Test

Before testing install dependencies npm i or yarn.

Run tests npm run test or yarn test

Explanation

How returned value of sort() implementation works:

Return value	sort order
> 0	sort a after b
< 0	sort a before b
=== 0	keep original order of a and b

So, the compare function has the following form:

function compareFn(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;
}