30 seconds of interviews
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此读数是使用Markdown-Builder构建的。
访谈令人生畏，甚至可以使经验最丰富的专家忘记在压力下的事情。审查并了解社区策划的访谈中通常会遇到哪些问题，这些问题是回答他们的，并为他们要问的任何事情做好了准备。通过汇总经验和现实世界的例子，您可以从紧张到为下一个大机会做好准备。
30秒的访谈是社区的努力，因此请随时以任何方式做出贡献。每个贡献都会有所帮助！
您有一个好主意还是知道一些不在列表中的很酷的问题？阅读贡献指南并提交拉动请求。
加入我们的吉特频道，以帮助项目的开发。
batches ，该功能返回可以从食谱烹饪的整个批次的最大数量。bind该函数在功能上等效于方法函数Function.prototype.bind 。setState的参数的目的是什么？children道具是什么？==和===有什么区别？0.1 + 0.2 === 0.3评估什么？map()和forEach()有什么区别？# ，除了最后四（4）个字符。this上下文？null和undefined有什么区别？pipe ，该功能管通过返回接受一个参数的函数来执行从左到右功能的组合。i++和前缀++i增量运算符有什么区别？this关键字是什么，它如何工作？'use strict'是什么，使用它有哪些关键好处？var ， let ， const和NO关键字语句之间有什么区别？setState的参数的目的是什么？children道具是什么？className而不是html中的class ？this上下文？alt属性的目的是什么？<script>标签上的defer和async属性是什么？<header>元素吗？ <footer>元素呢？<header> ， <article> ， <section> ， <footer>localStorage和sessionStorage 。rel="noopener"属性在哪里以及为什么使用？em和rem单位有什么区别？col-{n} / 12比率。@media属性的四种类型吗？无状态组件是其行为不取决于其状态的组件。无状态组件可以是功能或类成分。无状态的功能组件更易于维护和测试，因为它们可以保证在相同的道具下产生相同的输出。当不需要使用生命周期挂钩时，应首选无状态的功能组件。
this关键字。
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==和===有什么区别？三重等于（ === ）检查严格的平等，这意味着类型和值必须相同。另一方面，双等价（ == ）首先执行类型的胁迫，因此两个操作数均具有相同的类型，然后应用严格的比较。
==可能会产生不直觉的结果。
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元素是代表DOM节点或组件的普通JavaScript对象。元素是纯净的，永远不会变异，而且创造便宜。
组件是函数或类。组件可以具有状态并将道具作为输入，并将元素树作为输出返回（尽管它们可以代表通用容器或包装器，并且不一定要发射DOM）。组件可以启动生命周期方法的副作用（例如AJAX请求，DOM突变，与第三方库的接口），并且创建可能很昂贵。
const Component = ( ) => "Hello"
const componentElement = < Component />
const domNodeElement = < div />
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状态组件是其行为取决于其状态的组成部分。这意味着与纯函数组件不同，如果给出相同的道具，则组件的两个单独实例不一定会呈现相同的输出。
// 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()中初始化。
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这些州之一是一个Promise ：
悬而未决的承诺可以通过价值来实现，或者以理由拒绝（错误）。当这些选项中的任何一个都会发生时，相关的处理程序以诺言的方式排队。
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i++和前缀++i增量运算符有什么区别？两者都将变量值增加1。差异是他们评估的。
Postfix增量运算符在将其增量之前对其进行评估。
let i = 0
i ++ // 0
// i === 1前缀增量运算符在增量后评估该值。
let i = 0
++ i // 1
// i === 1
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batches ，该功能返回可以从食谱烹饪的整个批次的最大数量。 /**
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 }
)我们必须拥有可用的食谱的所有成分，并且数量超过或等于所需的单位数量。如果只有一种不可用或低于所需的成分，我们将无法进行一批。
使用Object.keys()将配方的成分返回作为数组，然后使用Array.prototype.map()将每个成分映射到可用单位的比率与配方所需的数量。如果根本不可用食谱所需的一种成分，则比率将与NaN进行评估，因此在这种情况下，逻辑或操作员可以将逻辑或运算符用于下降至0 。
使用差异...运算符将所有成分比的数组馈入Math.min()以确定最低比率。将整个结果传递到Math.floor()回合以返回整个批次的最大数量。
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它评估为"string" 。
typeof 0评估字符串"number" ，因此typeof "number"评估为"string" 。
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使用对象传播操作员...可以将对象自己的枚举属性复制到新对象中。这创建了物体的浅克隆。
const obj = { a : 1 , b : 2 }
const shallowClone = { ... obj }使用此技术，原型被忽略。另外，嵌套对象没有克隆，而是复制它们的引用，因此嵌套的对象仍然指与原始对象相同的对象。深度关闭更为复杂，以便有效克隆任何可能嵌套在对象中的对象（日期，regexp，function，set等）。
其他选择包括：
JSON.parse(JSON.stringify(obj))可用于深粘结一个简单的对象，但它是CPU密集型的，仅接受有效的JSON（因此它剥离功能并且不允许循环引用）。Object.assign({}, obj)是另一种选择。Object.keys(obj).reduce((acc, key) => (acc[key] = obj[key], acc), {})是另一个更详细的替代方案，它以更深入的深度显示了概念。
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同步意味着每个操作必须等待上一个操作完成。
异步意味着在仍在处理其他操作时可能会发生操作。
在JavaScript中，由于其单线程的性质，所有代码都是同步的。但是，异步操作不是程序的一部分（例如XMLHttpRequest或setTimeout ）是在主线程之外处理的，因为它们是由本机代码（浏览器API）控制的，但是程序的回调部分仍将同时执行。
alert屏蔽主线程之类的函数，以便在用户关闭之前没有注册用户输入。
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即使两个不同的对象可以具有相同值相同的属性，但使用==或===进行比较时，它们也不相等。这是因为它们是通过参考（内存中的位置）进行比较的，这与原始值不同，这些值按值进行了比较。
为了测试两个对象的结构是否相等，需要一个辅助功能。它将通过每个对象的自身属性进行迭代，以测试它们是否具有相同的值，包括嵌套对象。可选地，也可以通过将true作为第三参数进行测试对象的原型。
注意：此技术不会尝试测试除普通对象，数组，函数，日期和原始值以外的数据结构的等效性。
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是指攻击者将恶意脚本注入合法网站或Web应用程序的客户端代码注入。当应用程序未验证用户输入并自由注入动态HTML内容时，通常可以实现这一点。
例如，如果评论系统不验证或逃脱用户输入，则将处于危险之中。如果该评论包含未设计的HTML，则该注释可以将<script>标签注入其他用户会根据其知识执行的网站。
textContent而不是innerHTML可防止浏览器通过HTML解析器运行字符串，该解析器将在其中执行脚本。
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交叉原始资源共享或CORS是一种使用其他HTTP标头的机制，授予浏览器的权限，以从与网站Origin不同的原点的服务器访问资源。
交叉原始请求的一个示例是从http://mydomain.com提供的Web应用程序，该应用程序使用Ajax向http://yourdomain.com提出请求。
出于安全原因，浏览器限制了JavaScript发起的交叉原始HTTP请求。 XMLHttpRequest和fetch遵循相同的Origin策略，这意味着使用这些API的Web应用程序只能从同一来源请求HTTP Resources访问该应用程序的访问，除非来自其他原点的响应包括正确的CORS标头。
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DOM（文档对象模型）是一种跨平台API，将HTML和XML文档视为由节点组成的树结构。这些节点（例如元素和文本节点）是可以通过编程方式操纵的对象，并且对其进行的任何可见更改都会反映在文档中。在浏览器中，该API可用于JavaScript，可以操纵DOM节点以更改其样式，内容，文档中的位置或通过事件听众进行交互。
<head>带有defer属性，或者在DOMContentLoaded Event ockiter中。在构造DOM之后，应运行操纵DOM节点的脚本以避免错误。document.getElementById()和document.querySelector()是选择DOM节点的常见函数。innerHTML属性设置为新值可以通过HTML解析器运行字符串，从而提供了一种简单的方法，可以将动态HTML内容附加到节点。
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bind该函数在功能上等效于方法函数Function.prototype.bind 。 function example ( ) {
console . log ( this )
}
const boundExample = bind ( example , { a : true } )
boundExample . call ( { b : true } ) // logs { a: true }返回一个函数，该函数通过将其收集到其余的...运营商来接受任意数量的参数。从该函数中，返回使用Function.prototype.apply返回fn的结果，以将上下文和参数数组应用于函数。
const bind = ( fn , context ) => ( ... args ) => fn . apply ( context , args )
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var ， let ， const和NO关键字语句之间有什么区别？当变量分配之前不存在关键字时，如果不存在，它要么分配全局变量，要么重新分配已经声明的变量。在非图片模式下，如果尚未声明该变量，它将将变量分配为全局对象的属性（浏览器中的window ）。在严格的模式下，它将引发错误，以防止不必要的全局变量被创建。
var是在ES2015之前声明变量的默认语句。它创建了一个可以重新分配和重新播放的功能范围的变量。但是，由于缺乏块范围，如果变量在包含异步回调的循环中重复使用，则可能会导致问题，因为该变量将继续存在于块范围之外。
下面，到setTimeout回调执行时，循环已经完成， i变量为10 ，因此所有十个回调引用了功能范围中可用的相同变量。
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在ES2015中引入，这是声明变量的新首选方法，这些变量将在以后重新分配。试图重新汇集变量将引发错误。它是块拆分的，因此在循环中使用它可以使其范围范围内的迭代范围。
for ( let i = 0 ; i < 10 ; i ++ ) {
setTimeout ( ( ) => {
// logs 0, 1, 2, 3, ...
console . log ( i )
} )
} const是在ES2015中介绍的，是如果以后不会重新分配所有变量，即使对于将要突变的对象（只要对象的引用不更改），也是声明所有变量的新首选默认方法。它是块分割的，不能重新分配。
const myObject = { }
myObject . prop = "hello!" // No error
myObject = "hello" // Error let和const有一个称为时间死区（TDZ）的概念。虽然声明仍在提起，但在进入范围和声明无法访问的地方之间存在一段时间。var以及let解决它的常见问题，以及保留var的解决方案。var ，并更喜欢const作为所有变量的默认声明语句，除非以后将其重新分配，否则请使用let 。 let vs const
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事件代表团是将事件委派给单个共同祖先的技术。由于事件冒泡，事件通过在每个祖先元素上逐渐执行任何处理程序，直到可能正在聆听它的根来“冒泡” DOM树。
DOM事件提供了有关通过Event.target启动事件的元素的有用信息。这使父元素可以处理行为，就好像目标元素在听事件，而不是父母或父母本身的所有孩子。
这提供了两个主要好处：
而不是：
document . querySelectorAll ( "button" ) . forEach ( button => {
button . addEventListener ( "click" , handleButtonClick )
} )事件委托涉及使用条件来确保孩子目标与我们所需的元素匹配：
document . addEventListener ( "click" , e => {
if ( e . target . closest ( "button" ) ) {
handleButtonClick ( )
}
} )
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setState的参数的目的是什么？setState完成并渲染组件后，调用回调功能。由于setState是异步的，因此回调函数用于任何后操作。
setState ( { name : "sudheer" } , ( ) => {
console . log ( "The name has updated and component re-rendered" )
} ) setState完成后调用回调功能，并用于任何后操作。setState上的react文档
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JavaScript中有两个主要的句法类别：表达式和语句。第三个都是在一起，称为表达式语句。它们大致总结为：
一般的经验法则：
如果您可以打印或将其分配给变量，则是一个表达式。如果不能，这是一个陈述。
let x = 0
function declaration ( ) { }
if ( true ) {
}语句以执行操作但不产生价值的指示出现。
// Assign `x` to the absolute value of `y`.
var x
if ( y >= 0 ) {
x = y
} else {
x = - y
}上面代码中唯一的表达式是y >= 0它产生一个值，无论是true还是false 。代码的其他部分不会产生一个值。
表达产生一个值。它们可以传递到函数，因为解释器将其解析的价值取代。
5 + 5 // => 10
lastCharacter ( "input" ) // => "t"
true === true // => true 使用条件运算符以前用作表达式的一组语句的等效版本：
// Assign `x` as the absolute value of `y`.
var x = y >= 0 ? y : - y这既是表达式又是语句，因为我们将变量x （语句）称为评估（表达式）。
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根据在布尔语境中对其进行评估，价值要么是真实的，要么是虚假的。虚假意味着假和真实的意思是真实的。从本质上讲，它们是执行某些操作时被强制为true或false值。
JavaScript中有6个虚假值。他们是：
falseundefinednull"" （空字符串）NaN0 （ +0和-0 ）其他每个价值都被认为是真实的。
可以通过将价值传递到Boolean功能来检查价值的真实性。
Boolean ( "" ) // false
Boolean ( [ ] ) // true使用逻辑上没有的是一个快捷方式!操作员。使用!一旦将一个值转换为其反向布尔值等效物（即不是false是true），并且!再一次将转换，从而有效地将值转换为布尔值。
! ! "" // false
! ! [ ] // true
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初始化长度为n的空数组。使用Array.prototype.reduce()使用最后两个值的总和将值添加到数组中，但前两个值除外。
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评估什么？它评估为false ，因为JavaScript使用IEEE 754标准数学标准，并使用64位浮数。简而言之，这会导致精度错误，因为计算机在基本2中工作，而十进制为基础10。
0.1 + 0.2 // 0.30000000000000004解决此问题的一种解决方案是使用一个函数，该函数通过定义误差边距（Epsilon）值是否大致相等，即两个值之间的差异应小于。
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()和forEach()有什么区别？两种方法都通过数组的元素迭代。 map()通过在每个元素上调用回调函数并返回新数组，将每个元素映射到新元素。另一方面， forEach()调用每个元素的回调函数，但不会返回新数组。通常在对每次迭代产生副作用时通常使用forEach() ，而map()是一种常见的功能编程技术。
forEach()而无需返回值即可生成新数组。map()是将数据不可变的正确选择，而原始数组的每个值都映射到新数组。
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短路评估涉及从左到右评估和尽早停止评估的逻辑操作。
true || false在上面的示例中，使用逻辑或JavaScript不会查看第二个操作数false ，因为无论如何，表达式评估为true 。这被称为短路评估。
这也适用于逻辑和。
false && true这意味着您可以拥有一个表达式，该表达式在评估时会引发错误，并且不会引起问题。
true || nonexistentFunction ( )
false && nonexistentFunction ( )由于从左到右评估，多次操作仍然如此。
true || nonexistentFunction ( ) || window . nothing . wouldThrowError
true || window . nothing . wouldThrowError
true此行为的常见用例是设置默认值。如果第一操作数是虚假的，则将评估第二操作数。
const options = { }
const setting = options . setting || "default"
setting // "default"另一个常见的用例是仅在第一个操作数是真实情况下评估表达式。
// 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 )
)在上述情况下，如果e.target不或不包含匹配"button"选择器的元素，则该函数将不会被调用。这是因为第一台操作数将是虚假的，导致第二个操作数未进行评估。
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有时。由于JavaScript的自动分号插入，解释器将半洛龙置于大多数陈述之后。这意味着在大多数情况下可以省略半殖民物。
但是，在某些情况下需要它们。它们在块开始时不需要它们，但是如果他们遵循一条线，则是：
[ const previousLine = 3
; [ 1 , 2 , previousLine ] . map ( n => n * 2 )( const previousLine = 3
; ( function ( ) {
// ...
} ) ( )在上述情况下，解释器不会在3之后插入半隆，因此将3视为尝试对象属性访问或被调用为函数，这会丢弃错误。
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var foo = 1
var foobar = function ( ) {
console . log ( foo )
var foo = 2
}
foobar ( )由于提升，在调用console.log方法之前声明局部变量foo 。这意味着将局部变量foo作为参数传递给console.log()而不是在函数之外声明的全局变量。但是，由于该值没有带有变量声明，因此输出将undefined ，而不是2 。
strict模式
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提升是一种JavaScript机制，在编译阶段，将可变和函数声明放入内存中。这意味着无论函数和变量在何处声明，无论其范围是全球还是本地的，它们都将移至其范围顶部。
但是，该声明并没有提高价值。
以下片段：
console . log ( hoist )
var hoist = "value"等同于：
var hoist
console . log ( hoist )
hoist = "value"因此，记录hoist输出undefined到控制台，而不是"value" 。
提升还允许您调用函数声明，然后才能在程序中声明。
myFunction ( ) // No error; logs "hello"
function myFunction ( ) {
console . log ( "hello" )
}但是要警惕分配给变量的功能表达式：
myFunction ( ) // Error: `myFunction` is not a function
var myFunction = function ( ) {
console . log ( "hello" )
}
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在HTML中，属性名称均在所有小写字母中，并给出一个字符串，调用在某处定义的函数：
< 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 )
} )优点包括：
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?是的。 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.
例如，
< 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 )
} )优点包括：
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.
