for ( key in array) loops over array prototype

Question

I don't know how to word this problem exactly but I found this extremely wired.

Basically I did this test in chrome's developer tool console.

for (var request in [0,1,2]) { console.log(request);}

0
1
2
compare

the last four lines are all outputs from the for loop. during the for loop, request got the value compare.

I wonder if this is a bug in chrome.

`for..in` is not meant for arrays, use regular `for` loop: `var arr = [1,2,3]; for (var i=0; i — elclanrs, Oct 05 '13 at 23:08
this is exactly why you shouldn't use for..in to loop over arrays — John Dvorak, Oct 05 '13 at 23:09

score 2 · Accepted Answer · answered Oct 05 '13 at 23:13

2

for ... in ... iterates over the enumerable properties of an object, and is not intended for array indices. Array indices are also enumerable properties, but as you've discovered anything unsafely added to Array.prototype will be returned too.

To safely add a (non-enumerable) method to Array.prototype in ES5 browsers you can use Object.defineProperty, e.g.:

Object.defineProperty(Array.prototype, 'compare', {
    value: function() {
        ...
    }
});

This will stop for ... in from breaking, but it's still the wrong tool for the job when the variable of interest is an array.

answered Oct 05 '13 at 23:13

Alnitak

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thanks, Just confirmed that compare has been override by other teammember – Luka Yu Oct 05 '13 at 23:35
@LukaYu do hit him around the head. Also, one for yourself too, for the `for ... in` loop ;-) – Alnitak Oct 05 '13 at 23:44

score 1 · Answer 2 · edited May 23 '17 at 10:25

1

You're best off using an indexed for loop. For..in also enumerates over inherited properties etc.

var request = [0,1,2];
for (var i = 0; i < request.length; i++) {
   console.log(request[i]);
}

The top answer to this question:

stackoverflow previous answer

puts it better than I could:

edited May 23 '17 at 10:25

Community

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answered Oct 05 '13 at 23:16

Joseph Hopkins

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6

score 0 · Answer 3 · answered Oct 05 '13 at 23:24

in your case, the global "object-prototype" as a compare function declared for it, e.g...

object.prototype.compare = function (a,b) {return a === b}

...and so, whenever you iterate an object (an array being one kind of object) you also iterate over the "compare" function of it's prototype... which is a "member" of it.

score 0 · Answer 4 · answered Oct 05 '13 at 23:35

As others pointed out for .. in is not the best way to iterate thru array. If you insist on using it for some reason - use hasOwnProperty method to determine that property indeed belongs to the array:

var arr = [0,1,2];

for (var request in arr ) { 
    if (arr.hasOwnProperty(request)) console.log(request);
}

for ( key in array) loops over array prototype

4 Answers4