Ruphaa's Notes

How to pass a custom sort function in js and how does it work?


var numbers = [4, 2, 5, 1, 3];

numbers.sort(function (a, b) {
return a - b;


// [1, 2, 3, 4, 5]
function compare(a, b) {
if (a is less than b by some ordering criterion) {
return -1;
if (a is greater than b by the ordering criterion) {
return 1;
// a must be equal to b
return 0;

This is how the compare function works

If compareFunction is supplied, all non-undefined array elements are sorted according to the return value of the compare function (all undefined elements are sorted to the end of the array, with no call to compareFunction). If a and b are two elements being compared, then:

  • If¬†compareFunction(a, b)¬†returns¬†less than 0, sort¬†a¬†to an index lower than¬†b¬†(i.e.¬†a¬†comes first).
  • If¬†compareFunction(a, b)¬†returns 0, leave¬†a¬†and¬†b¬†unchanged with respect to each other, but sorted with respect to all different elements. Note: the ECMAscript standard does not guarantee this behavior, thus,¬†not all browsers (e.g. Mozilla versions dating back to at least 2003) respect this.
  • If¬†compareFunction(a, b)¬†returns greater than 0, sort¬†b¬†to an index lower than¬†a¬†(i.e.¬†b¬†comes first).
  • compareFunction(a, b)¬†must always return the same value when given a specific pair of elements¬†a¬†and¬†b¬†as its two arguments. If inconsistent results are returned, then the sort order is undefined.