Why does the typeof a numerical array index in a "for..in" loop considered a string? [duplicate]

Question

I noticed that in Javascript a variable used as the index in a for..in loop will be always a string even if I define it the following way:

var s_array = new Array();
s_array[0] = 'foo';
s_array[1] = 'bar';

for(i in s_array){
 alert(typeof(i)); // String
}

Why is it considered a string and not a number?

Quintin

Answer 1

The for(x in y) syntax is intended to iterate over the properties of an object (not the indexes of an array), and property names are always stored as strings.

The fact that it also works for arrays is a side effect of array elements being properties on the array object.

To understand the difference, consider this code:

var s_array = new Array();
s_array[0] = 'foo';
s_array[1] = 'bar';
s_array['foo'] = 'bar';

console.log("Object:");
for(i in s_array) {
 console.log(i);   
}
console.log("Array:");
for(var i = 0, l = s_array.length; i < l; i++) {
 console.log(i);   
}

which provides the following output:

Object:
0
1
foo
Array:
0
1

There's a foo property on the object, but it's not actually an element inside the array.

Contrastive

I find your wording slightly confusing: array indexes are not "mapped" to properties, array elements are properties. Arrays are just like other objects, there's nothing special about them. – Stoffel

@thg435 Good point, edited. I was trying to indicate that there's a slight difference between array elements as properties and other properties set on the object, but definitely could have worded it better. – Contrastive

first thank you all for the answers, so when I am asking for s_array[1] behind the scene.. there is some conversion of the number 1 to string? – Quintin

Answer 2

Arrays are essentially objects with managed set of indexed keys.

Since every key in an object is of type string hence it is a string as well.

Consider your array as :

{"0" : "foo" , "1" : "bar"}

So your

for(i in s_array){ alert(typeof(i)); }

can be read as

for each key in the s_array

Redbug

Answer 3

In js arrays are high-level, list-like objects (associative arrays).

indexes eventually gets coerced into a string by the JavaScript engine, anyway, through an implicit toString conversion.

source: MDN

Dentist