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.

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

The expression new C(arg1, arg2):

Assuming C is a JavaScript function (otherwise you get an error):

  1. Creates a new empty object (no properties)
  2. Sets the prototype of the new object to the value of the "prototype" property of C.
    • Note: The default value of prototype for a function is an object (automatically created when the function is declared) with its prototype set to Object.prototype and a constructor property pointing back to the function C.
    • Note: The terminology can be confusing. The property named "prototype" is not the same as the prototype of the object. Only functions have the property named "prototype", but all objects have a prototype.
  3. Calls the function C with 'this' set to the new object, and with the supplied arguments.
  4. If calling the function C returns an object, this object is the result of the expression. Otherwise the newly created object is the result of the expression.

An alternative to new in ECMAScript 5 would be to use the builtin Object.createObject method.

new C(arg1, arg2) would be equivalent to:

var obj = Object.createObject(C.prototype);
C.apply(obj, [arg1, arg2]);

Standard JavaScript does not allow you to explicitly set the prototype of an object, so Object.createObject cannot be implemented in the language itself. Some implementations does allow it through the non-standard property __proto__. In that case, new C can be simulated like this:

var obj = {};
obj.__proto__ = C.prototype;
C.apply(obj, [arg1, arg2]);