Asynchronous replace() for JavaScript

asyncReplace.js

String.prototype.asyncReplace = function (subStr, replacer, callback) {
	var str = this;
	var replacements = {};
	var pending = 0;
	var checkDone = function () {
		if (!--pending) {
			var result = str.replace(subStr, function (match) {
				var args = Array.prototype.slice.call(arguments, 0);
				var pos = args[args.length - 2];
				var key = pos + '-' + match.length;
				return replacements[key];
			});
			callback(null, result);
		}
	};
	str.replace(subStr, function (match) {
		pending++;
		var args = Array.prototype.slice.call(arguments, 0);
		var pos = args[args.length - 2];
		var key = pos + '-' + match.length;
		args.push(function (error, result) {
			if (error) {
				pending = Infinity;
				replacer = function () {};
				return callback(error);
			}
			replacements[key] = result;
			checkDone();
		});
		replacer.apply(null, args);
	});
	if (!pending) {
		setTimeout(function () {
			callback(null, str);
		}, 10);
	}
	return this;
};