Which takes less memory: a Javascript array or Javascript object?

Question

If I have a Javascript list which will have only numeric keys, which takes less memory?

var array = [];
array[0] = 'hello';
array[5] = 'world';
array[50] = 'foobar';

var obj = {};
obj[0] = 'hello';
obj[5] = 'world';
obj[50] = 'foobar';

I don't know a ton about Javascript engine internals, so...

The reason I ask is because that array, when converted to a string, will have a bunch of undefined's in the middle of it. Are those actually stored in some fashion, or is that just put in at string conversion?

Answer 1

An array is basically an ordered set of values associated with a single variable name.

In your example I think you try to do an associative array, and you should use object, Array is not meant to be used for key/value pairs.

Also the array length is indirecly increased when you assign a value to an index with higher length of the current array length:

var array = new Array();
array[99] = "Test";
// array.length is now 100

Check this detailed article on the subject.