Array.length gives incorrect length

Question

If I have an array having object as values at the indices like:

var a = [];
a[21] = {};
a[90] = {};
a[13] = {};
alert(a.length); // outputs 91

Answer 1

That's because length gives you the next index available in the array.

DOCS

arrayLength

If the only argument passed to the Array constructor is an integer between 0 and 2^32-1 (inclusive), this returns a new JavaScript array with length set to that number.

ECMA Specifications

Because you don't have inserted any element in the other keys than 21, 90, 13, all the remaining indexes contains undefined. DEMO

To get actual number of elements in the array:

var a = [];
a[21] = {};
a[90] = {};
a[13] = {};

var len = 0;

for (var i = 0; i < a.length; i++) {
  if (a[i] !== undefined) {
    len++;
  }
}
document.write(len);

Shorter version

var a = [];
a[21] = {};
a[90] = {};
a[13] = {};


for (var i = 0, len = 0; i < a.length; i++, a[i] !== undefined && len++);


document.write(len);

DEMO

EDIT

If the array contains large number of elements, looping to get its length is not the best choice.

As you've mentioned in the question, Object.keys(arr).length is the best solution in this case, considering that you don't have any properties added on that array. Otherwise, the length will not be what you might be expecting.(Thanks To @RobG)