I was recently going through the problems in LeetCode where I was faced with a problem that required sorting an array of numbers. Pretty easy, I immediately used the .sort() method

const nums = [1, 9, 4, 8];
nums.sort();
console.log(nums); // [1,4,8,9]

I ran the code and the test passed successfully. So I submitted the code but then I got an error. My code failed to pass all the tests 😧 ! If you've done LeetCode you're probably familiar with the rage I felt at the time.

This was the test case that my code failed

[74, 100, 60, 98, 8, 45, 6, 59, 69, 32, 93, 9]

Baffled with what was going on, I ran the code on my local machine and started using the Javascript developer's greatest asset - console.log().

[100, 32, 45, 59, 6, 60, 69, 74, 8, 9, 93, 98]

As soon as I saw this, I knew what was going on ! If no compare function is supplied to .sort() javascript coerces the array elements into strings and then sorts them in lexicographical manner. Damn it ! I knew this behavior and I still managed to fall for it.

The sort() method sorts the elements of an array in place and returns the sorted array. The default sort order is ascending, built upon converting the elements into strings, then comparing their sequences of UTF-16 code units values.

The Fix

I wrote a simple compare function to fix the bug.

nums.sort((a, b) => a - b);

console.log(nums);
// [6, 8, 9, 32, 45, 59, 60, 69, 74, 93, 98, 100]