Javascript Global Reference Error

Question

I got this question in an interview and I am curious as to why the two output different things:

(function() {
    console.log(bar);
    console.log(baz);
    foo();

    function foo() {
        console.log('aloha');
    }

    var bar = 2;
    baz = 3;
})();

ouputs:

undefined
Uncaught ReferenceError: baz is not defined

whereas:

(function() {
    console.log(bar);
    console.log(window.baz);
    foo();

    function foo() {
        console.log('aloha');
    }

    var bar = 2;
    baz = 3;
})();

outputs:

undefined
undefined
'aloha'

what is the difference in the way that baz and window.baz are referenced? I though globals were automatically attached to window?

I think the real problem is why logging `bar` doesn't throw error. At least I am curious about that. — alpakyol, Mar 07 '14 at 07:54
Why did you need to post all that other code when your question is just about the difference between `baz` and `window.baz`? — Barmar, Mar 07 '14 at 07:59
cause that was what I got asked verbatim. I guess I could have removed the other stuff. — moesef, Mar 07 '14 at 08:00
The interviewer was trying to make it hard for you by throwing in irrelevant junk to confuse you. Your goal when asking on SO should be the exact opposite. — Barmar, Mar 07 '14 at 08:09

score 1 · Accepted Answer · answered Mar 07 '14 at 07:56

A ReferenceError indicates that an invalid reference value has been detected (ECMA 5 15.11.6.3)

In practical terms, this means a ReferenceError will be thrown when JavaScript attempts to get the value of an unresolvable reference. (There are other cases where a ReferenceError will be thrown, most notably when running in ECMA 5 Strict mode. If you’re interested check the reading list at the end of this article)

For further reading take a look here.

thanks for the link. that's the kind of in-depth explanation I was looking for. — moesef, Mar 07 '14 at 08:03

shredmill · Answer 2 · 2014-03-07T08:03:57.867

0

Because function definitions are evaluated before execution, variable assignment expressions are not.

edited Mar 07 '14 at 08:03

answered Mar 07 '14 at 07:52

shredmill

283
3
10

So, why loggin `bar` doesn't throw error? – alpakyol Mar 07 '14 at 07:53
bar doesn't produce an error because of Javascripts Hoisting principle – moesef Mar 07 '14 at 07:59
I believe that var defs are also noted by the compiler before execution, however the assignment of the value is not. This would explain all the behavior. – shredmill Mar 07 '14 at 08:01

score 0 · Answer 3 · answered Mar 07 '14 at 07:52

0

This is because console.log(window.baz); is trying to find the variable inside window object where as console.log(baz) is trying to get a standalone variable, which is not defined yet.

answered Mar 07 '14 at 07:52

Santosh Pradhan

149
1
11

Javascript Global Reference Error

3 Answers3