Difference between window[] and eval() - Javascript

Question

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

Could you state that as a question, please? Specifically, what are you really asking? — Robusto, Jul 20 '10 at 20:55
I think it's pretty clear that the question is "please explain the situation to me". — Grumdrig, Jul 20 '10 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 ? — Norman, Jul 20 '10 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) — vol7ron, Apr 09 '12 at 04:00

score 14 · Accepted Answer · answered Jul 20 '10 at 21:12

14

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.

answered Jul 20 '10 at 21:12

Christian C. Salvadó

807,428
183
922
838

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 Jul 20 '10 at 21:49
1

@Norman, to access global variables use `window[prop]`, see [this question](http://stackoverflow.com/questions/86513/why-is-using-javascript-eval-function-a-bad-idea) to know why you shouldn't use `eval` when you don't need it.. – Christian C. Salvadó Jul 20 '10 at 22:00

score 6 · Answer 2 · answered Jul 20 '10 at 21:01

6

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 +" ]").

answered Jul 20 '10 at 21:01

Paul Grime

14,970
4
36
58

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. – Grumdrig Jul 20 '10 at 21:13
Yes, eval is almost always a bad idea. – Paul Grime Jul 29 '10 at 21:17

score 4 · Answer 3 · answered Jul 20 '10 at 21:07

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.

score 3 · Answer 4 · answered Jul 20 '10 at 21:11

"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"

Difference between window[] and eval() - Javascript

4 Answers4

Linked