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Convert string value into variable value - JavaScript

I have a string variable: id = '1';
I want to make a variable that holds the value of data.img + id + .dispWidth without making it a string.
For example, if id = '7'; then dw = data.img7.dispWidth;
How can I do this?

Use bracket notation to use dynamic property keys
dw = data['img' + id].dispWidth;

data['img' + id].dispWidth should do it. Although at that point you should probably consider whether you don't actually want an array.

creating variable names from parameters in javascript

I was wondering if it is possible to create variable names from parameters passed to a function in javascript. Something like this:
function createVar(number) {
var "number" + number;
}
createVar(1)
I'm new to Stack Overflow and programming, so any help would be appreciated.

You could attach this to the window object, but note it will be global. For example:
function createVar(varName, value) {
window[varName] = value;
}
createVar("test", "Hello World");
alert(test); // Outputs "Hello World".

It is possible to interpret Object as associative array where you specify index and get value by name of index ( hash ):
var x = Array();
x[number] = value;
Single variable name is for programmer, and the code would be hard to maintain and understand when you set variable dynamically in code.

Honestly, I don't see why this would ever be useful, because every time you want to use the variable you'd have to search for it with your number argument.
However, you can do it, albeit not the exact way you had described:
function createVar(number){
eval("var number" + number.toString() + ";");
}
however, this variable will only be accessible within the function, to make it global assign to the window object:
function createVar(number){
window["number" + number] = 15; // creates "global" variable
}
As I've stated before, however, I don't see this being useful, [i]ever[/i], if you want to stratify values by numbers you'd be much better off with an array.

Accessing an array by index within an object

I'm trying to access a particular array by it's index within an object within an object (sorry, this may be the wrong terminology).
var person = {
name: ["Tom", "Mike", "Sally"],
hair: {
style: ["bob", "weave", "mullet"],
length: ["long","short","medium"]
}
}
getDetail(length);
function getDetail(det) {
var answer = person.hair.det[1];
console.log("Hair " + det + " is " + answer) //outputs: "Hair length is long"
}
When I do this, I am getting an error of "Can't read property '1' of undefined". Which tells me it isn't passing the 'det' variable correctly. If I take that out and put length instead, it works.
What am I missing?
Thanks!

The problem is that in your case you should be passing a string or a variable to your getDetail() function (length by itself is none, as it is not defined previously, nor is quoted), also there's the fact that if you want to use a variable to indicate the property/key of an object, you should use this type of syntax: object["property"]. You can change your code to:
getDetail('length');
function getDetail(det) {
var answer = person.hair[det][1];
console.log("Hair " + det + " is " + answer) //outputs: "Hair length is long"
}

When you say getDetail(length), that's looking for a variable called length, and passing it into the function. Since it doesn't exist, the function gets undefined.
HOWEVER, when you write "myObjectVariableName.etcetera", it will specifically look for an index called "etcetera" - so it won't even be using your det argument - it's instead looking for the "det" index.
You can, instead, write either of the following.
myObjectVariableName["myIndexName"]
var myString = "myIndexName";
myObjectVariableName[myString]
Both of those will both have the same result.
So, to get the expected result, what you want to do is pass the string "length" into getDetail, and then replace .det with [det] (darkajax seems to have written what I was going for, but I'm hoping my explanation also helps make sense of why it wasn't working)

Dynamic Javascript - Is This Valid?

can someone tell me if this is valid javascript? I know you couldnt do this sort of thing in c# but js is a much looser language..
var arrayToUse = "arr" + sender.value;
for (i = 0; i <= arrayToUse.length; i++) {
// something..
}
specifically - the dynamic generation of the array name..
update..
so i have an array called arrMyArray which is initialised on document ready. sender.value = "MyArray" - but could be something else eg MyArray2
I want to dyanimcally iterate over the array that is indicated by the sender.value value.

Yes, this is entirely valid.
arrayToUse will be a string (regardless of the value of sender.value — it will be converted to a string), and i will iterate from 0 to the string's length).
One minor note: it should be for (**var** i = 0; …), otherwise i will be treated as a global variable, which will almost certainly end badly if you've got multiple loops running at the same time.
Edit: you want to get the array based on the name? In that case you've got to look it up in whatever context the array is defined.
If it's a global array, use window.
For example:
var arrayName = "arr" + sender.value;
var array = window[arrayName];
…

To get a variable name defined by a variable, you need to use eval, like so:
var arrayToUse = eval("arr" + sender.value);
However, you must be very careful with this, because controlling sender.value would allow someone to hijack your entire application this way. You should usually try to find another solution.
If the variable is defined at the globally, you can look it up as window["arr" + sender.value] instead. This is still not ideal, but is less of a security risk.

What you need to do is access a variable with the name "arr" + sender.value. Accessing the variable whose contents are "arr + sender.value doesn't do what you want -- that's just a string.
To access the variable with that name, you can look it up as a global (globals are members of the window object in the browser):
window["arr" + sender.value]
This is safer and faster than using eval() because it doesn't run code in a JavaScript execution context to evaluate the string -- it just looks up a variable in the window object with that name.

constructing javascript variable names at runtime

someFunction(link) {
someOtherFunction('div' + link);
}
By calling someFunction("Test"), the string "divTest" gets passed to someOtherFunction(). But I want the value of the variable "divTest" to be passed.
How can that be done?

Make your variables members of an object. Then you can use [] to access the objects members using a string:
var byname = {
divabc: ...,
divxyz: ...
};
function someFunction(link) {
someOtherFunction(byname['div'+link]);
}
someFunction('abc'); // calls someOtherFunction(byname.divabc)

For this kind of dynamic construction/access of variable names you should use the alternative object notation where:
object.member === object["member"]
This way you could construct your variable name as a string and use it inside square brackets for accessing object members.

eval will do this, but it's usually indicative of some other problem with the program when you want to synthesize identifiers like this. As Ionut says it's better to use the [] notation. I like to link to this whenever questions like this come up.

You should be able to accomplish this with the 'eval' function.

Try this:
var divFoo = "bar";
function someFunction(link) {
someOtherFunction(this['div' + link]);
}
function someOtherFunction(value) {
alert(value);
}
someFunction("Foo");

As wybiral said, all you need is eval:
someFunction(link) {
someOtherFunction(eval('(div' + link + ')');
}
Basically what it does is evaluates the contents of a string as code. Obviously eval is a dangerous little tool since it allows executing arbitrary code so take care when using it.