Strings as keys of array

Question

When using strings as keys of an array, console is showing that the array without these declared values and while iterating by this values where keys are string aren't displayed? , although i can get value of them.

>> var arr = [ 0, 1, 2, 3 ];
   undefined

>> arr["something"] = "aught";
   "aught"

>> arr
   [0, 1, 2, 3]

>> arr["something"]
   "aught"

>> for( var i = arr.length; i--; console.log( arr[ i ] ) );
   3
   2
   1
   0

I understand that arrays are objects which has implemented some kind of 'enumerate' interface in JavaScript's engine.

Most interesting is that interpreter isn't throwing either warning or error, so I spent some time of searching for where data could be lost.

Darin Dimitrov · Accepted Answer · 2011-12-25 16:38:50Z

109

In javascript there are 2 type of arrays: standard arrays and associative arrays

[ ] - standard array - 0 based integer indexes only
{ } - associative array - javascript objects where keys can be any strings

So when you define:

var arr = [ 0, 1, 2, 3 ];

you are defining a standard array where indexes can only be integers. When you do arr["something"] since something (which is what you use as index) is not an integer you are basically defining a property to the arr object (everything is object in javascript). But you are not adding an element to the standard array.

answered Dec 25, 2011 at 16:38

Darin Dimitrov

1.0m275 gold badges3.3k silver badges3k bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Darin Dimitrov Over a year ago

@abuduba, because there is nothing wrong :-) Your code is perfectly valid javascript. It's ambiguous to the reader since you are mixing the 2 type of arrays but it is valid. Why would you want the interpreter to indicate that something is wrong when there is nothing wrong.

jfriend00 Over a year ago

This is mostly a terminology issue, but javascript does NOT generally call {} an associative array. They call it an object that has properties. Don't most people agree that it's best not to confuse things by calling a javascript object an associative array?

David Hellsing Over a year ago

Note: If you try to use defineProperty on the array like Object.defineProperty.call(arr,'something','aught'); it will throw an error.

abuduba Over a year ago

Youre right. Key something is property of object instead index of array => arr.something. Maybe I'am too long without sleep .. I forgot that. Thanks @David - Nice, i didnt know about it

Jean-Luc Barat Over a year ago

It's really strange because in console, var a = []; a['test'] = ""; a; displays ['test': ""] therefore a.length is empty as expected. Also it's strange because if we add something like this a[0] = 0; both keys 'o' and 'test' has a property of 'a', It's hard to understand!

fanaugen · Accepted Answer · 2012-03-30 13:04:06Z

16

for( var i = arr.length; i--; console.log( arr[ i ] ) );

This will only give you the numeric indices, of course, but you can still loop over both numeric indices and string keys of your array like this:

for (var x in arr) {
    console.log(x + ": " + arr[x]);
}
/* (console output):
     0: 0
     1: 1
     2: 2
     3: 3
     something: aught
*/

answered Mar 30, 2012 at 13:04

fanaugen

1,1272 gold badges11 silver badges23 bronze badges

1 Comment

KhaledMohamedP Over a year ago

so, lets add this Array.prototype.somethingCool = "not really"; and then run your for-in-loop !!

Collectives™ on Stack Overflow

Strings as keys of array

2 Answers 2

5 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related