JavaScript:How new works

The new operator takes a function F and arguments: new F(arguments...). It does three easy steps:

1. Create the instance of the class. It is an empty object with its __proto__ property set to F.prototype.
2. Initialize the instance. The function F is called with the arguments passed and this set to be the instance.
3. Return the instance

Now that we understand what the new operator does, we can implement it in Javascript.

 

 function New (f) {
/*1*/  var n = { '__proto__': f.prototype };
       return function () {
/*2*/    f.apply(n, arguments);
/*3*/    return n;
       };
}

 

And just a small test to see that it works.

 

 

function Point(x, y) {
  this.x = x;
  this.y = y;
}
Point.prototype = {
  print: function () { console.log(this.x, this.y); }
};
 
var p1 = new Point(10, 20);
p1.print(); // 10 20
console.log(p1 instanceof Point); // true
 
var p2 = New (Point)(10, 20);
p2.print(); // 10 20
console.log(p2 instanceof Point); // true