What values can a constructor return to avoid returning this?

Question

What are the exact circumstances for which a return statement in Javascript can return a value other than this when a constructor is invoked using the new keyword?

Example:

function Foo () {   return something; }  var foo = new Foo (); 

If I'm not mistaken, if something is a non-function primitive, this will be returned. Otherwise something is returned. Is this correct?

In other words, what values can something take to cause (new Foo () instanceof Foo) === false?

Answer 1

The exact condition is described on the [[Construct]] internal property, which is used by the new operator:

From the ECMA-262 3rd. Edition Specification:

13.2.2 [[Construct]]

When the [[Construct]] property for a Function object F is called, the following steps are taken:

1. Create a new native ECMAScript object.
2. Set the [[Class]] property of Result(1) to "Object".
3. Get the value of the prototype property of F.
4. If Result(3) is an object, set the [[Prototype]] property of Result(1) to Result(3).
5. If Result(3) is not an object, set the [[Prototype]] property of Result(1) to the original Object prototype object as described in 15.2.3.1.
6. Invoke the [[Call]] property of F, providing Result(1) as the this value and providing the argument list passed into [[Construct]] as the argument values.
7. If Type(Result(6)) is Object then return Result(6).
8. Return Result(1).

Look at steps 7 and 8, the new object will be returned only if the type of Result(6) (the value returned from the F constructor function) is not an Object.

Answer 2

Concrete examples http://jsbin.com/zivivucahi/1/edit?html,js,console,output

/* ECMA 262 v 5 http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf "4.3.2 primitive value member of one of the types Undefined, Null, Boolean, Number, Symbol, or String as defined in clause 6" */  var Person = function(x){   return x;  };   console.log(Person.constructor); console.log(Person.prototype.constructor); console.log(typeof(Person)); console.log(typeof(Person.prototype));  function log(x){   console.log(x instanceof Person);   console.log(typeof x);   console.log(typeof x.prototype); }  log(new Person(undefined)); log(new Person(null)); log(new Person(true)); log(new Person(2)); log(new Person(""));  //returns a function not an object log(new Person(function(){}));   //implementation? //log(new Person(Symbol('%')));