Explain the Object object in detail in JS

This article is about the basic content of JS learning, learning the prototype concept of Object object, friends who are interested can refer to it.

Object Concept

In JavaScript, all reference types are objects. For example, in function Foo () {}, Foo itself is a reference to an object.

Create object method literal method new constructor function declaration Object.create

Literal object

javascript language level to quickly create objects The instance of

var obj = {foo: 'foo', bar: 'bar'}; // Object对象字面量
var obj2 = [obj, 'foo', 'bar']; // Array数组字面量
var obj3 = /^[a-zA-Z0-9]$/; // RegExp正则字面量
var obj4 = function(){}; // Function函数字面量

new constructor

is passed through the constructor of the built-in object, or a custom function. Use the new operator to create an object and execute the constructor method.

var obj = new Object();
var obj2 = new Array(1000);
var obj3 = new RegExp('^[a-zA-Z0-9]$');
var obj4 = new Function('a', 'b', 'return a + b;');

Function declaration

The object created by function declaration. Function belongs to a special object.

function Foo() {}
Foo instanceof Object;
Foo instanceof Function;

Object.create

Pass in an object as the prototype of the returned object, create a new object, and point the prototype of the new object to the passed in object.

var foo = {
 'foo': 'foo',
 'bar': 'bar'
};
var o = Object.create(foo); // o.__proto__ = foo
console.log(o.foo); // o.__proto__.foo

Use Object.create(null) You can return a dictionary object.

var o = Object.create(null);
o instanceof Object; // return false;
o.toString(); // Uncaught TypeError

Object prototype

Every Objects have a built-in __proto__ attribute that points to the prototype attribute of the function that constructed it. The constructor's

prototype.constructor points to the constructor Function native. The process of searching for an object's attributes consists of the following parts:

Search for the data descriptor (writable, value) or access descriptor (getter, setter) of the object's attributes. If found, return the corresponding value. If the query cannot be found, proceed to step 2. Find whether the value of the object property is explicitly defined (can be detected by Object.getOwnPropertyNames). If the object property is defined, the defined value is returned. If not, proceed to step 3. Find the hidden prototype of the object__proto__The properties of the object, the rules are the same as steps 1 and 2. If it has not been found, repeat step 3 until __proto__ is null.

The specific case is shown in the figure below:

#Detecting object prototype

Test an object in its prototype chain Whether there is a prototype attribute of a constructor in

instanceof Object.prototype.isPrototypeOf

instanceof

operator, language-level detection of the prototype of the object Whether the chain contains the prototype of the constructor

function Foo () {}
Foo instanceof Function; // return true
Foo instanceof Object; // return true

Simulation instanceof Find whether the constructor of the object's prototype chain contains the passed-in constructor, __proto__ In some specific browsing The device has a function attribute that is exposed to the user

function Bar () {}
function isInstanceof (obj, Constructor) {
 if (obj === null) {
 return false;
 }
 // 忽略 string, number, boolean, null, undefined 类型干扰
 if (!~['object', 'function'].indexOf(typeof obj)) {
 return false;
 }

 var prototype = obj.__proto__;
 while(prototype) {
 if (prototype.constructor === Constructor) {
  return true;
 }
 prototype = prototype.__proto__;
 }
 return false;
}
isInstanceof(Bar, Function);
isInstanceof(Bar, Object);

isPrototypeOf

The prototype object of the constructor is used to detect whether there is a constructor in the prototype chain of the target object. Prototype object.

function Baz () {}
var baz = new Baz();
Baz.prototype.isPrototypeOf(baz);
Function.prototype.isPrototypeOf(baz);
Object.prototype.isPrototypeOf(baz);

Get the object prototype Object.getPrototypeOf __proto__

var o = {};
var prototype = Object.getPrototypeOf(o);
console.log(prototype === Object.prototype); // return true
// 部分浏览器有效
var o2 = {};
console.log(o2.__proto__ === Object.prototype); // return true

Set the object prototype Object.create Object.setPrototypeOf

Object.create

Return an object and set its prototype

function Foo () {}
 function Bar () {}
 Foo.prototype.foo = 'foo';
 Bar.prototype = Object.create(Foo.prototype); 
 Bar.prototype.constructor = Bar; // 修正原型链的constructor
 var o = new Bar();
 console.log(o.foo); // return foo;
 console.log(o instanceof Bar); // return true

Object.setPrototypeOf

Set the implicit prototype of the object directly__proto__

function Foo () {}
 Foo.prototype.name = 'foo';
 var o = Object.create(null);
 Object.setPrototypeOf(o, Foo.prototype);
 console.log(o.name); // return foo

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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