Why
for (i in [1, 2, 3]) console.log(typeof(i), i);
gives this:
[Log] string 0
[Log] string 1
[Log] string 2
I've expected numbers.
@ Safari 7.0 (9537.71), Mac OS X 10.9
2 Answers
var arr = [1, 2, 3];
for (var i in arr)
console.log(typeof arr[i], arr[i]);
You're getting strings rather than numbers because for..in loops iterate keys/properties (in this case, the Array's indices) rather than values.
To get the value of each key, i, you'll have to use a property accessor, arr[i].
Though, why strings at all rather than the original number indices is because, with current standards, all properties are strings.
console.log(Object.keys( ['foo', 'bar', 'baz'] )); // ["0", "1", "2"]
Any value can actually be used with property accessors, but it'll be converted ToString() before it's actually used as a key/property.
6) Let propertyNameString be ToString(propertyNameValue).
Maps and Symbols are currently planned to be the exceptions to that.
Also, you may find "Why is using “for…in” with array iteration such a bad idea?" of interest and at least consider using a simple for loop instead:
var arr = [1, 2, 3];
for (var i = 0, l = arr.length; i < l; i++)
console.log(typeof arr[i], arr[i]);
Though, for future reference, the upcoming ECMAScript 6 standard has added for..of loops, which should iterate as you were expecting.
for (var i of [1, 2, 3])
console.log(typeof i, i); // number, 1, 2, 3
answered Dec 11, 2013 at 14:33
Jonathan Lonowski
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3 Comments
akaRem
The Q was: why strings? Ok, it's easy to guess that I get "indexes" instead of values. But the fact that indexes are strings was very unexpected. Now I understand that it's a "bug by design". .. Thank you for your post, it's well-formed and very useful, but it's not the exact answer for my question.
Jonathan Lonowski
@akaRem Ah. Sorry. I guess I read it more as: "why those strings?" I've added an explanation for "why strings?" Though, "bug by design" is an odd choice of description. I can understand wanting to disagree with the behavior being what it is, but it's still well-defined and standardized. So, it's not a "bug."
akaRem
Ok, it's just bad design. :)
That is because an Array in Javascript is a special Object with property keys (which are strings) used as indices.
you are iterating that Array like an Object and because of that i is seen as a property key, a string.
To iterate in the right way an Array you have to use the following:
for( var i=0; i < [1,2,3].length; i++){ ... }
inkeyword is always for keys/properties, not values.for-initerates keys and theinoperator checks for the existence of keys. To get a value, you need to use a property accessor:arr[i].