. </ p >
}
</ div >
)
} Logical AND && and the ternary ? : operator replace the if / else statements.
function App ( { messages , isVisible } ) {
return (
< div >
{ messages . length > 0 ? (
< h2 > You have { messages . length } unread messages. </ h2 >
) : (
< h2 > You have no unread messages. </ h2 >
) }
{ isVisible && < p > I am visible. </ p > }
</ div >
)
}
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getDerivedStateFromProps : Executed before rendering on the initial mount and all component updates. Used to update the state based on changes in props over time. Has rare use cases, like tracking component animations during the lifecycle. There are only few cases where this makes sense to use over other lifecycle methods. It expects to return an object that will be the the new state, or null to update nothing. This method does not have access to the component instance either.
componentDidMount : Executed after first rendering and here all AJAX requests, DOM or state updates, and set up eventListeners should occur.
shouldComponentUpdate : Determines if the component will be updated or not. By default, it returns true. If you are sure that the component doesn't need to render after state or props are updated, you can return a false value. It is a great place to improve performance as it allows you to prevent a rerender if component receives new prop.
getSnapshotBeforeUpdate : Invoked right after a component render happens because of an update, before componentDidUpdate . Any value returned from this method will be passed to componentDidUpdate .
componentDidUpdate : Mostly it is used to update the DOM in response to prop or state changes.
componentWillUnmount : It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
componentDidCatch : Used in error boundaries, which are components that implement this method. It allows the component to catch JavaScript errors anywhere in the child component tree (below this component), log errors, and display a UI with error information.
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Keys are a special string attribute that helps React identify which items have been changed, added or removed. They are used when rendering array elements to give them a stable identity. Each element's key must be unique (eg IDs from the data or indexes as a last resort).
const todoItems = todos . map ( todo => < li key = { todo . id } > { todo . text } </ li > )<li> tag. <li> element, if you extract list items as components.
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Callback refs are preferred over the findDOMNode() API, due to the fact that findDOMNode() prevents certain improvements in React in the future.
// Legacy approach using findDOMNode()
class MyComponent extends Component {
componentDidMount ( ) {
findDOMNode ( this ) . scrollIntoView ( )
}
render ( ) {
return < div />
}
}
// Recommended approach using callback refs
class MyComponent extends Component {
componentDidMount ( ) {
this . node . scrollIntoView ( )
}
render ( ) {
return < div ref = { node => ( this . node = node ) } />
}
} findDOMNode() .
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Fragments allow a React component to return multiple elements without a wrapper, by grouping the children without adding extra elements to the DOM. Fragments offer better performance, lower memory usage, a cleaner DOM and can help in dealing with certain CSS mechanisms (eg tables, Flexbox and Grid).
render ( ) {
return (
< React . Fragment >
< ChildA />
< ChildB />
< ChildC />
</ React . Fragment >
) ;
}
// Short syntax supported by Babel 7
render ( ) {
return (
< >
< ChildA />
< ChildB />
< ChildC />
</ >
) ;
}
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this context in React component classes? In JavaScript classes, the methods are not bound by default. This means that their this context can be changed (in the case of an event handler, to the element that is listening to the event) and will not refer to the component instance. To solve this, Function.prototype.bind() can be used to enforce the this context as the component instance.
constructor ( props ) {
super ( props ) ;
this . handleClick = this . handleClick . bind ( this ) ;
}
handleClick ( ) {
// Perform some logic
}bind approach can be verbose and requires defining a constructor , so the new public class fields syntax is generally preferred: handleClick = ( ) => {
console . log ( 'this is:' , this ) ;
}
render ( ) {
return (
< button onClick = { this . handleClick } >
Click me
</ button >
) ;
}this (referring to the component instance) is preserved: < button onClick = { e => this . handleClick ( e ) } > Click me </ button > Note that extra re-rendering can occur using this technique because a new function reference is created on render, which gets passed down to child components and breaks shouldComponentUpdate / PureComponent shallow equality checks to prevent unnecessary re-renders. In cases where performance is important, it is preferred to go with bind in the constructor, or the public class fields syntax approach, because the function reference remains constant.
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Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
Class components become error boundaries if they define either (or both) of the lifecycle methods static getDerivedStateFromError() or componentDidCatch().
class ErrorBoundary extends React . Component {
constructor ( props ) {
super ( props )
this . state = { hasError : false }
}
// Use componentDidCatch to log the error
componentDidCatch ( error , info ) {
// You can also log the error to an error reporting service
logErrorToMyService ( error , info )
}
// use getDerivedStateFromError to update state
static getDerivedStateFromError ( error ) {
// Display fallback UI
return { hasError : true } ;
}
render ( ) {
if ( this . state . hasError ) {
// You can render any custom fallback UI
return < h1 > Something went wrong. </ h1 >
}
return this . props . children
}
}
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A higher-order component (HOC) is a function that takes a component as an argument and returns a new component. It is a pattern that is derived from React's compositional nature. Higher-order components are like pure components because they accept any dynamically provided child component, but they won't modify or copy any behavior from their input components.
const EnhancedComponent = higherOrderComponent ( WrappedComponent )
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When the application is running in development mode, React will automatically check for all props that we set on components to make sure they are the correct data type. For incorrect data types, it will generate warning messages in the console for development mode. They are stripped in production mode due to their performance impact. Required props are defined with isRequired .
For example, we define propTypes for component as below:
import PropTypes from "prop-types"
class User extends React . Component {
static propTypes = {
name : PropTypes . string . isRequired ,
age : PropTypes . number . isRequired
}
render ( ) {
return (
< h1 > Welcome, { this . props . name } </ h1 >
< h2 > Age , { this . props . age }
)
}
} propTypespropTypes is not mandatory. However, it is a good practice and can reduce bugs.
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Context provides a way to pass data through the component tree without having to pass props down manually at every level. For example, authenticated user, locale preference, UI theme need to be accessed in the application by many components.
const { Provider , Consumer } = React . createContext ( defaultValue )
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Refs provide a way to access DOM nodes or React elements created in the render method. Refs should be used sparringly, but there are some good use cases for refs, such as:
Refs are created using React.createRef() method and attached to React elements via the ref attribute. In order to use refs throughout the component, assign the ref to the instance property within the constructor:
class MyComponent extends React . Component {
constructor ( props ) {
super ( props )
this . myRef = React . createRef ( )
}
render ( ) {
return < div ref = { this . myRef } />
}
}Refs can also be used in functional components with the help of closures.
React.createRef() and attach to elements via the ref attribute.
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children prop? children is part of the props object passed to components that allows components to be passed as data to other components, providing the ability to compose components cleanly. There are a number of methods available in the React API to work with this prop, such as React.Children.map , React.Children.forEach , React.Children.count , React.Children.only and React.Children.toArray . A simple usage example of the children prop is as follows:
function GenericBox ( { children } ) {
return < div className = "container" > { children } </ div >
}
function App ( ) {
return (
< GenericBox >
< span > Hello </ span > < span > World </ span >
</ GenericBox >
)
}
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Portal are the recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.
ReactDOM . createPortal ( child , container ) The first argument ( child ) is any renderable React child, such as an element, string, or fragment. The second argument ( container ) is a DOM element.
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alt attribute on images? The alt attribute provides alternative information for an image if a user cannot view it. The alt attribute should be used to describe any images except those which only serve a decorative purpose, in which case it should be left empty.
alt attribute.alt tags to understand image content, so they are considered important for Search Engine Optimization (SEO).. at the end of alt tag to improve accessibility.
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Browsers have a cache to temporarily store files on websites so they don't need to be re-downloaded again when switching between pages or reloading the same page. The server is set up to send headers that tell the browser to store the file for a given amount of time. This greatly increases website speed and preserves bandwidth.
However, it can cause problems when the website has been changed by developers because the user's cache still references old files. This can either leave them with old functionality or break a website if the cached CSS and JavaScript files are referencing elements that no longer exist, have moved or have been renamed.
Cache busting is the process of forcing the browser to download the new files. This is done by naming the file something different to the old file.
A common technique to force the browser to re-download the file is to append a query string to the end of the file.
src="js/script.js" => src="js/script.js?v=2"The browser considers it a different file but prevents the need to change the file name.
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<header> elements? What about <footer> elements? Yes to both. The W3 documents state that the tags represent the header( <header> ) and footer( <footer> ) areas of their nearest ancestor "section". So not only can the page <body> contain a header and a footer, but so can every <article> and <section> element.
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<header> , <article> , <section> , <footer> <header> is used to contain introductory and navigational information about a section of the page. This can include the section heading, the author's name, time and date of publication, table of contents, or other navigational information.
<article> is meant to house a self-contained composition that can logically be independently recreated outside of the page without losing its meaning. Individual blog posts or news stories are good examples.
<section> is a flexible container for holding content that shares a common informational theme or purpose.
<footer> is used to hold information that should appear at the end of a section of content and contain additional information about the section. Author's name, copyright information, and related links are typical examples of such content.
<form> and <table>
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rel="noopener" attribute used? The rel="noopener" is an attribute used in <a> elements (hyperlinks). It prevents pages from having a window.opener property, which would otherwise point to the page from where the link was opened and would allow the page opened from the hyperlink to manipulate the page where the hyperlink is.
rel="noopener" is applied to hyperlinks.rel="noopener" prevents opened links from manipulating the source page.
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defer and async attributes on a <script> tag?If neither attribute is present, the script is downloaded and executed synchronously, and will halt parsing of the document until it has finished executing (default behavior). Scripts are downloaded and executed in the order they are encountered.
The defer attribute downloads the script while the document is still parsing but waits until the document has finished parsing before executing it, equivalent to executing inside a DOMContentLoaded event listener. defer scripts will execute in order.
The async attribute downloads the script during parsing the document but will pause the parser to execute the script before it has fully finished parsing. async scripts will not necessarily execute in order.
Note: both attributes must only be used if the script has a src attribute (ie not an inline script).
< script src =" myscript.js " > </ script >
< script src =" myscript.js " defer > </ script >
< script src =" myscript.js " async > </ script > defer script in the <head> allows the browser to download the script while the page is still parsing, and is therefore a better option than placing the script before the end of the body.defer .async .defer if the DOM must be ready and the contents are not placed within a DOMContentLoaded listener.
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In HTML, the attribute name is in all lowercase and is given a string invoking a function defined somewhere:
< button onclick =" handleClick() " > </ button >In React, the attribute name is camelCase and are passed the function reference inside curly braces:
< button onClick = { handleClick } /> In HTML, false can be returned to prevent default behavior, whereas in React preventDefault has to be called explicitly.
< a href =" # " onclick =" console.log('The link was clicked.'); return false " /> function handleClick ( e ) {
e . preventDefault ( )
console . log ( "The link was clicked." )
}
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Some of the key differences are:
<DOCTYPE>checked="checked" instead of checked )
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The DOM (Document Object Model) is a cross-platform API that treats HTML and XML documents as a tree structure consisting of nodes. These nodes (such as elements and text nodes) are objects that can be programmatically manipulated and any visible changes made to them are reflected live in the document. In a browser, this API is available to JavaScript where DOM nodes can be manipulated to change their styles, contents, placement in the document, or interacted with through event listeners.
<head> with a defer attribute, or inside a DOMContentLoaded event listener. Scripts that manipulate DOM nodes should be run after the DOM has been constructed to avoid errors.document.getElementById() and document.querySelector() are common functions for selecting DOM nodes.innerHTML property to a new value runs the string through the HTML parser, offering an easy way to append dynamic HTML content to a node.
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HTML specifications such as HTML5 define a set of rules that a document must adhere to in order to be “valid” according to that specification. In addition, a specification provides instructions on how a browser must interpret and render such a document.
A browser is said to “support” a specification if it handles valid documents according to the rules of the specification. As of yet, no browser supports all aspects of the HTML5 specification (although all of the major browser support most of it), and as a result, it is necessary for the developer to confirm whether the aspect they are making use of will be supported by all of the browsers on which they hope to display their content. This is why cross-browser support continues to be a headache for developers, despite the improved specificiations.
HTML5 defines some rules to follow for an invalid HTML5 document (ie, one that contains syntactical errors)
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localStorage and sessionStorage .With HTML5, web pages can store data locally within the user's browser. The data is stored in name/value pairs, and a web page can only access data stored by itself.
Differences between localStorage and sessionStorage regarding lifetime:
localStorage is permanent: it does not expire and remains stored on the user's computer until a web app deletes it or the user asks the browser to delete it.sessionStorage has the same lifetime as the top-level window or browser tab in which the data got stored. When the tab is permanently closed, any data stored through sessionStorage is deleted. Differences between localStorage and sessionStorage regarding storage scope: Both forms of storage are scoped to the document origin so that documents with different origins will never share the stored objects.
sessionStorage is also scoped on a per-window basis. Two browser tabs with documents from the same origin have separate sessionStorage data.localStorage , the same scripts from the same origin can't access each other's sessionStorage when opened in different tabs.
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The BEM methodology is a naming convention for CSS classes in order to keep CSS more maintainable by defining namespaces to solve scoping issues. BEM stands for Block Element Modifier which is an explanation for its structure. A Block is a standalone component that is reusable across projects and acts as a "namespace" for sub components (Elements). Modifiers are used as flags when a Block or Element is in a certain state or is different in structure or style.
/* block component */
. block {
}
/* element */
. block__element {
}
/* modifier */
. block__element--modifier {
}Here is an example with the class names on markup:
< nav class =" navbar " >
< a href =" / " class =" navbar__link navbar__link--active " > </ a >
< a href =" / " class =" navbar__link " > </ a >
< a href =" / " class =" navbar__link " > </ a >
</ nav > In this case, navbar is the Block, navbar__link is an Element that makes no sense outside of the navbar component, and navbar__link--active is a Modifier that indicates a different state for the navbar__link Element.
Since Modifiers are verbose, many opt to use is-* flags instead as modifiers.
< a href =" / " class =" navbar__link is-active " > </ a >These must be chained to the Element and never alone however, or there will be scope issues.
. navbar__link . is-active {
}
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CSS preprocessors add useful functionality that native CSS does not have, and generally make CSS neater and more maintainable by enabling DRY (Don't Repeat Yourself) principles. Their terse syntax for nested selectors cuts down on repeated code. They provide variables for consistent theming (however, CSS variables have largely replaced this functionality) and additional tools like color functions ( lighten , darken , transparentize , etc), mixins, and loops that make CSS more like a real programming language and gives the developer more power to generate complex CSS.
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col-{n} / 12 ratio of the container. < div class =" row " >
< div class =" col-2 " > </ div >
< div class =" col-7 " > </ div >
< div class =" col-3 " > </ div >
</ div > Set the .row parent to display: flex; and use the flex shorthand property to give the column classes a flex-grow value that corresponds to its ratio value.
. row {
display : flex;
}
. col-2 {
flex : 2 ;
}
. col-7 {
flex : 7 ;
}
. col-3 {
flex : 3 ;
}
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@media properties?all , which applies to all media type devicesprint , which only applies to printersscreen , which only applies to screens (desktops, tablets, mobile etc.)speech , which only applies to screenreaders @media rule
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Content : The inner-most part of the box filled with content, such as text, an image, or video player. It has the dimensions content-box width and content-box height .
Padding : The transparent area surrounding the content. It has dimensions padding-box width and padding-box height .
Border : The area surrounding the padding (if any) and content. It has dimensions border-box width and border-box height .
Margin : The transparent outer-most layer that surrounds the border. It separates the element from other elements in the DOM. It has dimensions margin-box width and margin-box height .
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em and rem units? Both em and rem units are based on the font-size CSS property. The only difference is where they inherit their values from.
em units inherit their value from the font-size of the parent elementrem units inherit their value from the font-size of the root element ( html ) In most browsers, the font-size of the root element is set to 16px by default.
em and rem units
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CSS sprites combine multiple images into one image, limiting the number of HTTP requests a browser has to make, thus improving load times. Even under the new HTTP/2 protocol, this remains true.
Under HTTP/1.1, at most one request is allowed per TCP connection. With HTTP/1.1, modern browsers open multiple parallel connections (between 2 to 8) but it is limited. With HTTP/2, all requests between the browser and the server are multiplexed on a single TCP connection. This means the cost of opening and closing multiple connections is mitigated, resulting in a better usage of the TCP connection and limits the impact of latency between the client and server. It could then become possible to load tens of images in parallel on the same TCP connection.
However, according to benchmark results, although HTTP/2 offers 50% improvement over HTTP/1.1, in most cases the sprite set is still faster to load than individual images.
To utilize a spritesheet in CSS, one would use certain properties, such as background-image , background-position and background-size to ultimately alter the background of an element.
background-image , background-position and background-size can be used to utilize a spritesheet.
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The General Sibling Selector ~ selects all elements that are siblings of a specified element.
The following example selects all <p> elements that are siblings of <div> elements:
div ~ p {
background-color : blue;
} The Adjacent Sibling Selector + selects all elements that are the adjacent siblings of a specified element.
The following example will select all <p> elements that are placed immediately after <div> elements:
div + p {
background-color : red;
}
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Assuming the browser has already determined the set of rules for an element, each rule is assigned a matrix of values, which correspond to the following from highest to lowest specificity:
When two selectors are compared, the comparison is made on a per-column basis (eg an id selector will always be higher than any amount of class selectors, as ids have higher specificity than classes). In cases of equal specificity between multiple rules, the rules that comes last in the page's style sheet is deemed more specific and therefore applied to the element.
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A focus ring is a visible outline given to focusable elements such as buttons and anchor tags. It varies depending on the vendor, but generally it appears as a blue outline around the element to indicate it is currently focused.
In the past, many people specified outline: 0; on the element to remove the focus ring. However, this causes accessibility issues for keyboard users because the focus state may not be clear. When not specified though, it causes an unappealing blue ring to appear around an element.
In recent times, frameworks like Bootstrap have opted to use a more appealing box-shadow outline to replace the default focus ring. However, this is still not ideal for mouse users.
The best solution is an upcoming pseudo-selector :focus-visible which can be polyfilled today with JavaScript. It will only show a focus ring if the user is using a keyboard and leave it hidden for mouse users. This keeps both aesthetics for mouse use and accessibility for keyboard use.
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WCAG stands for "Web Content Accessibility Guidelines". It is a standard describing how to make web content more accessible to people with disabilities They have 12-13 guidelines and for each one, there are testable success criteria, which are at three levels: A, AA, and AAA. The higher the level, the higher the impact on the design of the web content. The higher the level, the web content is essentially more accessible by more users. Depending on where you live/work, there may be regulations requiring websites to meet certain levels of compliance. For instance, in Ontario, Canada, beginning January 1, 2021 all public websites and web content posted after January 1, 2012 must meet AA compliance.
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ARIA stands for "Accessible Rich Internet Applications", and is a technical specification created by the World Wide Web Consortium (W3C). Better known as WAI-ARIA, it provides additional HTML attributes in the development of web applications to offer people who use assistive technologies (AT) a more robust and interoperable experience with dynamic components. By providing the component's role, name, and state, AT users can better understand how to interact with the component. WAI-ARIA should only be used when an HTML element equivalent is not available or lacks full browser or AT support. WAI-ARIA's semantic markup coupled with JavaScript works to provide an understandable and interactive experience for people who use AT.
An example using ARIA:
<div
role="combobox"
aria-expanded="false"
aria-owns="ex1-grid"
aria-haspopup="grid"
id="ex1-combobox">
...
</div>
Credit: W3C's ARIA 1.1 Combobox with Grid Popup Example
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The Accessibility Tree is a structure produced by the browser's Accessibility APIs which provides accessibility information to assistive technologies such as screen readers. It runs parallel to the DOM and is similar to the DOM API, but with much fewer nodes, because a lot of that information is only useful for visual presentation. By writing semantic HTML we can take advantage of this process in creating an accessible experience for our users.
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Landmark roles is a way to identify different sections of a page like the main content or a navigation region. The Landmarks helps assistive technology users to navigate a page, allowing them skip over areas of it.
Zum Beispiel,
< div id =" header " role =" banner " > Header of the Page </ div >
< div id =" content " role =" main " > Main Content Goes Here </ div >
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fs . readFile ( filePath , function ( err , data ) {
if ( err ) {
// handle the error, the return is important here
// so execution stops here
return console . log ( err )
}
// use the data object
console . log ( data )
} )Zu den Vorteilen gehören:
As you can see from below example, the callback is called with null as its first argument if there is no error. However, if there is an error, you create an Error object, which then becomes the callback's only parameter. The callback function allows a user to easily know whether or not an error occurred.
This practice is also called the Node.js error convention , and this kind of callback implementations are called error-first callbacks .
var isTrue = function ( value , callback ) {
if ( value === true ) {
callback ( null , "Value was true." )
} else {
callback ( new Error ( "Value is not true!" ) )
}
}
var callback = function ( error , retval ) {
if ( error ) {
console . log ( error )
return
}
console . log ( retval )
}
isTrue ( false , callback )
isTrue ( true , callback )
/*
{ stack: [Getter/Setter],
arguments: undefined,
type: undefined,
message: 'Value is not true!' }
Value was true.
*/
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REST (REpresentational State Transfer) is a software design pattern for network architecture. A RESTful web application exposes data in the form of information about its resources.
Generally, this concept is used in web applications to manage state. With most applications, there is a common theme of reading, creating, updating, and destroying data. Data is modularized into separate tables like posts , users , comments , and a RESTful API exposes access to this data with:
Here is an example of the URL and HTTP method with a posts resource:
/posts/ => GET/posts/new => POST/posts/:id => PUT/posts/:id => DELETE
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getData ( function ( a ) {
getMoreData ( a , function ( b ) {
getMoreData ( b , function ( c ) {
getMoreData ( c , function ( d ) {
getMoreData ( d , function ( e ) {
// ...
} )
} )
} )
} )
} ) Refactoring the functions to return promises and using async/await is usually the best option. Instead of supplying the functions with callbacks that cause deep nesting, they return a promise that can be await ed and will be resolved once the data has arrived, allowing the next line of code to be evaluated in a sync-like fashion.
The above code can be restructured like so:
async function asyncAwaitVersion ( ) {
const a = await getData ( )
const b = await getMoreData ( a )
const c = await getMoreData ( b )
const d = await getMoreData ( c )
const e = await getMoreData ( d )
// ...
}There are lots of ways to solve the issue of callback hells:
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The event loop handles all async callbacks. Callbacks are queued in a loop, while other code runs, and will run one by one when the response for each one has been received.
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XSS refers to client-side code injection where the attacker injects malicious scripts into a legitimate website or web application. This is often achieved when the application does not validate user input and freely injects dynamic HTML content.
For example, a comment system will be at risk if it does not validate or escape user input. If the comment contains unescaped HTML, the comment can inject a <script> tag into the website that other users will execute against their knowledge.
textContent instead of innerHTML prevents the browser from running the string through the HTML parser which would execute scripts in it.
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MIT. Copyright (c) Stefan Feješ.
