🎯 summary

1. Create an empty object
2. An internal property of an empty object__proto__Assigned to the constructorprototypeattribute
3. ConstructorthisPointing to an empty object
4. Execute the constructor internal code
5. Returns the new object

Detailed instructions

The new operation goes through the following steps:

1. Create an empty object

   • An empty Object is an instance of Object, i.e{} 。

   let obj = {}
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2. The empty object’s internal __proto__ attribute is assigned to the constructor’s Prototype attribute

   • This is done to link the empty object to the correct stereotype

   function Foo(num) {
     this.number = num
   }
   
   obj.__proto__ = Foo.prototype
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3. Point the constructor’s this to an empty object

   • That is, this inside the constructor is assigned an empty object so that the constructor will be executed correctly later.

   Foo.call(obj, 1)
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4. Execute the constructor internal code

   • Add properties and methods to empty objects.

5. Returns the new object

   • If the constructor internally returns a reference type value via a return statement, the new operation eventually returns the reference type value. Otherwise return the newly created object.
   • Reference type values: All values except basic type values (numeric, string, Boolean, NULL, undefined, Symbol).

Emulate the new operator

The following myNew function emulates the behavior of the new operator

function myNew(func, ... args) {
  let obj = {}
  obj.__proto__ = func.prototype
  let res = func.apply(obj, args)
  return res instanceof Object ? res : obj
}

function Foo(num) {
  this.number = num
}

let foo1 = myNew(Foo, 1)
console.log(foo1 instanceof Foo)  // true
console.log(foo1.number)  / / 1

let foo2 = new Foo(2)
console.log(foo2 instanceof Foo)  // true
console.log(foo2.number)  / / 2
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The instanceof operator is used to determine whether the return value of the constructor is an instanceof Object, that is, a reference type value. This is because all reference type values are instances of Object, the base type of all reference type values.