As you can see, both Script tags start off by defining self-executing functions. I did this to make sure that nothing was accidentally scoping to the Window object without my understanding. Within the source-code's self-executing function, I define another function, whisperSweetNothings(). This whisperSweetNothings() function creates a closure with its context, which happens to house the variable, "description." This source code is then, itself, eval()'d within the primary code's self-execution function, which happens to house another variable, "name." The whisperSweetNothings() function references both the "description" and "name" variables, which will only work if has both contexts in its closure chain.
When we run the above code, we get the following console output:
Erika, you are looking quite beautiful today.
As you can see, the whisperSweetNothings() function properly referenced the description variable (beautiful) in its defining context as well as the name variable (Erika) in its calling context. Had I done the same thing with the Function() constructor, the Name variable would have errored-out as undefined.
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Everytime I've seen eval, it's been followed by 'thats a security threat'. I suppose the idea is that code could be injected into the string prior to eval then executed as if it was running on your page. This would create a possible XSS attack.
Do you or anyone else have any concerns over security using eval?
The only way I can see eval() being a concern is if you are eval()'ing data that comes back over an HTTP request (ala AJAX). If you are eval() code that you wrote, then it should be of no concern. If someone is able to intercept *that* kind of call, then they already way behind enemy lines and probably can execute any kind of code they want.
As far as the HTTP-delivered JSON data that gets eval()'d, yeah, I suppose if someone intercepts your request and messes with your JSON data, it could be malicious.
I'm a naive developer. I just want to say that this post is "super cool".
Thanks my man. I am glad you liked it!
Really nice blog with easy to understand instructions.
Thanks for spending time to write this blog entry.
My pleasure - just happy you are finding value in it.
Cool Article, Thank you Ben
It's probably the "console.log()" call that is tripping you up. That is part of the FireBug Firefox plugin. If you want this to run in another browser, you have to remove that call (perhaps replace with an alert() call).
what if i use
var text = eval("TeamName_" + teamCount);
*team count can b any integer value.
In this case, the string will concatenate to something like TeamName_7... then, the eval() method will evaluate that value as if it were a variable name, returning said variable's value into the new variable, "text."
Its really very helpful.First time i understand the use of eval in this scenario.
Thanks for giving time for my problem and for your support too.
One more question ben.In my problem we can simply use
var text = "TeamName_" + teamCount;
then why use
var text = eval("TeamName_" + teamCount);
Actually i want to know the difference between the two. Is their any advantage if using eval?
sorry for so many questions in one..hope you will not get irritate.
To answer your comparison question, imagine I had this code:
var girl1 = "Jill";
var girl2 = "Kim";
var data1 = ("girl" + 1);
var data2 = eval("girl" + 2);
... when I run this, my data variables will contain the following values:
data1 ==> "girl1"
data2 ==> "Kim"
Does that help at all? You only need to use eval() when you want to dynamically evaluate some code.
Thank you so much Ben. I got my answer. Really very helpful.I was searching it for so long..
But Ben as i saw its very useful,can i use eval() frequently?
You can use eval() when ever you want; but, I don't think you will have to use it that often. Typically, you can find better ways to reference variables.
"eval()" has capabilities not otherwise realizable.
It has the disadvantage that it is slow.
Very interesting post! You encoruraged me to use eval wich i had in mind to implement a sort of templating of js code in wich i can use to pass by reference context variables to pseudo functions (blocks)! Very powerfull! Eval is not evil!
I am trying to use eval to bring the entire jQuery library into global scope. I have escaped all quotes and backslashes although I keep getting an error: Can't convert an undefined object. I also removed all comments and newlines. I ran it through jslint and there were no errors.
Does anyone have an idea as to why I get the undefined variable?
I really enjoyed this post, though the security issue is very real, for some reason, only the most malicious hackers seem to know this. I had some sites defaced through this function 2 months ago. (disclaim) I'll do my best to explain without inadvertently posting a "how to destroy the web with eval" guide. Please delete this if you feel it's instruction negatively effects security. (/disclaim)
There are a couple things I left out of the attack scenario intentionally, to avoid spreading bad practices, I hope I conveyed the risk, it's a security issue that unfortunately can't easily be patched in the stack, developers need to understand that eval's utility lies in server side and non public facing applications, and open to the anonymous public is a time bomb at best.
I don't like how some developers say things like 'NEVER use eval'. There is a logic behind this train of through, but I think it's exaggerated; there are certain specific (albeit unusual) problems for which eval is an ideal solution. The trick is check/escape all user input before you run it to mitigate the risk of code injection; just as you would do with an SQL query.
Thank a ton. That was a valuable information.
ben you made this look extremely simple .Thanks for your time
<script id="source" type="text/source-code">
var description = "beautiful"
", you are looking quite " +
var name = "Erika";
eval( document.all.source.innerHTML )();