Reduce used for undelimit

Old undelimit without reduce

String.prototype.unDelimit = function(delimeters) {
	delimeters = delimeters || ["&", "="];
	if (delimeters.length === 1)
		return this.split(delimeters[0]);
	var items = this.split(delimeters[0]),
		o = {},
		i = 0,
		j = items.length,
		subItem;
	for (i; i<j; i++) {
		subItem = items[i].split(delimeters[1]);
		subItem[1] = subItem[1] || "";
		if (o[subItem[0]]) {
			if (isArray(o[subItem[0]])) {
				o[subItem[0]].push(subItem[1]);
			} else {
				o[subItem[0]] = [o[subItem[0]], subItem[1]];
			}
		} else {
			o[subItem[0]] = subItem[1];
		}
	}
	return o;
}

"a=1&b=2&b=3".unDelimit(['&', '=']);

undelimit.js

function undelimit(string, delimiter1, delimiter2) {
	var parts = string.split(delimiter1);
	return parts.reduce(function(accumulator, currentValue, index, array) {
		var parts = currentValue.split(delimiter2); // [a, 1]
		var key = parts[0];
		var value = parts[1];

		if(accumulator[key]) {
			var accumulatorValue = accumulator[key];
			if(typeof accumulatorValue == 'string') {
				accumulator[key] = [accumulatorValue];
			}
			accumulator[key].push(value);
		} else {
			accumulator[key] = value;
		}
		return accumulator;
	}, {});
}

undelimit("a=1&b=2&b=3", '&', '=');