11

I've been using both in javascript ... really don't know the difference. Googling always shows results for the "window object" or "opening a new window in javascript" so couldn't find anything there.

 eval("v"+e)
 window["v"+e]

Sometimes window works for me and at other times eval works ....

So what's the difference between eval() and window[] ?

Sorry for the newbie question though !

Norman

Improve this question
5
  • 2
    Could you state that as a question, please? Specifically, what are you really asking? Commented Jul 20, 2010 at 20:55
  • Whatever the answer, don't use eval! Commented Jul 20, 2010 at 21:09
  • I think it's pretty clear that the question is "please explain the situation to me". Commented Jul 20, 2010 at 21:14
  • @ jasongetsdown - small question ... why shouldn't eval be used ? Is there a better way to access local variables like so var c = eval("ev"+d); where d=1 locally (this value is passed to a function) and I want to set the value of c equal to the value of local varable ev1 ? Commented Jul 20, 2010 at 21:53
  • @Norman: the answer to that is, if you're new and using eval, you're probably doing something wrong. Eval does have a purpose, but newcomers probably aren't using it for that. (this is an old question that was recently brought up, so this question is meant for others) Commented Apr 9, 2012 at 4:00

4 Answers 4

Reset to default
15

Another point that has not been addressed is that eval will resolve the variable reference using the caller variable environment, for example:

var foo = "global";

(function () {
  var foo = "local";
  alert(eval("foo")); // alerts "local"
  alert(window["foo"]); // alerts "global"
})();

So as you can see, is not completely equivalent.

If you simply want to reference a global variable, I would recommend you to use the window[prop] approach and avoid surprises.

Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

So in order to access local variables use window otherwise use eval ? ... why do so many people discourage the use of eval? is there another way to access local variables like so eval("ev"+d); where d=1 locally and I want to access the value of local varable ev1 ?
@Norman, to access global variables use window[prop], see this question to know why you shouldn't use eval when you don't need it..
7

eval() interprets arbitrary javascript statements, whereas with window you are accessing a property of the window object.

In your example, you seem to be using a property name in both eval() and window[]. As the global scope in a browser is the same as the window object's scope they will evaluate to the same thing.

You can think of your eval("v"+e) statement as being equivalent to eval("window['v'" + e +" ]").

Improve this answer

2 Comments

Well put. But to make sure its clear - eval is the bad way to do it; the other is the good way to do it.
Yes, eval is almost always a bad idea.
5

Both return a global variable's value. The difference is that if the global variable is undefined, you will get an error on executing eval() whereas window['variableName'] will return undefined(not an error) because accessing an undefined property is not an error but accessing an undefined variable is an error.

Improve this answer

Comments

4

"v"+e -> string

eval(x) -> evaluates the string x, containing javascript expression

window[x] -> returns window's property with the same name, as tha value of x is. this in fact can be a global variable

therefore, when you have a global variable v1 = "foo", and e = 1, then eval("v"+e) and window["v" + e] both return "foo"

Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.