Javascript array returns last entry for all

Question

I am trying to set a multi-dimensional array in JavaScript but it returns the last entry for all data. I am expecting the alert to return 444 but it returns 555.

Any ideas?

function test() {
    var arr = [,];
    arr[1]="qtest0";
    arr[1,0]=444;
    arr[2,0]=555;
    arr[2]="qtest1";
    alert(arr[1]);
    alert(arr[1,0]);
}

What are you expecting arr[1,0] and the others like it to do? You're basically only accessing the index after the comma. The index before is ignored. — user1106925
– user1106925, Commented Jul 5, 2015 at 3:29
...JavaScript doesn't have true multidimensional arrays. You can create nested "arrays of arrays", but they need to be created manually. — user1106925
– user1106925, Commented Jul 5, 2015 at 3:30
This is multi dimen array [[1,2],[3,4],[[5,6],[7,8]]] yours is I dont know where you learned that from array — Trash Can
– Trash Can, Commented Jul 5, 2015 at 3:42
@Dummy - Your example isn't a multidimensional array, it is an array of arrays. Similar, but not quite the same thing. Other languages allow multidimensional arrays with a syntax similar to the OP's. — nnnnnn
– nnnnnn, Commented Jul 5, 2015 at 3:48
@nnnnnn multi dimen arrays are arrays of arrays, what's your point? I have never seen any language that allows for the creation of multi dimen arrays using the OP's syntax — Trash Can
– Trash Can, Commented Jul 5, 2015 at 3:50

Community · Accepted Answer · 2017-05-23 11:43:58Z

6

This statement:

var arr = [,];

Does not create a multidimensional array. It creates a sparse array with one empty slot in it. And this statement:

arr[1,0]=444;

Does not assign to a two-dimensional array index. Rather, the expression 1,0 simply evaluates to 0 (this is how the somewhat obscure comma operator works). So arr[1,0] is exactly the same as arr[0].

Javascript does not natively support multidimensional arrays. Rather, you should have an array whose elements are themselves arrays:

var arr = [];
arr[1] = [];
arr[1][0] = 444;

For more information on multidimensional arrays, see this question.

edited May 23, 2017 at 11:43

CommunityBot

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answered Jul 5, 2015 at 3:36

Thom Smith

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