Detailed explanation of JavaScript constructors and prototypes with examples

This article brings you relevant knowledge about JavaScript, which mainly introduces related issues about constructors and prototypes, as well as prototype-related content. Let’s take a look at it together. I hope it will be helpful to everyone. helpful.

[Related recommendations: JavaScript video tutorial, web front-end]

1. Constructor and prototype

1.1 Use prototype to solve the problem of memory waste

Disadvantages: There is the problem of memory waste,

If there are two objects or more, the space of a complex data type will be opened multiple times

Constructor prototypeprototypeThe prototype object mainly solves the problem of memory waste. The function assigned by the constructor through the prototype is owned by all objects shared. Every constructor in JavaScript has a prototype property that points to another object. This prototype is an object, and all properties and methods of the prototype object will be owned by the constructor. At this time, you can use prototype to put methods in it for use by all instance objects of the object.

Object prototype

_ proto_ Object Every object will have an attribute

_ proto_

Points to the prototype prototype object of the constructor. The reason why our object can use the properties and methods of the constructor prototype prototype object is because the object has _ proto_ The existence of prototype.

sy's _ proto_ and sym's _ proto_ are the same method search rules: first look at sy, whether there is a sing method on the sym object, and if so, execute this Sing on the object, because there is _ _ proto _ _, go to the constructor function prototype object Prototype, and go to the constructor function prototype object Prototype to find the sing method (Simply put, I use it if I have it. , if not, go to the prototype object and look for it

) When the function and object are created, a property will be automatically created, both of which point to the same empty object

1.2 constructor constructor constructor Constructor

Object prototype (_

proto

_) and constructor (prototype) prototype object have a property

constructorproperty, constructor We call it a constructor because it refers back to the constructor itself.

Both of them have constructors. If the prototype is added in the form of an object, then the prototype pointer has changed and points to a new one. The prototype object of this new object does not point to the construor. You need to point it manually

2. Prototype chain

2.1 Member search rules in js

When accessing the properties (including methods) of an object, first check whether the object itself has the property. If not, search for its prototype (that is, the prototype prototype object pointed to by ._ proto _). If it doesn't exist yet, look for the prototype of the prototype object (the prototype object of Object). And so on until Object is found (null).

2.2 Prototype object this points to

1. In the constructor, this points to the object instance shanyu

<script>
        function Person(uname, uage) {
            this.uname = uname;
            this.uage = uage;
        }
        // 声明一个变量然后验证this指向是不是和实例化对象相同
        var that;
        Person.prototype.skill = function () {
            console.log('我会吃饭');
            that = this;
        }
        var shanyu = new Person('山鱼', 30);
        shanyu.skill();
        console.log(that === shanyu);
    </script>

2.3 Extending built-in objects

You can extend the original built-in objects with customized methods through prototype objects.

 <script>
        //  自定义对象应用，给Array添加一个自定义方法
        Array.prototype.sum = function () {
            var sum = 0;
            for (var i = 0; i < this.length; i++) {
                sum += this[i];
            }
            return sum;
        }
        var arr = [1, 2, 3, 4];
        console.log(arr.sum());
    </script>

Arrays and string built-in objects cannot be prototyped Object overwriting operation Array.prototype = {} If this operation is performed, the original method will be overwritten. The only way is Array.prototype. method name = function(){}.

3.call function

Call this function and modify the this pointer when the function is running. There are three parameters: thisArg The object pointed to by the current calling function this arg1, arg2 Other parameters passed

    <script>
        function sing(x,y) {
            console.log("a~a~给我已被忘情水");
            console.log(this);
            console.log(x+y);
        }
        var fn = {
            name: '山鱼'
        }
        // call()可以改变这个函数的this指向此时这个函数的this就指向了o这个对象
        sing.call(fn,1,2)
    </script>

4. Inherit

through us When typing, you can see that this points to Son, that is, Son uses the uname and uage in the parent constructor

##4.1 Utilizing prototype object inheritance

There was no extends before es6, so you can use the combination of

constructor and model object to perform inheritance operations

 <script>
        function Father(uname, uage) {
            this.uname = uname;
            this.uage = uage;
        }
        Father.prototype.eat = function () {
            console.log("我爱吃鸡腿");
        }
        // 子构造函数
        Son.prototype = new Father();
        Son.prototype.constructor = Son;
        function Son(uname, uage, swing) {
            Father.call(this, uname, uage);
            this.swing = swing;
        }
        Son.prototype.student = function () {
            console.log('我爱学习！！');
        }
        // 要向使用父亲原型对象里面的方法，可以实例化一下Father，然后
        // 这时候Son的this指向到了Father，所以我们要用constructor将this指回到Son
        var son = new Son('山鱼妹', 18, "游泳冠军！！！");
        // console.log(son.eat());
        console.log(son);
        console.log(Father.prototype);
    </script>

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JavaScript video tutorial, web front-end]

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