Object inside of array -- works in one scope but not in another?

Question

Ok I've been learning some of the more advanced aspects of Javascript and now trying to use this I'm stuck.

Here is my code:

function Data(){} function init(state){ var item; item=new Data(); item.fieldrid=17; item.description='foo'; state.push(item); }; function findInState(state,fieldrid) { for (var item in state) { alert(item.fieldrid); //prints undefined if (item.fieldrid == fieldrid) { return item; } } return null; } var s=[]; init(s); alert(s[0].fieldrid); //prints 17 (expected) alert(findInState(s,17).fieldrid); //exception here. function returns null.

A running example is here at jsbin

Why does this not work? I would expect the alert in findInState to yield 17 but instead it yields undefined.

What am I doing wrong?

deceze · Accepted Answer · 2010-06-10 01:20:11Z

3

for (var item in state)

You should not loop over an array using for..in.
It works as expected when using for (var i = 0; i < state.length; i++).

See https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Statements/for...in:

Although it may be tempting to use this as a way to iterate over an Array, this is a bad idea.

answered Jun 10, 2010 at 1:20

deceze♦

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5 Comments

BalusC Over a year ago

Actually, you can loop over it, the var item then only holds the array index. You should then use state[item] instead of item in the remnant of the for block. Needless to say that this is indeed not the preferred way to loop over an array :)

Earlz Over a year ago

@balus so then what is "the preferred way"? Is there some short cut I'm missing other than the C style for(;;) syntax?

deceze Over a year ago

@Earlz No, for(;;) is the preferred way.

BalusC Over a year ago

Use for(;;). The for in may return other properties than array index and the iteration order is arbitrary. You need to do additional checks. Also see the link in the answer, there's a whole paragraph at bottom of Description chapter.

Christian C. Salvadó Over a year ago

@Earlz: See also this answer I've posted earlier for more details about why you should avoid for-in with array-like objects...

user347594 · Accepted Answer · 2010-06-10 01:32:29Z

In

alert(item.fieldrid); //prints undefined

You need to access the array state[item] so before that line, prepend

item = state[item];

I think, that I'll do.

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