query string to javascript object

qs.js

/* globals console */
'use strict';

function parseQS(str) {
  var strArr = String(str)
    .replace(/^&/, '')
    .replace(/&$/, '')
    .split('&');
  var sal = strArr.length;
  var i;
  var j;
  var ct;
  var p;
  var lastObj;
  var lastIter;
  var undef;
  var chr;
  var tmp;
  var key;
  var value;
  var postLeftBracketPos;
  var keys;
  var keysLen;
  var obj;
  var arr = {};

  function fixStr(str) {
    return decodeURIComponent(str.replace(/\+/g, '%20'));
  }

  function isInt(n) {
    return Number(n) === n && n % 1 === 0;
  }

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

var j1 = parseQS('a=123&b[0][c]=c1&b[1][d]=d1');
var j2 = parseQS('a=123&b%5B0%5D%5Bc%5D=c1&b%5B1%5D%5Bd%5D=d1');

console.log(j1); // { a: '123', b: [ { c: 'c1' }, { d: 'd1' } ] }
console.log(j2); // { a: '123', b: [ { c: 'c1' }, { d: 'd1' } ] }

console.log(j1.b[0].c); // c1
console.log(j2.b[1].d); // d1