Javascript: case sensitive string compare

22nd January 2021

Getting case sensitive string compare in Javascript isn’t as trivial as it may seem at first:

// Returns ["Action", "activity", "alpha"],
// although ["activity", "alpha", "Action"] is expected instead
["alpha", "Action", "activity"].sort((a, b) => {
    return a.localeCompare(b, { sensitivity: 'case' });
});

The jury is still out on wheter this is an oversight in the spec or the common vendor implementations, in the meanwhile, a manual comparer does the trick:

function caseSensitiveCompare(a, b) {
  // Sort character by character, return early if possible
  for (let ii = 0; ii < Math.max(a.length, b.length); ii++) {
    const aChar = a.charAt(ii);
    const bChar = b.charAt(ii);

    // If inputs match up to here, but lengths don't match, sort by length
    if (!(aChar && bChar)) {
      return a.length - b.length;
    }

    // If we meet a differing character, return early
    const comp = aChar.localeCompare(bChar);
    if (comp !== 0) {
      return comp;
    }
  }
  // If we found nothing to do, the strings are equal
  return 0;
};

The above snippet covers full string sensitivity while still relying on localeCompare() for the grunt work.

// Returns ["activity", "alpha", "Action"] as expected
["alpha", "Action", "activity"].sort(caseSensitiveCompare);