The "Look and Say Sequence" in JavaScript. See: https://www.youtube.com/watch?v=ea7lJkEhytA

look-and-say.js

function lookAndSay(num) {
	var look = /(1+|2+|3+)/g;
	function say(match, position) {
		return match.length.toString() + match.substring(0, 1);
	}
	
	return num.toString().replace(look, say);
}


// Usage
var num = 1;
for (var i:int = 0; i < 20; i++) {
    num = lookAndSay(num);
    console.log(num);
}

The script assumes the following:

No digits other than 1, 2, and 3 appear in the sequence, unless the seed number contains such a digit or a run of more than three of the same digit.
-- From Wikipedia: Look and Say Sequence

but can easily be expanded to allow for "non-standard" seeds.