Now support up to 999 trillion. (fixed a flooring error with ~~ and removed unneeded console.log)

in_words.js

// Converts a number to english words
// Based on https://gist.github.com/bcamarda/3001102
// Refactored and made some changes to support numbers to a hundred (US) trillion
// github.com/zz85


var inWords = (function() {

	var numberHash = {
		0:"", 1:"one", 2:"two", 3:"three", 4: "four", 5: "five", 6: "six", 7: "seven", 8: "eight", 9: "nine", 10: "ten",
		11: "eleven", 12: "twelve", 13: "thirteen", 14: "fourteen", 15: "fifteen", 16: "sixteen", 17: "seventeen", 18: "eighteen",
		19: "nineteen", 20: "twenty", 30: "thirty", 40: "forty", 50: "fifty", 60: "sixty", 70: "seventy", 80: "eighty", 90: "ninety"
	};

	var groups = ['' ,'thousand', 'million', 'billion', 'trillion'];
	var values = [10e-1, 10e2, 10e5, 10e8, 10e11];

	var convertUnder100 = function(n) {
		if (n in numberHash) return numberHash[n];
		return numberHash[~~(n / 10) * 10] + " " + numberHash[n % 10];
	}

	var convertUnder1000 = function(n) {
		if (n >= 100) return numberHash[~~(n / 100)] + " hundred " + convertUnder100(n % 100).trim();
		return convertUnder100(n % 100).trim();
	}

	return function(n){
		if (n == 0) return 'zero';
		var word = '';
		var i = groups.length, v;
		while (i--) {
			v = values[i];
			if (n >= v) word += convertUnder1000(~~(n / v % 1000)).trim() + " " + groups[i] + " ";
		}
		return word.trim();
	};

})();

// Tests

function test(number) {
	console.log(number, inWords(number));
}

[0, 1, 10, 11, 15, 20, 42, 50, 99, 100, 101, 111, 121, 200, 999,	
1000, 1001, 1010, 1100, 1234, 123456, 123456789, 123456789123, 123123123123123123,
999999999999999].forEach(test);