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Understanding Sort Function in JavaScript

February 11, 2022

Sort Function

  • Used to sort arrays in JavaScript.
  • This method mutates the original array.
  • By default, the order is ascending but we can change the order.
  • We can also provide our compare function to implement custom sorting.

It Gets Tricky

What happens when you sort [22, 1, 8, 45, 11, 2, 1] using the default .sort()?

You'd expect [1, 1, 2, 8, 11, 22, 45], but you get [1, 1, 11, 2, 22, 45, 8].

Why did the sort function not work as expected?

The reason is that the sort method converts the elements into strings and then compares the sequences of their UTF-16 code unit values.

So 1 is converted to "1". When we compare two strings like "1" and "22", the char code is compared — char code of 1 is 49, whereas char code of 2 is 50. Because 49 < 50, we get 1, 1, 11 before 2, 22.

This is where the compare function comes to the rescue.

Compare Function

The compare function takes two arguments. These arguments are taken from pairs of array elements at indices [0,1], [1,2], ... [arrayLength-2, arrayLength-1] to compare each pair of elements.

The sorting of elements is based on the return value of the compare function for each comparison.

If the function returns a value:

  • < 0, then a comes before b.
  • > 0, then the position of a and b is swapped.
  • === 0, then there is no change in position.

Complete Sort Function for Numbers

Ascending Order

const arr = [22, 1, 8, 45, 11, 2, 1];
arr.sort((a, b) => a - b);
// [1, 1, 2, 8, 11, 22, 45]

Descending Order

const arr = [22, 1, 8, 45, 11, 2, 1];
arr.sort((a, b) => b - a);
// [45, 22, 11, 8, 2, 1, 1]

Sort Function for Strings

There is no need for a compare function when sorting strings. We can use the default sort method to sort strings.

const arr = ["banana", "apple", "cherry"];
arr.sort();
// ["apple", "banana", "cherry"]
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