The prototype in JavaScript

Everything in JavaScript is an object, and many objects have prototype this member.

To understand this value of the opportunity or to interview the front-end, the interviewer asked me you know how to inherit JS, I face confused force. Now think about it is too little to know, even the introduction is not.

I tried it first. What is the prototype of the Origin object of everything?

Console.log (Object.prototye);

　

As a result, some of these functions are still well-acquainted, such as valueof (), toString ().

What I now know is that the prototype is useful for initializing the members of an object when new is created.

For example

function A () {}a.prototype = {    foo:function () {        return 123;    }};　　var B = new A (); Console.log (B.foo ()); "123"

The above operation is equivalent to using the A function to initialize the B object, at initialization time the members inside the a.prototype copy to the B (Foo function).

At this point, if you look at B.prototype, you will return to undefined. Note B has no prototype this value. And a why does it have this value?

First look at what A.prototype looks like:

function A (ID) {
This.id=id;
}
Console.log (A.prototype);

The process of creating a function can actually be written like this:

var a=new Function ("id", "this.id=id");

So it is clear that a, created by the object of function, I guess, initializes the A.prototype within the function.

And here I stumbled upon the difference between these two methods of creating a function, and if you use the second one, that A.prototype is long:

var a=new Function ("id", "this.id=id"); Console.log (A.prototype);

The constructor in prototype points to an anonymous function, not a.

In fact, at initialization time, the A function itself is also called as a constructor function.

function A (ID) {    this.id = ID;} var B = new A ("Id1"); Console.log (b.id);    "Id1"

Prototype member constructor also deserves attention. This value is typically the owner of the prototype, or A.prototype.constructor=a.

function A (ID) {    this.id = ID;} var B = new A ("Id1"); Console.log (a.prototype.constructor);        "Function A (id) {this.id = ID;}" Console.log (b.constructor);    Ditto

Each object will have a value of __proto__, which will point to the prototype that created its own object.

function A (ID) {    this.id = ID;}　　var B = new A ("Id1"); Console.log (b.__proto__); Display the contents of A.prototype

So follow this prototype chain to always find Object.prototype.

There are also two functions isprototypeof and hasownproperty briefly introduced:

isPrototypeOf checks whether an object is in the prototype chain of another object.

function A (ID) {    this.id = ID;} A.prototype.boo=3;var B = New A ("Id1"); Console.log (a.prototype.isprototypeof (b));    "True" Console.log (Object.prototype.isPrototypeOf (b));    "True"

hasOwnProperty checks whether a member is non-inherited.

function A (ID) {    this.id = ID;} A.prototype.boo = 3;var B = new A ("Id1"); Console.log (B.hasownproperty ("id"));   "True" Console.log (B.hasownproperty ("Boo"));    "False"

About the inheritance relationship in Java there are a lot of things to explore, I just know a little bit of fur, I hope in the future study can further understand the mystery.

The prototype in JavaScript