Modified version of parse_str() from phpjs - http://phpjs.org/functions/parse_str/ - Source: https://github.com/kvz/phpjs/blob/master/functions/strings/parse_str.js

parse_str.js

function parse_str(str, array) {
	//       discuss at: http://phpjs.org/functions/parse_str/
	//      original by: Cagri Ekin
	//      improved by: Michael White (http://getsprink.com)
	//      improved by: Jack
	//      improved by: Brett Zamir (http://brett-zamir.me)
	//      bugfixed by: Onno Marsman
	//      bugfixed by: Brett Zamir (http://brett-zamir.me)
	//      bugfixed by: stag019
	//      bugfixed by: Brett Zamir (http://brett-zamir.me)
	//      bugfixed by: MIO_KODUKI (http://mio-koduki.blogspot.com/)
	// reimplemented by: stag019
	//         input by: Dreamer
	//         input by: Zaide (http://zaidesthings.com/)
	//         input by: David Pesta (http://davidpesta.com/)
	//         input by: jeicquest
	//      modified by: 2015-04-13 corey gilmore
	//                 -  indent using tabs (personal preference)
	//                 -  always return a parsed object, never put variables in the global scope
	//                 -  strip leading ? and & from the passed string
	// --------------------------------------------------
	//             note: When a particular argument has been passed, and the returned value is different parse_str of PHP. For example, a=b=c&d====c
	//             test: skip
	//        example 1: var arr = {};
	//        example 1: parse_str('first=foo&second=bar', arr);
	//        example 1: $result = arr
	//        returns 1: { first: 'foo', second: 'bar' }
	//        example 2: var arr = {};
	//        example 2: parse_str('str_a=Jack+and+Jill+didn%27t+see+the+well.', arr);
	//        example 2: $result = arr
	//        returns 2: { str_a: "Jack and Jill didn't see the well." }
	//        example 3: var abc = {3:'a'};
	//        example 3: parse_str('abc[a][b]["c"]=def&abc[q]=t+5');
	//        returns 3: {"3":"a","a":{"b":{"c":"def"}},"q":"t 5"}

	var strArr = String(str)
		.replace(/^[?&]/, '')
		.replace(/&$/, '')
		.split('&'),
		sal = strArr.length,
		i, j, ct, p, lastObj, obj, lastIter, undef, chr, tmp, key, value,
		postLeftBracketPos, keys, keysLen,
		fixStr = function (str) {
			return decodeURIComponent(str.replace(/\+/g, '%20'));
		};

	if (!array) {
		array = {};
	}

	for (i = 0; i < sal; i++) {
		tmp = strArr[i].split('=');
		key = fixStr(tmp[0]);
		value = (tmp.length < 2) ? '' : fixStr(tmp[1]);

		while (key.charAt(0) === ' ') {
			key = key.slice(1);
		}
		if (key.indexOf('\x00') > -1) {
			key = key.slice(0, key.indexOf('\x00'));
		}
		if (key && key.charAt(0) !== '[') {
			keys = [];
			postLeftBracketPos = 0;
			for (j = 0; j < key.length; j++) {
				if (key.charAt(j) === '[' && !postLeftBracketPos) {
					postLeftBracketPos = j + 1;
				} else if (key.charAt(j) === ']') {
					if (postLeftBracketPos) {
						if (!keys.length) {
							keys.push(key.slice(0, postLeftBracketPos - 1));
						}
						keys.push(key.substr(postLeftBracketPos, j - postLeftBracketPos));
						postLeftBracketPos = 0;
						if (key.charAt(j + 1) !== '[') {
							break;
						}
					}
				}
			}
			if (!keys.length) {
				keys = [key];
			}
			for (j = 0; j < keys[0].length; j++) {
				chr = keys[0].charAt(j);
				if (chr === ' ' || chr === '.' || chr === '[') {
					keys[0] = keys[0].substr(0, j) + '_' + keys[0].substr(j + 1);
				}
				if (chr === '[') {
					break;
				}
			}

			obj = array;
			for (j = 0, keysLen = keys.length; j < keysLen; j++) {
				key = keys[j].replace(/^['"]/, '')
					.replace(/['"]$/, '');
				lastIter = j !== keys.length - 1;
				lastObj = obj;
				if ((key !== '' && key !== ' ') || j === 0) {
					if (obj[key] === undef) {
						obj[key] = {};
					}
					obj = obj[key];
				} else {
					// To insert new dimension
					ct = -1;
					for (p in obj) {
						if (obj.hasOwnProperty(p)) {
							if (+p > ct && p.match(/^\d+$/g)) {
								ct = +p;
							}
						}
					}
					key = ct + 1;
				}
			}
			lastObj[key] = value;
		}
	}
	return array;
}