Javascript Objects Detailed explanation

Create an object

•Direct Object Measurement

 var o = { foo : "bar" }

•Constructor function

 var o = new Object();

•Prototype inheritance

 var p = Object.create(o);

Class inheritance

Javascript objects have their own properties and inherited properties.

• When querying the attribute x of object o, first look for the attribute x in o. If it is not found, look for the x attribute in the prototype object of o until x or an object whose prototype is null is found.

• When assigning a value to the x attribute of object o, if there is already a property x in o, change the value of x. If there is no property x in o, create an x ​​attribute for o and assign a value.

•That is, the prototype chain will only work when querying.

 var O = { x : 1 };function P() { this.y = 2; }P.prototype = O;var t = new P(); console.log(t); console.log('x' in t);//true console.log(t.hasOwnProperty('x'));//false

You can use in or hasOwnProperty to determine whether there are properties in the object.

Object properties

•Transfer through object properties

You can use for..in to traverse the properties of an object

When using for..in, iterates over the properties on the prototype chain. The traversal order is to traverse with breadth priority

Therefore, using hasOwnProperty can determine whether it is an object's own attribute.

•Properties of object properties

Use Object.getOwnPropertyDescriptor() to get the descriptor for the object-specific property

Writable means whether the object attribute is writable

For example

 var o = { foo : 'bar'}Object.defineProperty(o, "foo", { writable : false });o.foo = 'world';console.log(o.foo);// Still output bar

Enumerable means whether the object attribute is enumerable

For example

The enumerable of the length and other attributes in Array is false, so

 for (p in Array) { console.log(p);}

Nothing output

Configurable means whether the configurability and enumeration of properties can be modified.

These configuration properties can be defined using Object.defineProperties.

Object.defineProperty(o, "foo", { writable : false });

Get means a method to obtain object properties

Set represents a method to set object properties

Example

 var book = { _year: 2004, edition: 1};Object.defineProperty(book, "year", { get: function () { console.log('get year'); return this._year; }, set: function (newValue) { console.log('set year'); if (newValue > 2004) { this._year = newValue; this.edition += newValue - 2004; } }});book.year = 2005;//Console output 'set year'console.log(book.year);//Console outputs the values ​​of 'get year' and year

Object method

toString converts objects into strings. The default conversion will be something like [object Object], so if you need to convert it to json format, you can use JSON.stringify

valueOf needs to be used when converting objects to other types. Similarly, there is nothing worth saying about the default conversion.

Executable object

You can create an executable object by the following method

 function bar(o) { var f = function() { return "Hello World!"; } o.__proto__ = f.__proto__; f.__proto__ = o; return f;}var o = { x: 5 };var foo = bar(o);console.log(foo());console.log(foo.x);console.log(typeof foo);//function

It can be used as an object (with a prototype chain) or as a function to call it directly