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 to uncamelize
 * @param string Word separator
 * @return string Uncamelized text
 */
function uncamelize(text, separator) {
 
  // Assume separator is _ if no one has been provided.
  if(typeof(separator) == "undefined") {
    separator = "_";
  }
 
  // Replace all capital letters by separator followed by lowercase one
  var text = text.replace(/[A-Z]/g, function (letter) {
    return separator + letter.toLowerCase();
  });
 
  // Remove first separator (to avoid _hello_world name)
  return text.replace("/^" + separator + "/", '');
 
}

Awesome, thank you.

More elegance if you want :

function uncamelize(text, separator = "_")  {
 
 return text.replace(/[A-Z]/g,  (letter) => separator + letter.toLowerCase())
              .replace("/^" + separator + "/", '');
 
}

This is nice, but doesn't fully work. Some problems:

• Building a RegExp on the fly with simple strings doesn't work. One need to use the RegExp class for that. I was able to get exactly the format it is trying to avoid: uncamelize('FooBar'); yields _foo_bar in the current version.
• It doesn't handle numbers and leave them concatenated with the previous word.
• It doesn't handle abbreviations (but this is a hard one :) ). uncamelize('HTTPS'); will result into h_t_t_p_s (actually, _h_t_t_p_s in the current version). The expected behavior would be to get https, but as I said, this is a hard one, because there are legit usages of consecutive capital letters in the camel case, like getANumber.

I'm opening a "pull request" with some suggestions on how to fix the first two problems.

It seems to not be possible to offer you a pull request in the Gist interface... In any case, my suggestions can be found here: https://gist.github.com/schrodervictor/bbbd360a496fee4ac267f9abd20855d1

BTW, the original version is much more "elegant" and easy to understand (it doesn't even need all those comments) than the one suggested by @abdennour. There's absolutely no need to transform it into an one-liner. This is the job of the Minifiers. Keep the source code readable by humans, please.