index.js

/**
 * Object#toString() ref for stringify().
 */

var toString = Object.prototype.toString;

/**
 * Array#indexOf shim.
 */

var indexOf = typeof Array.prototype.indexOf === 'function'
  ? function(arr, el) { return arr.indexOf(el); }
  : function(arr, el) {
      for (var i = 0; i < arr.length; i++) {
        if (arr[i] === el) return i;
      }
      return -1;
    };

/**
 * Array.isArray shim.
 */

var isArray = Array.isArray || function(arr) {
  return toString.call(arr) == '[object Array]';
};

/**
 * Object.keys shim.
 */

var objectKeys = Object.keys || function(obj) {
  var ret = [];
  for (var key in obj) ret.push(key);
  return ret;
};

/**
 * Array#forEach shim.
 */

var forEach = typeof Array.prototype.forEach === 'function'
  ? function(arr, fn) { return arr.forEach(fn); }
  : function(arr, fn) {
      for (var i = 0; i < arr.length; i++) fn(arr[i]);
    };

/**
 * Array#reduce shim.
 */

var reduce = function(arr, fn, initial) {
  if (typeof arr.reduce === 'function') return arr.reduce(fn, initial);
  var res = initial;
  for (var i = 0; i < arr.length; i++) res = fn(res, arr[i]);
  return res;
};

/**
 * Cache non-integer test regexp.
 */

var isint = /^[0-9]+$/;

function promote(parent, key) {
  if (parent[key].length == 0) return parent[key] = {};
  var t = {};
  for (var i in parent[key]) t[i] = parent[key][i];
  parent[key] = t;
  return t;
}

function parse(parts, parent, key, val) {
  var part = parts.shift();
  // end
  if (!part) {
    if (isArray(parent[key])) {
      parent[key].push(val);
    } else if ('object' == typeof parent[key]) {
      parent[key] = val;
    } else if ('undefined' == typeof parent[key]) {
      parent[key] = val;
    } else {
      parent[key] = [parent[key], val];
    }
    // array
  } else {
    var obj = parent[key] = parent[key] || [];
    if (']' == part) {
      if (isArray(obj)) {
        if ('' != val) obj.push(val);
      } else if ('object' == typeof obj) {
        obj[objectKeys(obj).length] = val;
      } else {
        obj = parent[key] = [parent[key], val];
      }
      // prop
    } else if (~indexOf(part, ']')) {
      part = part.substr(0, part.length - 1);
      if (!isint.test(part) && isArray(obj)) obj = promote(parent, key);
      parse(parts, obj, part, val);
      // key
    } else {
      if (!isint.test(part) && isArray(obj)) obj = promote(parent, key);
      parse(parts, obj, part, val);
    }
  }
}

/**
 * Merge parent key/val pair.
 */

function merge(parent, key, val){
  if (~indexOf(key, ']')) {
    var parts = key.split('[')
      , len = parts.length
      , last = len - 1;
    parse(parts, parent, 'base', val);
    // optimize
  } else {
    if (!isint.test(key) && isArray(parent.base)) {
      var t = {};
      for (var k in parent.base) t[k] = parent.base[k];
      parent.base = t;
    }
    set(parent.base, key, val);
  }

  return parent;
}

/**
 * Parse the given obj.
 */

function parseObject(obj){
  var ret = { base: {} };
  forEach(objectKeys(obj), function(name){
    merge(ret, name, obj[name]);
  });
  return ret.base;
}

/**
 * Parse the given str.
 */

function parseString(str){
  return reduce(String(str).split('&'), function(ret, pair){
    var eql = indexOf(pair, '=')
      , brace = lastBraceInKey(pair)
      , key = pair.substr(0, brace || eql)
      , val = pair.substr(brace || eql, pair.length)
      , val = val.substr(indexOf(val, '=') + 1, val.length);

    // ?foo
    if ('' == key) key = pair, val = '';
    if ('' == key) return ret;

    return merge(ret, decode(key), decode(val));
  }, { base: {} }).base;
}

/**
 * Parse the given query `str` or `obj`, returning an object.
 *
 * @param {String} str | {Object} obj
 * @return {Object}
 * @api public
 */

exports.parse = function(str){
  if (null == str || '' == str) return {};
  return 'object' == typeof str
    ? parseObject(str)
    : parseString(str);
};

/**
 * Turn the given `obj` into a query string
 *
 * @param {Object} obj
 * @return {String}
 * @api public
 */

var stringify = exports.stringify = function(obj, prefix) {
  if (isArray(obj)) {
    return stringifyArray(obj, prefix);
  } else if ('[object Object]' == toString.call(obj)) {
    return stringifyObject(obj, prefix);
  } else if ('string' == typeof obj) {
    return stringifyString(obj, prefix);
  } else {
    return prefix + '=' + encodeURIComponent(String(obj));
  }
};

/**
 * Stringify the given `str`.
 *
 * @param {String} str
 * @param {String} prefix
 * @return {String}
 * @api private
 */

function stringifyString(str, prefix) {
  if (!prefix) throw new TypeError('stringify expects an object');
  return prefix + '=' + encodeURIComponent(str);
}

/**
 * Stringify the given `arr`.
 *
 * @param {Array} arr
 * @param {String} prefix
 * @return {String}
 * @api private
 */

function stringifyArray(arr, prefix) {
  var ret = [];
  if (!prefix) throw new TypeError('stringify expects an object');
  for (var i = 0; i < arr.length; i++) {
    ret.push(stringify(arr[i], prefix + '[' + i + ']'));
  }
  return ret.join('&');
}

/**
 * Stringify the given `obj`.
 *
 * @param {Object} obj
 * @param {String} prefix
 * @return {String}
 * @api private
 */

function stringifyObject(obj, prefix) {
  var ret = []
    , keys = objectKeys(obj)
    , key;

  for (var i = 0, len = keys.length; i < len; ++i) {
    key = keys[i];
    if ('' == key) continue;
    if (null == obj[key]) {
      ret.push(encodeURIComponent(key) + '=');
    } else {
      ret.push(stringify(obj[key], prefix
        ? prefix + '[' + encodeURIComponent(key) + ']'
        : encodeURIComponent(key)));
    }
  }

  return ret.join('&');
}

/**
 * Set `obj`'s `key` to `val` respecting
 * the weird and wonderful syntax of a qs,
 * where "foo=bar&foo=baz" becomes an array.
 *
 * @param {Object} obj
 * @param {String} key
 * @param {String} val
 * @api private
 */

function set(obj, key, val) {
  var v = obj[key];
  if (undefined === v) {
    obj[key] = val;
  } else if (isArray(v)) {
    v.push(val);
  } else {
    obj[key] = [v, val];
  }
}

/**
 * Locate last brace in `str` within the key.
 *
 * @param {String} str
 * @return {Number}
 * @api private
 */

function lastBraceInKey(str) {
  var len = str.length
    , brace
    , c;
  for (var i = 0; i < len; ++i) {
    c = str[i];
    if (']' == c) brace = false;
    if ('[' == c) brace = true;
    if ('=' == c && !brace) return i;
  }
}

/**
 * Decode `str`.
 *
 * @param {String} str
 * @return {String}
 * @api private
 */

function decode(str) {
  try {
    return decodeURIComponent(str.replace(/\+/g, ' '));
  } catch (err) {
    return str;
  }
}