How to uncamelize a string in Javascript? Example: "HelloWorld" --> "hello_world"

uncamelize.js

/**
 * Uncamelize a string, joining the words by separator character.
 *
 * @param {string} text - Text to uncamelize
 * @param {string} [separator='_'] - Word separator
 * @returns {string} Uncamelized text
 */
function uncamelize(text, separator) {
  // Assume separator is _ if no one has been provided.
  if('undefined' === typeof separator) {
    separator = '_';
  }

  // Replace all capital letters and group of numbers by the
  // separator followed by lowercase version of the match
  var text = text.replace(/[A-Z]|\d+/g, function(match) {
    return separator + match.toLowerCase();
  });

  // Remove first separator (to avoid _hello_world name)
  return text.replace(new RegExp('^' + separator), '');
}

separator = "_" in the function definition would save you an unnecessary check, but otherwise this works great!