Sorting Arrays in Javascript

TIL how the Array.prototype.sort() sorts by default, and how to pass a comparator function in to the sort method if a different sort order is desired.

I needed to sort an array of integers today using javascript. I knew arrays had a sort method baked in to the prototype, so I figured this would be well suited for sorting my integer array…WRONG! See below.

var arr = [20, 3000, 3, 100];
arr.sort();

//the array is now sorted as follows
// [100, 20, 3, 3000]

What is going on???

Array.prototype.sort() default sorts by ascii value. My integers were being converted to strings and compared as such. In the case above, integers with a 1 as their first digit come first in ascii…then 2…then 3…

To do a proper integer sort, you need to pass a comparator function to the Array.prototype.sort() method that dictates how sorting should be accomplished. The comparator takes 2 arguments (a, b) which represents two adjacent elements in the array to be sorted.

1. If 0 is returned from the comparator, nothing is done to the positions of ‘a’ and ‘b’ in the array
2. If a value greater than 0 is returned, ‘b’ comes first positionally in the array relative to ‘a’
3. If a value less than 0 is returned, ‘a’ comes first positionally in the array relative to ‘b’

A proper integer sort is achieved by the following:

var arr = [20, 3000, 3, 100];
arr.sort(function(a, b){
	return a - b;
});

//the array is now sorted as follows
// [3, 20, 100, 3000]

The comparator function above returns values that comply to the 3 rules above, and will sort the integer values appropriately.

See the following reference from Mozilla on Array.prototype.sort() for additional info.