Arr.sort((a,b) => a-b); explanation

Arr.sort((a,b) => a-b); explanation
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Hello everybody, I’m just going throw algorithms and solved Where do I Belong algorithm. I did everything but I didn’t know how to sort by numbers to get a normal order and not to have 10 or 20 before 3 or 4. And I found arr.sort((a,b) => a-b); this code and it works but can somebody explain me what exactly (a,b) a-b do or check in this arrow function for me to better understand?

function getIndexToIns(arr, num) {
  // Find my place in this sorted array.
  arr.sort((a,b) => a-b);
  num = arr.indexOf(num);
  return num;

getIndexToIns([40, 60], 50);


When you sort an array with .sort(), it assumes that you are sorting strings. When sorting numbers, the default behavior will not sort them properly.

The function that you pass tells how to sort the elements. It takes two parameters (a and b) that represent any two elements from the array. How the elements will be sorted depends on the function’s return value:

  • if it returns a negative value, the value in a will be ordered before b.
  • if it returns 0, the ordering of a and b won’t change.
  • if it returns a positive value, the value in b will be ordered before a.

When you pass the function (a, b) => a - b, you’re telling the .sort() function to sort the numbers in ascending order.


Not sure if you are familiar with the arrow function syntax. Just in case, here’s a short explanation:
(a, b) => a - b is a shortcut notation for:

function (a, b){
  return a - b;

Function sort takes a comparison function as a parameter. This is what the sorting algorithm uses repeatedly to compare two elements in the array and decide which one goes to the left of the array and which goes to the right.

If you’re sorting numbers in ascending order, the smallest goes to the left. If you’re sorting in descending order, the biggest goes to the left.

The function needs to return -1 for a to be sorted to the left of b, 1 to be sorted to the right of b and 0 to be considered equal.

In this case a - b returns

  • a negative number if a is smaller than b – so a will be sorted to the left of b
  • a positive number if a is bigger than b – so a will be sorted to the right of b
  • zero if they are equal – so it doesn’t matter which one comes first

If you wanted to sort in descending order, i.e. bigger numbers first, you’d need your function to return b - a to invert the logic.

Using a comparison function also allows you to sort your numbers in a completely different way to serve your purpose. You could for instance sort all the odd numbers first then the even numbers (the example doesn’t care about the internal order within odd and even numbers):

function compare(a, b){
  let mod_a = a % 2;
  let mod_b = b % 2;
  return mod_b - mod_a;

And of course, this is not limited to numbers. You can sort just about any data with your own arbitrary criteria. For instance you can sort strings based on their lowercase value instead of the default Unicode, or ignore ‘a’ and ‘the’ at the beginning. You can sort an array of objects representing your music collection by artist, or by album name, or by date, or all of the above.


Thank you both for good answers :slight_smile: