phpjs/functions/strings/strtolower.js

php.js offers community built php functions in javascript

function strtolower(str) {
  // From: http://phpjs.org/functions
  // +   original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +   improved by: Onno Marsman
  // *     example 1: strtolower('Kevin van Zonneveld');
  // *     returns 1: 'kevin van zonneveld'
  return (str + '').toLowerCase();
}