Introduction to the principle of new calling function in JS

This article mainly introduces the principle of new calling function in JS. It has certain reference value. Now I share it with you. Friends in need can refer to it.

Constructor is often used in JavaScript to create Object (calling a function through the new operator), what exactly happens when calling a function using new? Let’s look at a few examples before explaining what’s going on behind the scenes.

1) Look at three examples

1.1 No return statement

There is no return statement at the end of the constructor. This is also the default situation when using the constructor. Finally, a new object will be returned, as follows:

function Foo(age) {
  this.age = age;
}

var o = new Foo(111);
console.log(o);

This is a common process of using a constructor to create an object, and what is printed is {age: 111}.

1.2 return object type data

Constructor lastreturnObject type data:

function Foo(age) {
  this.age = age;

  return { type: "我是显式返回的" };
}

var o = new Foo(222);
console.log(o);

What is printed is{type: 'I It is explicitly returned'}, that is to say, all the work before return is done in vain, and finally the object after return is returned.

1.3 Return basic type data

Does that mean that as long as there is return at the end of the constructor body, the data after return will be returned?

Let’s take a look at the situation of returning basic type data:

function Foo(age) {
  this.age = age;

  return 1;
}

var o = new Foo(333);
console.log(o);

What is printed is {age: 333}, and the effect when there is no return Same. It’s different from expectations. Please see the analysis below for the principle behind it.

2) The principle behind

2.1 The case of non-arrow functions

When using the new operator to create an object, the ES5 official documentation is in Function definition The following definition is made in section 13.2.2 [[Construct]]:

When the [[Construct]] internal method for a Function object F is called with a possibly empty list of arguments, the following steps are taken:

1. Let obj be a newly created native ECMAScript object.

2. ##Set all the internal methods of obj as specified in 8.12.

3. Set the [[Class] ] internal property of obj to Object.

4. Set the [[Extensible]] internal property of obj to true.

5. Let proto be the value of calling the [[Get]] internal property of

   F with argument "prototype".

##If Type(proto) is Object, set the
6. [[Prototype]] internal property of obj to proto

   .

If Type(proto) is not Object, set the [[Prototype]] internal property of obj to the standard built-in
7. Object prototype object

   as described in 15.2.4.

Let result be the result of calling the [[Call]] internal property of F,
8. providing obj as the this value

   and providing the argument list passed into [[Construct]] as args.

9. If Type(result) is Object then return result

   .

Return obj.
See steps 8 and 9:

8) Call function

F

, assign its return value to result; among them, the actual parameters when F is executed are passed to [[Construct]] (that is, F itself) parameter, F internal this points to obj; 9) If result
is Object type, returns result;

This also explains that if the constructor explicitly returns the object type, it will return the object directly instead of returning the originally created Object.

Finally look at step 10:

10) If

F

returns not an object type (step 9 is not true), return the created object obj.

If the constructor does not explicitly return the object type (explicitly returns the basic data type or does not return it directly), the initially created object is returned.

2.2 The case of arrow function

What if the constructor is an arrow function?

There is no

[[Construct]]

method in the arrow function, and it cannot be called using new, and an error will be reported. NOTICE:

[[Construct]]

refers to the constructor itself. Relevant specifications are mentioned in the official documentation of ES6, but the official documentation since ES6 is extremely difficult to understand and will not be described here.

3) The complete process of new calling function

3.1 Chinese description and related code analysis

Any function except arrow functions can be used

new

What happens behind the call is very clear in English in the previous section, and it is described in Chinese as follows: <p>1）创建 ECMAScript 原生对象 <code>obj；  
2）给 obj 设置原生对象的内部属性；（和原型属性不同，内部属性表示为 [[PropertyName]]，两个方括号包裹属性名，并且属性名大写，比如常见 [[Prototype]]、[[Constructor]]）  
3）设置 obj 的内部属性 [[Class]] 为 Object；  
4）设置 obj 的内部属性 [[Extensible]] 为 true；  
5）将 proto 的值设置为 F 的 prototype 属性值；  
6）如果 proto 是对象类型，则设置 obj 的内部属性 [[Prototype]] 值为 proto；（进行原型链关联，实现继承的关键）  
7）如果 proto 是不对象类型，则设置 obj 的内部属性 [[Prototype]] 值为内建构造函数 Object 的 prototype 值；（函数 prototype 属性可以被改写，如果改成非对象类型，obj 的 [[Prototype]] 就指向 Object 的原型对象）  
8）9）10）见上节分析。（决定返回什么）

对于第 7 步的情况，见下面代码：

function Foo(name) {
  this.name = name;
}

var o1 = new Foo("xiaoming");
console.log(o1.__proto__ === Foo.prototype); // true

// 重写构造函数原型属性为非对象类型，实例内部 [[Prototype]] 属性指向 Object 原型对象
// 因为实例是一个对象类型的数据，默认会继承内建对象的原型，
// 如果构造函数的原型不满足形成原型链的要求，那就跳过直接和内建对象原型关联
Foo.prototype = 1;
var o2 = new Foo("xiaohong");
console.log(o2.__proto__ === Foo.prototype); // false
console.log(o2.__proto__ === Object.prototype); // true

3.2 更简洁的语言描述

若执行 new Foo()，过程如下：

1）创建新对象 o；  
2）给新对象的内部属性赋值，关键是给[[Prototype]]属性赋值，构造原型链（如果构造函数的原型是 Object 类型，则指向构造函数的原型；不然指向 Object 对象的原型）；  
3）执行函数 Foo，执行过程中内部 this 指向新创建的对象 o；  
4）如果 Foo 内部显式返回对象类型数据，则，返回该数据，执行结束；不然返回新创建的对象 o。

4）几点说明

4.1 判断是否是 Object 类型

关于一个数据是否是 Object 类型，可以通过 instanceof 操作符进行判断：如果 x instanceof Object 返回 true，则 x 为 Object 类型。

由上可知，null instanceof Object 返回 false，所以 null 不是 Object 类型，尽管typeof null 返回 "Object"。

4.2 instanceof 原理

instanceof 的工作原理是：在表达式 x instanceof Foo 中，如果 Foo 的原型（即 Foo.prototype）出现在 x 的原型链中，则返回 true，不然，返回 false。

因为函数的原型可以被改写，所以会出现在 x 通过 Foo new 出来之后完全改写 Foo 的原型 x instanceof Foo 返回 false 的情况。因为实例创建之后重写构造函数原型，实例指向的原型已经不是构造函数的新的原型了，见下面代码：

const Foo = function() {};

const o = new Foo();

o instanceof Foo; // true

// 重写 Foo 原型
Foo.prototype = {};
o instanceof Foo; // false

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