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Get nth element of a key in an object

I'd like to return the third node (hello3.com) of the key hello.com in javascript object.
nodes = {
"hello.com":
{
id:"hello1.com",
id2:"hello2.com",
id3:"hello3.com"
}
}
I know that I can fetch all the key/values like this:
newobject = nodes["hello.com"];
but how would I get the third. I'm aware that you can't count on the order in an object. If not, can I pull just the third by maybeb id3.

You answered your own question when you said that you can't count on the properties of an object to be in any certain order. If your properties are sequential in nature (your properties were counting up in your example), then I would suggest trying to use an Array.
nodes = {
"hello.com": [
"hello1.com",
"hello2.com",
"hello3.com"
]
};
In the above example, you would access the 3rd property with
nodes["hello.com"][2]
The double bracket notation is because "hello.com" is in quotes to allow a . in the name. If the key didn't require quotes, like helloCom as an example, you could use
nodes.helloCom[2]
Beyond this, if you name your keys sequentially, then you can impose an "order". It's not that any property is literally before or after another, but rather that you have informed yourself of what order YOU intend them to be.

You can try this,
nodes = {
"hello.com": {
id: "hello1.com",
id2: "hello2.com",
id3: "hello3.com"
}
}
console.log(nodes["hello.com"]["id3"]);

Use:
nodes['hello.com'].id3 or nodes['hello.com']['id3']
Both are corrent way to get id3 data from given object

BY INDEX :
About accessing by index, you can not achieve it directly. the closest you can get is array of keys but that also do not guarantee the order of keys returned. See this answer provided on other thread.
for (var i in nodes["hello.com"]) { console.log(i);//logs id,id2,id3 };
BY NODENAME:
nodes["hello.com"] returns object. You can use key to access the value by
1) using dot notation:
nodes["hello.com"].id3
2) or by bracket notation
nodes["hello.com"]["id3"]

Try one of the following expressions
nodes["hello.com"]["id3"]
or
nodes["hello.com"].id3

Accessing object in array using string

I have this code:
var dictionary=[
apple={name:"apple",pos:"noun",d:"",c:["fruit"]},
banana={name:"banana",pos:"noun",d:"",c:["fruit"]}
];
How could I access one of the objects in the array by using a string of its name. In the way that you could access an object as
object['propertyName']
is there a way to do something similar with an array? I want to access it in a way like
dictionary["apple"].pos
//Want to return "noun"
Is there a simple way to do something like that with an array, and if not is there an alternative method that I could use?

The way you're generating your dictionary s wrong; it's syntactically valid, but it's almost certainly not what you intended to do. It's not binding the key apple to that object. Rather, it's defining an implicit (global) variable named apple and assigning the object to that, as well as the first element of the array.
Try this:
var dictionary= {
apple: {name:"apple",pos:"noun",d:"",c:["fruit"]},
banana: {name:"banana",pos:"noun",d:"",c:["fruit"]}
};
console.log(dictionary["apple"].pos); // "noun"
// This also works:
console.log(dictionary.apple.pos); // "noun"

How to access fields in JSON object by index

I know this isn't the best way to do it, but I have no other choice :(
I have to access the items in JSONObject by their index. The standard way to access objects is to just wirte this[objectName] or this.objectName. I also found a method to get all the fields inside a json object:
(for (var key in p) {
if (p.hasOwnProperty(key)) {
alert(key + " -> " + p[key]);
}
}
(Soruce : Loop through Json object).
However there is no way of accessing the JSONfields directly by a index. The only way I see right now, is to create an array, with the function above, get the fieldname by index and then get the value by fieldname.
As far as I see it, the p (in our case the JSON file must be an iteratable array to, or else the foreach loop wouldn't work. How can I access this array directly? Or is it some kind of unsorted list?

A JSON Object is more like a key-value-map; so, yes, it is unsorted. The only way to get around is the index->property name map you've already mentioned:
var keysbyindex = Object.keys(object);
for (var i=0; i<keysbyindex.length; i++)
alert(object[keysbyindex[i]]);
But why would you need these indexes? A unsorted map also has no length property, as an Array had. Why don't you use the for-in-loop
var counter = 0; // if you need it
for (var key in object) {
alert(object[key])
counter++;
}
? If you have a parsed JSON object, i.e. a plain JS Object, you won't have to worry about enumerable prototype properties.

Based on Bergis anserwer this is my solution:
var keysbyindex = Object.keys(this);
alert(this[keysbyindex[index]]);
return this[keysbyindex[index] || ""];
However, I think (not tested) it's extremly bad regaring performace and shouldn't be used! But desperate times require desperate measures.....

I don't think you can actually achieve this without creating your own parsing of JSON. You're writing that you want to go trough a JSON-object, but what you're actually trying to do is go trough a plain old Javascript object. Json is simply a string-representation used to transfer/store said object, and in here lies the main problem: the parser that transforms the string into an actual object (ie. the browser in most cases) can chose to ignore the order it finds the properties if it want to. Also, different browsers might have different approaches to parsing JSON for all you know. If they simply use a hash-map for the object that it's simple to loop through it, but the order won't be dependent on the order of the keys in the file, but rather the keys themselves.
For example, if you have the json {"b":"b","a":"a"} and do the for in loop, under some implementations you might end up with a comming first, and in others you might end up with b.

var jsn = {keyName: 'key value result come here...'};
var arr = jsn ? $.map(jsn, function (el) { return el }) : [0];
console.log(arr[0])
$('.result').text(arr[0]);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<span class="result"></span>

How do I add JSON object as new level to another JSON object?

I have a code that gets in the end collection of two JSON objects, something like this.
var jsonL1 = {"holder1": {}}
var jsonL2 = {"section":"0 6","date":"11/12/13"}
I would like to insert jsonL2 inside jsonL1.holder1 and merge it to one JSON object.
Desired output
{
"holder1": {
"section": "0 6",
"date": "11/12/13"
}
}
How can I do that?

It is as easy as:
L1.holder1 = L2
I removed the "json" from the variable names, as #patrick already said, you are dealing not with "JSON objects" but with object literals.
See also: There is no such thing as a JSON object
You also might want to learn more about objects in JavaScript.

If you want the first object to reference the second, do this:
jsonL1.holder1 = jsonL2;
If you wanted a copy of the second in the first, that's different.
So it depends on what you mean by merge it into one object. Using the code above, changes to jsonL2 will be visible in jsonL1.holder, because they're really just both referencing the same object.
A little off topic, but to give a more visual description of the difference between JSON data and javascript object:
// this is a javascript object
var obj = {"section":"0 6","date":"11/12/13"};
// this is valid JSON data
var jsn = '{"section":"0 6","date":"11/12/13"}';

How to use JSON to re-build the Javascript Object?

I have an object like this:
var someObj = Class.create ({
initialize: function(objName){
this.objName = objName;
}
});
I can use
o = new someObj("objName");
to make an obj. I can use Object.toJSON(o) to change the o to become a JSON String,
but I want the JSON String convert back to someObj, so, I use eval() to pass the
JSON String to become an object, but the question is, it can become a JS Obj,
but the constructor of "o" is not someObj. How can I eval the JSON String by using
"someObj" as the constructor?

JSON strings cannot represent objects with member functions, so the only thing you will get out of a JSON string is raw data. Assuming the toJSON method results in a JSON string representing an object with all the non-function members of your class instance, you should be able to take the resulting object and attach the prototype to get all the functions back. For example, using jQuery's handy extend function:
var o = new someObj("objName");
var json = Object.toJSON(o);
var json_obj = eval(json);
$.extend(json_obj, someObj.prototype);
json_obj.someMethodDefinedOnsomeObj()
Depending on how the framework you are using to represent classes in JavaScript makes use of the prototypal object model, your milage may very with the above example. Also, using eval() creates a security hole, so if you do not trust where the JSON string is coming from, you should use a different de-serialization method. Just for full coverage, here is how I did it with raw prototypes:
function Animal(name){
this.name = name;
}
Animal.prototype.talk = function(){
console.log("My name is "+this.name);
}
var a = new Animal("Brendan Eich");
a.talk();
var json = '{name: "Tim Berners-Lee"}'
var b = eval(b);
$.extend(b, Animal.prototype);
b.talk();
In a firebug console this produces the output:
My name is Brendan Eich
My name is Tim Berners-Lee

See JSON revivers at http://json.org/js.html
var myObject = JSON.parse(myJSONtext, reviver);
The optional reviver parameter is a
function that will be called for every
key and value at every level of the
final result. Each value will be
replaced by the result of the reviver
function. This can be used to reform
generic objects into instances of
pseudoclasses, or to transform date
strings into Date objects.

you're using prototypejs right? i've always found that tricky and ended up just making my own initializer that read in an object that had been evaled or the json string itself.