Question

Someone in the question Is there any reason to manually return in a constructor function mentioned that he thought that returning arbitrary objects in constructor calls may have been introduced in a recent version of Javascript.

I am unable to access alternate browsers right now, and I can’t find any information about this, so what version introduced this, or has it existed since forever?

If it hasn’t been around for long, then what browser versions will support it?

Answer 1

If you look in ECMA-262 ed 1 (which is still available online from the ECMA site) you will see:

11.2.2 The new operator

The production NewExpression : new NewExpression is evaluated as follows:

1. Evaluate NewExpression.
2. Call GetValue(Result(1)).
3. If Type(Result(2)) is not Object, generate a runtime error.
4. If Result(2) does not implement the internal [[Construct]] method, generate a runtime error.
5. Call the [[Construct]] method on Result(2), providing no arguments (that is, an empty list of arguments).
6. If Type(Result(5)) is not Object, generate a runtime error.
7. Return Result(5).

And then in §15.3.2.1 #18 the [[Construct]] method is explained (in the following, F is a newly constructed Function object):

18. Set the [[Construct]] property of F to a method that, when it is invoked, constructs a new object whose [[Prototype]] property is equal
to the value of F.prototype at the time the [[Construct]] method is
invoked (but if this value is not an object then the value of
Object.prototype is used), then invokes F as a function (using its
[[Call]] property) with the new object as the this value and the
arguments given to the [[Construct]] method as the arguments. If the
result of invoking the [[Call]] method is an object, that object
becomes the result of the invocation of the [[Construct]] method;
otherwise the new object becomes the result of the invocation of the
[[Construct]] method.

So a constructor returning this by default has been in ECMAScript since the beginning.