Question

if I have function like this:

function Apple(){
    this.color = "green";
    return this;
}

When creating object like this:

var my_obj = new Apple();

is that line return this; necessary and/or is it valid by language reference?

Answer 1

No, returning this is not necessary, but it is valid. If the returned value is an object, new will return that object instead of the newly created object.

See points 11.2.2 and 13.2.2 of ECMAScript 5:

The new operator calls the internal [[Construct]] method on the constructor (usually a function):

11.2.2 The new Operator

The production NewExpression : new NewExpression is evaluated as follows:

1. Let ref be the result of evaluating NewExpression.
2. Let constructor be GetValue(ref).
3. If Type(constructor) is not Object, throw a TypeError exception.
4. If constructor does not implement the [[Construct]] internal method, throw a TypeError exception.
5. Return the result of calling the [[Construct]] internal method on constructor, providing no arguments (that is, an empty list of arguments).

The [[Construct]] internal method of functions is described in point 13.2.2:

13.2.2 [[Construct]]

When the [[Construct]] internal method for a Function object F is called with a possibly empty list of arguments, the following steps are taken:

1. Let obj be a newly created native ECMAScript object.
2. Set all the internal methods of obj as specified in 8.12.
3. Set the [[Class]] internal property of obj to “Object”.
4. Set the [[Extensible]] internal property of obj to true.
5. Let proto be the value of calling the [[Get]] internal property of F with argument “prototype”.
6. If Type(proto) is Object, set the [[Prototype]] internal property of obj to proto.
7. If Type(proto) is not Object, set the [[Prototype]] internal property of obj to the standard built-in Object prototype
   object as described in 15.2.4.
8. Let result be the result of calling the [[Call]] internal property of F, providing obj as the this value and providing the argument list passed into [[Construct]] as args.
9. If Type(result) is Object then return result.
10. Return obj.