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Difference between window[] and eval() - Javascript

Answer 1

+4 A:

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 +" ]").

Paul Grime 2010-07-20 21:01:08

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 2010-07-20 21:13:01

Yes, eval is almost always a bad idea.

Paul Grime 2010-07-29 21:17:20

Answer 2

+2 A:

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.

Marimuthu Madasamy 2010-07-20 21:07:01

**clever answer

mykhal 2010-07-20 21:10:49

Answer 3

A:

"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"

mykhal 2010-07-20 21:11:14

Answer 4

+1 A:

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.

CMS 2010-07-20 21:12:14

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 2010-07-20 21:49:34

@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..

CMS 2010-07-20 22:00:52

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Difference between window[] and eval() - Javascript

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