The $.param( ) inverse function in JavaScript / jQuery

Question

You should use jQuery BBQ's deparam function. It's well-tested and documented.

This is a slightly modified version of a function I wrote a while ago to do something similar.

var QueryStringToHash = function QueryStringToHash  (query) {
  var query_string = {};
  var vars = query.split("&");
  for (var i=0;i<vars.length;i++) {
    var pair = vars[i].split("=");
    pair[0] = decodeURIComponent(pair[0]);
    pair[1] = decodeURIComponent(pair[1]);
        // If first entry with this name
    if (typeof query_string[pair[0]] === "undefined") {
      query_string[pair[0]] = pair[1];
        // If second entry with this name
    } else if (typeof query_string[pair[0]] === "string") {
      var arr = [ query_string[pair[0]], pair[1] ];
      query_string[pair[0]] = arr;
        // If third or later entry with this name
    } else {
      query_string[pair[0]].push(pair[1]);
    }
  } 
  return query_string;
};

How about this short functional approach?

function parseParams(str) {
    return str.split('&').reduce(function (params, param) {
        var paramSplit = param.split('=').map(function (value) {
            return decodeURIComponent(value.replace(/\+/g, ' '));
        });
        params[paramSplit[0]] = paramSplit[1];
        return params;
    }, {});
}

Example:

parseParams("this=is&just=an&example") // Object {this: "is", just: "an", example: undefined}

My answer:

function(query){
  var setValue = function(root, path, value){
    if(path.length > 1){
      var dir = path.shift();
      if( typeof root[dir] == 'undefined' ){
        root[dir] = path[0] == '' ? [] : {};
      }

      arguments.callee(root[dir], path, value);
    }else{
      if( root instanceof Array ){
        root.push(value);
      }else{
        root[path] = value;
      }
    }
  };
  var nvp = query.split('&');
  var data = {};
  for( var i = 0 ; i < nvp.length ; i++ ){
    var pair = nvp[i].split('=');
    var name = decodeURIComponent(pair[0]);
    var value = decodeURIComponent(pair[1]);

    var path = name.match(/(^[^\[]+)(\[.*\]$)?/);
    var first = path[1];
    if(path[2]){
      //case of 'array[level1]' || 'array[level1][level2]'
      path = path[2].match(/(?=\[(.*)\]$)/)[1].split('][')
    }else{
      //case of 'name'
      path = [];
    }
    path.unshift(first);

    setValue(data, path, value);
  }
  return data;
}

I am using David Dorward's answer, and realized that it doesn't behave like PHP or Ruby on Rails how they parse the params:

1) a variable is only an array if it ends with [], such as ?choice[]=1&choice[]=12, not when it is ?a=1&a=2

2) when mulitple params exist with the same name, the later ones replaces the earlier ones, as on PHP servers (Ruby on Rails keep the first one and ignore the later ones), such as ?a=1&b=2&a=3

So modifying David's version, I have:

function QueryStringToHash(query) {

  if (query == '') return null;

  var hash = {};

  var vars = query.split("&");

  for (var i = 0; i < vars.length; i++) {
    var pair = vars[i].split("=");
    var k = decodeURIComponent(pair[0]);
    var v = decodeURIComponent(pair[1]);

    // If it is the first entry with this name
    if (typeof hash[k] === "undefined") {

      if (k.substr(k.length-2) != '[]')  // not end with []. cannot use negative index as IE doesn't understand it
        hash[k] = v;
      else
        hash[k.substr(0, k.length-2)] = [v];

    // If subsequent entry with this name and not array
    } else if (typeof hash[k] === "string") {
      hash[k] = v;  // replace it

    // If subsequent entry with this name and is array
    } else {
      hash[k.substr(0, k.length-2)].push(v);
    }
  } 
  return hash;
};

which is tested fairly thoroughly.