Return true if string is inside another string, locale independent.

substring_exists.js

function substringExists(haystack, needle)
{
  size_haystack = haystack.length;
  size_needle = needle.length;
  loop = size_haystack - size_needle;
  for (i=0; i < loop + 1; i++) {
    sub = haystack.substr(i, size_needle);
    c = sub.localeCompare(needle, {}, {sensitivity: 'base'});
    if (c === 0) {
      return true;
    }
  }
  return false;
}

console.log(substringExists("A Maçã é uma fruta estranha.", "maça"));