picoCreator picoCreator - 25 days ago 7
Javascript Question

How does the new operator work in JavaScript?

Probably the least understood part of JavaScript, standing beside the prototype chain.

So the question is: how does...

new dataObj(args);


...actually create an object, and define its prototype chain/constructors/etc?

Best is to show an alternative, to fully understand this keyword.

CMS CMS
Answer

The new operator uses the internal [[Construct]] method, and it basically does the following:

  • Initializes a new native object
  • Sets the internal [[Prototype]] of this object, pointing to the Function prototype property.
    • If the function's prototype property is not an object (a primitive values, such as a Number, String, Boolean, Undefined or Null), Object.prototype is used instead.
  • After creating the object, it calls the function, providing the object as its this value.
  • If the return value of the called function, is a primitive, the object created internally is returned.
  • Otherwise, if an object is returned, the object created internally is lost.

An equivalent implementation of what the new operator does, can be expressed like this (assuming that the ECMAScript 5 Object.create method is available):

function NEW(f) {
  var obj, ret, proto;

  // Check if `f.prototype` is an object, not a primitive
  proto = Object(f.prototype) === f.prototype ? f.prototype : Object.prototype;

  // Create an object that inherits from `proto`
  obj = Object.create(proto);

  // Apply the function setting `obj` as the `this` value
  ret = f.apply(obj, Array.prototype.slice.call(arguments, 1));

  if (Object(ret) === ret) { // the result is an object?
    return ret;
  }
  return obj;
}

// Example usage:
function Foo (arg) {
  this.prop = arg;
}
Foo.prototype.inherited = 'baz';

var obj = NEW(Foo, 'bar');
obj.prop;          // 'bar'
obj.inherited;     // 'baz'
obj instanceof Foo // true