How can I concatenation parameter name out of 2 strings?

Question

I have the following variable:

var MyVar = "8";

I have 2 strings for example:

var foo = "My";
var bar = "Var";

Now I want to alert MyVar value, meaning to alert "8" and not "MyVar"

alert(foo + bar) // returns "MyVar"

`alert(eval(foo + bar))`. But beware, this method is dangerous. For instance, imagine that either variable does not contain a part of a variable identifier, but evil code. — Rob W, Mar 18 '13 at 15:57
`eval` is only dangerous if the page is dynamic server side, and users have a way of interacting with the content (forums, comments, etc.) because then XSS is a concern. Users always can run whatever code they wish on a site, regardless of `eval` being present or not. The only time `eval` would be dangerous is when user's content is put into a function that calls `eval` within the function, so that other users would run the malicious script when the loaded the hackers post or comment or whatever. — howderek, Mar 18 '13 at 16:12

score 5 · Answer 1 · answered Mar 18 '13 at 15:57

5

This is a rare case where eval will be needed:

Code

var MyVar = "8",
    foo = "My",
    bar = "Var";
alert(eval(foo + bar))

Link: http://jsfiddle.net/howderek/wcVNU/

answered Mar 18 '13 at 15:57

howderek

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Why is this downvoted? It's the only correct answer (matching the OPs question) on this page – Rob W Mar 18 '13 at 15:58
+1. I clearly misunderstood the question whereas you did not. :) – David L Mar 18 '13 at 15:59
3

Okay, I removed the downvote. My point is, if you need "dynamic variables" and eval, you're probably doing it wrong. An object is the way to go in most cases. – bfavaretto Mar 18 '13 at 16:05
+1. You can even eval in a variable : `var value = eval(foo + bar);` – Florian F. Mar 18 '13 at 16:05
@bfavaretto I agree, I can't see why the OP would need to do what he is doing, but `eval` is the way to do it. – howderek Mar 18 '13 at 16:07

score 4 · Answer 2 · answered Mar 18 '13 at 15:58

4

Assuming it's a global variable:

alert(window[foo + bar])

But you're probably better off using objects and properties for that. Object properties can also be accessed with bracket notation:

var obj = {
    MyProp : 8
};
var foo = "My";
var bar = "Prop";
alert(obj[foo + bar]);

answered Mar 18 '13 at 15:58

bfavaretto

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2

Now, what if it's not a global variable? – Rob W Mar 18 '13 at 15:59
1

What if it is a mix of global and non-global? – howderek Mar 18 '13 at 16:01
1

Hmmm, why am I getting undefined when I try this? – j08691 Mar 18 '13 at 16:02

travis · Accepted Answer · 2013-03-18T16:43:37.730

1

Without changing the context to much of what you are doing you can use the eval function. However, you have to be very careful with it.

var MyVar = 8;

var foo = "My";
var bar = "Var";

alert(eval(foo + bar));

Depending on what your doing though there are a lot of ways to do this. If you assign MyVar to be part of some context such as this, or window you can simply lookup the value with the key as the variable name.

Window Context

(function () {
  window.MyVar = 8;

  var foo = "My";
  var bar = "Var";

  alert(window[foo+bar]);
})();

Function Context

new (function () {
  this.MyVar = 8;

  var foo = "My";
  var bar = "Var";

  alert(this[foo+bar]);
})();

Object Context

(function () {
  var obj = {}
  obj.MyVar = 8;

  var foo = "My";
  var bar = "Var";

  alert(obj[foo+bar]);
})();

edited Mar 18 '13 at 16:43

answered Mar 18 '13 at 16:02

travis

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Your function context is actually global. – bfavaretto Mar 18 '13 at 16:09
1

You'r right, care to elaborate as I'm not sure why. Do anonymous functions take on global context? – travis Mar 18 '13 at 16:19
2

Inside a function, `this` will be the global object unless (a) the function is called as an object method (then `this` will be the object), or (b) the function is called as a constructor, with the `new` operator (in which case `this` will point to the new object being constructed). https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/this – bfavaretto Mar 18 '13 at 16:37

How can I concatenation parameter name out of 2 strings?

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