JavaScript you don't Know (c)--object

First, the grammar

Two forms of definition: text form and construction form.

// text Form var myObj = {  Key:value}; // Construction Form var New  = value;

Second, type

Objects are the basis of JavaScript.

1) Basic type

There are six main types in JavaScript (the term is "language type"):

String, number,boolean,null, Undefined, object

2) built-in objects

There are also some object subtypes in JavaScript, often referred to as built-in objects.

String, number, Boolean, Object, Function, Array, Date, REGEXP, Error

The engine can automatically convert some of the underlying types into the corresponding built-in objects, and then it can invoke the object's properties or methods.

// Convert a string to a string object var strprimitive = "I am a String"//   // "M"//  Convert numbers to Number object var num = 42.359.toFixed (2); Console.log (num) ; // 42.36

Third, the content

1) Properties

var myObject =2//  2 Property access //  2 Key Access

. A syntax is often referred to as " property access ", and ["a"] syntax is often referred to as " key access ".

2) Copying objects

There is an ingenious method of copying JSON security (that is, it can be serialized as a JSON string and can parse an object with exactly the same structure and value based on the string):

var newObj = Json.parse (json.stringify (someobj));

Shallow copy is easy to understand and has much less problem than deep copy, so ES6 defines object.assign (..) method to achieve shallow replication.

3) Property Descriptor

Starting with ES5, all attributes have a property descriptor. Can be implemented by means of object.defineproperty () .

var myObject ="A", {  2,  true,  true ,   true  //  2

Vue.js is the way to achieve tracking changes.

4) Getter and setter

In ES5, the getter and setter sections can be used to override the default action, but only on a single property and cannot be applied across the entire object.

varMyObject = {  //define a getter for aget A () {return2; }};object.defineproperty (MyObject,//target Object"B",//Property name{//Descriptor    //set a getter for BGetfunction() {      return  This. A * 2    },    //Make sure B appears in the object's property listEnumerabletrue}); Console.log (MYOBJECT.A); //2Console.log (myobject.b);//4

5) The existence of

The in operator checks whether the property is in the object and its [[Prototype]] prototype chain.

hasOwnProperty (..) Only checks if the property is in the MyObject object and does not check the [[Prototype]] chain.

There is a previous "JavaScript in typeof, ToString, Instanceof, constructor and in" made some comparisons.

var myObject = {  2};(in//  truein//   myobject.hasownproperty (//  true//  false

Four, traverse

Unlike arrays, normal objects do not have a built-in @ @iterator, so they cannot be automatically completed for. of traversal.

But you can define @ @iterator for any object you want to traverse.

varMyObject ={A:2, B:3};object.defineproperty (MyObject, Symbol.iterator, {enumerable:false, writable:false, Configurable:true, Value:function() {    varn \ This; varIDX = 0; varKS =Object.keys (o); return{Next:function() {        return{value:o[ks[idx++]], done: (IDX>ks.length)};  }    }; }});//Manually traverse MyObjectvarit =Myobject[symbol.iterator] (); It.next ();//{value:2, done:false}It.next ();//{value:3, done:false}It.next ();//{value:undefined, done:true}//use for. of Traverse MyObject for(varV of MyObject) {Console.log (v);}//2//3

JavaScript you don't Know (c)--object