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Setting dynamically an Object in JavaScript

It seems complicated for me.
First, I have this list:
liste_path_categories.push(
{ index: null
, letter: "letter1"
, type: key
, picture_url: "url1"
, id_categ: null
, response: "Answer here"
});
What I want is to extract from this big list an object in this form:
data["String1"]["String2"]= String3
With :
String1=list_path_categories[i].letter
String2=list_path_categories[i].id_categ
String3=list_path_categories[i].response
example:
data['A']['12'] : "A_answer"
To declare the data i make this:
var data = new Object(new Object);
How I can set all the values in data?

You can use the Array.forEach method to iterate through liste_path_categories and construct your data object.
Example:
var liste_path_categories = [];
var data = {};
liste_path_categories.push(...);
...
liste_path_categories.push(...);
liste_path_categories.forEach(function(element) {
data[element.letter] = {};
data[element.letter][element.id_categ] = element.response;
});
jsFiddle example : http://jsfiddle.net/3ZvNf/

Your question is pretty vague but do you mean something like this?
Setting a dynamic property in an object wich belongs to another object?
data['A']['12'].answer = "A_answer"

Instead of using strings, you have to use the variables in your property access:
var data = {};
if (!data[String1]) {
data[String1] = {}; // make sure that data[String1] exists and is an object
}
data[String1][String2] = String3;
If you want to do this for elements in the array, you have to iterate over the array.
P.S.: I recommend to use more expressive variable names than StringX.

first create the constructor (in OOP terminology):
var ctor_object = function(letter,id_categ,response)
{
this.letter = letter;
this.id_cated = id_categ;
this.response = response;
}
(in genereal you should omit the ctor_ syntax and name it directly after the name of the class of your object)
then use your constructor upon your list of categories:
var length = liste_path_categories.length,
element = null;
for (var i = 0; i < length; i++)
{
element = liste_path_categories[i];
my_obj = new ctor_object(element.letter,element.id_categ,element.reponse)
// Do something with my_obj
}

Find specific key value in array of objects

This is the code:
var groups = {
"JSON":{
"ARRAY":[
{"id":"fq432v45","name":"Don't use me."},
{"id":"qb45657s","name":"Use me."}
]
}
}
I want to get the name value where the id is "qb45657s" how could this be accomplished? I figured the obvious loop through all of the array and check if it's equal but is there an easier way?
Edit: I cannot change "Array" to an object because I need to know the length of it for a different function.

You can simply filter on the given id:
groups["JSON"]["ARRAY"].filter(function(v){ return v["id"] == "qb45657s"; });
This will return [{"id":"qb45657s","name":"Use me."}]

Assuming you had a valid JSON string like this (note I say valid, because you need an enclosing {} or [] to make it valid):
var json = '{"JSON":{
"ARRAY":[
{"id":"fq432v45","name":"Don't use me."},
{"id":"qb45657s","name":"Use me."}
]
}
}';
You would just parse it into an actual object like this:
var jsonObj = JSON.parse(json); // makes string in actual object you can work with
var jsonArray = jsonObj.JSON.ARRAY; // gets array you are interested in
And then search for it like:
var needle = 'qb45657s';
var needleName;
for (var i = 0; i < jsonArray.length; i++) {
if (jsonArray[i].id === needle) {
needleName = jsonArray[i].name;
}
}

How to split a URL string with parameters into an array using JavaScript

I'm trying to break up a string like this one:
fname=bill&mname=&lname=jones&addr1=This%20House&...
I want to end up with an array indexed like this
myarray[0][0] = fname
myarray[0][1] = bill
myarray[1][0] = mname
myarray[1][1] =
myarray[2][0] = lname
myarray[2][1] = jones
myarray[3][0] = addr
myarray[3][1] = This House
The url is quite a bit longer than the example. This is what I've tried:
var
fArray = [],
nv = [],
myarray = [];
fArray = fields.split('&');
// split it into fArray[i]['name']="value"
for (i=0; i < fArray.length; i++) {
nv = fArray[i].split('=');
myarray.push(nv[0],nv[1]);
nv.length = 0;
}
The final product is intended to be in 'myarray' and it is, except that I'm getting a one dimensional array instead of a 2 dimensional one.
The next process is intended to search for (for example) 'lname' and returning the index of it, so that if it returned '3' I can then access the actual last name with myarray[3][1].
Does this make sense or am I over complicating things?

Your line myarray.push(nv[0],nv[1]); pushes two elements to the array myarray, not a single cell with two elements as you expect (ref: array.push). What you want is myarray.push( [nv[0],nv[1]] ) (note the brackets), or myarray.push(nv.slice(0, 2)) (ref: array.slice).
To simplify your code, may I suggest using Array.map:
var q = "foo=bar&baz=quux&lorem=ipsum";
// PS. If you're parsing from a-tag nodes, they have a property
// node.search which contains the query string, but note that
// it has a leading ? so you want node.search.substr(1)
var vars = q.split("&").map(function (kv) {
return kv.split("=", 2);
});
For searching, I would suggest using array.filter:
var srchkey = "foo";
var matches = vars.filter(function (v) { return v[0] === srchkey; });
NB. array.filter will always return an array. If you always want just a single value, you could use array.some or a bespoke searching algorithm.

for (var i = 0; i < fArray.length; i++) {
nv = fArray[i].split('=');
myarray.push([nv[0],nv[1]]);
}
nv.length = 0; is not required, since you're setting nv in each iteration of the for loop.
Also, use var i in the for-loop, otherwise, you're using / assigning a global variable i, that's asking for interference.

How to parse JSON data when the property name is not known in advance?

Here is my response code in jQuery:
var response = $.parseJSON(response);
for (var i = 0; i < response.groupIds.length; i++) {
console.log(response.groupIds[i], i);
}
Each response.groupIds[i] is of the form {"unknown name":"unknown value"}.
I wish to access both of these bits of data in javascript, how do I accomplish this when I don't know in advance what e.g. unknown name is?

Use Object.keys to retrieve a full list (array) of key names. A polyfill is available here.
var group = response.groupIds[i];
var allPropertyNames = Object.keys(group);
for (var j=0; j<allPropertyNames.length; j++) {
var name = allPropertyNames[j];
var value = group[name];
// Do something
}
Your question's response format contains only one key-value pair. The code can then be reduced to:
var group = response.groupIds[i];
var name = Object.keys(group)[0]; // Get the first item of the list; = key name
var value = group[name];
If you're not interested in the list, use a for-i-in loop with hasOwnProperty. The last method has to be used, to exclude properties which are inherit from the prototype.
for (var name in group) {
if (group.hasOwnProperty(name)) {
var value = group[name];
// Do something
}
}

Use a for..in loop:
for( x in response.groupIds[i]) {
// x is now your unknown key
// response.groupIds[i][x] is the unknown value
}
Since there is only one property of the object, that'll work nicely.

How to separate the values from two dimension array in js?

jQuery.get("ChkNewRspLive.php?lastmsgID=" + n, function(newitems){
//some code to separate values of 2d array.
$('#div1').append(msgid);
$('#div2').append(rspid);
});
Let's say the value of newitems is [["320","23"],["310","26"]]
I want to assign "320" and "310" to var msgid.
I want to assign "23" and "26" to var rspid.
How to do that?
I tried to display newitems and the output is "Array". I tried to display newitems[0] and the output is blank.
If I redeclare var newitems = [["320","23"],["310","26"]]; it works. So I guess the variable newitems from jQuery.get is something wrong. Is it I cannot pass the array from other page to current page through jQuery directly?
Regarding the array on other page, if echo json_encode($Arraytest); the output is [["320","23"],["310","26"]] but if echo $Arraytest; the output is Array. How do I pass the array from other page to currently page by jQuery.get?

I don't totally understand the question but I'm going to assume you want the values in an array, as two values can't be stored in one (scalar) variable simultaneously.
jQuery.get("ChkNewRspLive.php?lastmsgID=" + n, function(newitems){
//some code to separate values of 2d array.
var msgid = [],
rspid = [];
for( i = 0 ; i < newitems.length ; i++){
msgid[msgid.length] = newitems[i][0];
rspid[rspid.length] = newitems[i][1];
}
//msgid now contains ["320","310"]
//rspid now contains ["23","26"]
});
Bear in mind those are in the function scope. If you want to use them outside of that scope instantiate them outside. see: closure

You can use pluck from underscore.js: http://documentcloud.github.com/underscore/#pluck
var msgid = _(newitems).pluck(0)
var rspid = _(newitems).pluck(1)

Try this:
function getArrayDimension(arr, dim) {
var res = [];
for(var i = 0; i < arr.length; i++) {
res.push(arr[i][dim]);
}
return res;
}
var newitems = [["320","23"],["310","26"]];
var msgid = getArrayDimension(newitems, 0);
var rspid = getArrayDimension(newitems, 1);
msgid and rspid are arrays holding the 'nth' dimention.
Tnx