Do you know what js does when new an object?

Preface

In JavaScript, an instance object can be created through the new operator, and this instance object inherits the properties and methods of the original object. Therefore, the significance of new's existence is that it implements inheritance in JavaScript, not just instantiates an object.

The role of new

Let us first understand the role of new through examples. The examples are as follows:

function Person(name) {
  this.name = name
}
Person.prototype.sayName = function () {
  console.log(this.name)
}
const t = new Person('小明')
console.log(t.name)  // 小明
t.sayName()  // 小明

From the above examples we can draw The following conclusion:

• new The instance object created through the constructor Person can access the properties in the constructor.

• new The instance created through the constructor Person can access the properties in the constructor prototype chain, that is, through new Operators, instances and constructors are connected through the prototype chain.

Constructor Person does not explicitly return any value (default returns undefined), if What happens if we make it return a value?

function Person(name) {
  this.name = name
  return 1
}
const t = new Person('小明')
console.log(t.name)  // 小明

The constructor in the above example returned 1, but this return value is of no use, and the result is exactly the same as the previous example. We can draw another conclusion:

If the constructor returns the original value, then the return value is meaningless.

Let’s try what happens when the object is returned:

function Person(name) {
  this.name = name
  return {age: 23}
}
const t = new Person('小明')
console.log(t)  // { age: 23 }
console.log(t.name)  // undefined

Through the above example, we can find that when the return value is an object, the return value will be returned normally. go out. We once again came to a conclusion:

If the return value of the constructor is an object, then the return value will be used normally.

Summary: These two examples tell us that constructors should try not to return values. Because returning the original value will not take effect, returning the object will cause the new operator to have no effect.

To implement new

First of all, we need to know what js does when using the new operator:

1. js internally An object is created
2. This object can access the properties on the constructor prototype, so the object needs to be connected to the constructor
3. This inside the constructor is assigned the value of this new object (i.e. this points to the new object)
4. The return of the original value needs to be ignored, and the returned object needs to be processed normally

After knowing the steps, we can start to implement the function of new:

function _new(fn, ...args) {
  const newObj = Object.create(fn.prototype);
  const value = fn.apply(newObj, args);
  return value instanceof Object ? value : newObj;
}

The test example is as follows:

function Person(name) {
  this.name = name;
}
Person.prototype.sayName = function () {
  console.log(this.name);
};

const t = _new(Person, "小明");
console.log(t.name);  // 小明
t.sayName();  // 小明

The above is about the role of the new operator in JavaScript and how to implement a new operator.

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