I was experimenting with the new keyword and I can't find an explanation for this behavior. Let's say we have a function returning an integer:

(In firebug)

>>> function x() { return 2; }
>>> x()
2
>>> new x()
x { }

But if the function returns an array :

>>> function y() { return [2]; }
>>> y()
[2]
>>> new y()
[2]

Why is that ?

I was experimenting with the new keyword and I can't find an explanation for this behavior. Let's say we have a function returning an integer:

(In firebug)

>>> function x() { return 2; }
>>> x()
2
>>> new x()
x { }

But if the function returns an array :

>>> function y() { return [2]; }
>>> y()
[2]
>>> new y()
[2]

Why is that ?

• javascript
• arrays
• function
• new-operator

• 2 Off-topic: I know you're just experimenting, but it's worth getting in the habit early and if you're experimenting you may not (yet) know this: The overwhelming convention in JavaScript code is that the names of constructor functions (functions you use with new rather than just calling directly) start with a capital letter, and the names of all other functions start with a lower-case letter. So Foo would be a constructor function, and foo would not. – T.J. Crowder Commented Jun 27, 2011 at 9:51
• Is this valid javascript? Why have you called x() on its own with no semi-colon? – Exitos Commented Jun 27, 2011 at 9:53
• 1 @Pete2k: Looks like kbok is playing around in a JavaScript console. Also, semicolons are optional in JavaScript, statements can also be terminated by an EOL. – mu is too short Commented Jun 27, 2011 at 9:54
• @Pete2k: Why should it not be valid? It's a function. Semi-colons are optional. – Felix Kling Commented Jun 27, 2011 at 9:54
• this is because I'm typing it in the firebug console. – slaphappy Commented Jun 27, 2011 at 9:55

2 Answers 2

The new operator has an interesting behavior: It returns the object created by the operator unless the constructor function returns a different object. Any non-object return value of the constructor function is ignored, which is why when you return 2 you don't see this.

Here's what happens when you say new x():

1. The interpreter creates a new blank object.
2. It sets the object's underlying prototype to x.prototype.
3. It calls x with this set to the new object.
4. In the normal case, x doesn't return anything and the result of the new expression is the new object created in step 1. But, if x returns a non-null object reference, then that object reference is the result of the new expression rather than the object created in step 1. Any other kind of return value (null, primitive numbers, primitive strings, undefined, etc.) is ignored; it has to be a non-null object reference to take precedence over the object new created.

This special treatment given to object references by the new operator lets you substitute a different object for the one new created. This can be handy in some limited situations, but the vast majority of the time, a function designed to be used with new (called a constructor function) shouldn't return anything at all.

For some light reading (hah!), this is covered by Section 13.2.2 ("[[Construct]]") of the specification (HTML; PDF), which is referenced by Section 11.2.2 ("The new operator").

Because an array is an object but 2 is not.

If you call a function with the new keyword, it has to return an object. If you don't do that explicitly, it automatically returns this (which is an empty object that inherits from funcName.prototype).