Generates a unique string based on the timestamp. Scales across multiple instances if `prepend` argument is set to a unique value. Both produce a similar value, one in base 62 and the other in base 16. Preference comes down to speed and size.

uniqueId_base62.js

// Slower, larger source, produces a 9 character string
var uniqueId = (function() {
	var ALPHA_NUMERIC = ["0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z"];
	
	function toAlphaNumeric(n) {
		var len = ALPHA_NUMERIC.length,
			digits = "";
		
		while (n !== 0) {
			var r = n % len;
			n = (n - r) / len;
			digits = ALPHA_NUMERIC[r] + digits;
		}
	
		return digits;
	}
	
	var lastTimestamp = null,
		counter = 0;

	return function(prepend) {
		if (prepend == null) prepend = "";
		
		var ts = toAlphaNumeric(Date.now());
		if (lastTimestamp !== ts) {
			counter = 0;
			lastTimestamp = ts;
		}
		
		counter++;
		var cnt = toAlphaNumeric(counter);
		while (cnt.length < 2) { cnt = 0 + cnt; }

		return prepend + ts + cnt;
	}
})();

uniqueId_hex.js

// Faster, smaller source, produces a 14 character string
var uniqueId = (function() {
	var lastTimestamp = null,
		counter = 0;

	return function(prepend) {
		if (prepend == null) prepend = "";
		
		var ts = Date.now().toString(16);
		if (lastTimestamp !== ts) {
			counter = 0;
			lastTimestamp = ts;
		}
		
		counter++;
		var cnt = counter.toString(16);
		while (cnt.length < 3) { cnt = 0 + cnt; }

		return prepend + ts + cnt;
	}
})();