javascript - create anonymous methods using "eval" - browser specific behaviour - Stack Overflow

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I working with jQuery and i needed to generate an anonymous method with the eval() function.

The following lines worked with Opera but not with IE, FF, Chrome:

var callbackStr = "function(){alert('asdf');}";  
var callback = eval(callbackStr);  
callback();

This code works with all Browsers:

var callbackStr = "var callback = function(){alert('asdf');}";  
eval(callbackStr);  
callback();  

You see, I already solved my problem. But I want to know, what exactly is happening. Can anybody explain this behaviour to me, or tell me where i can find further information?

(PS: I know this page.)

I working with jQuery and i needed to generate an anonymous method with the eval() function.

The following lines worked with Opera but not with IE, FF, Chrome:

var callbackStr = "function(){alert('asdf');}";  
var callback = eval(callbackStr);  
callback();

This code works with all Browsers:

var callbackStr = "var callback = function(){alert('asdf');}";  
eval(callbackStr);  
callback();  

You see, I already solved my problem. But I want to know, what exactly is happening. Can anybody explain this behaviour to me, or tell me where i can find further information?

(PS: I know this page.)

• javascript
• callback
• eval

Share Improve this question Follow edited Jul 14, 2010 at 11:31 Pranay Rana 177k3737 gold badges243243 silver badges265265 bronze badges asked Jul 14, 2010 at 11:28 ChriglChrigl 64866 silver badges2323 bronze badges 1

• 1 No idea but why do you need to eval it, why can't you just set the function in a variable? – Ashley Commented Jul 14, 2010 at 11:31

Add a ment  | 

4 Answers 4

Sorted by: Reset to default 7

The reason is the same as the reason you need to wrap parentheses around JSON strings before using eval on them - eval treats { as a token, the start of a statement eval assumes the anonymous functions is a FunctionDeclaration. Using parentheses allows eval to treat what's inside as an expression:

var callbackStr = "(function(){alert('asdf');})";  
var callback = eval(callbackStr);  
callback();
// -> alerts 'asdf';

As for why Opera behaves differently here, I have no idea. As for your use of eval, there's probably a better way (there nearly always is).

I dug out a quote from the spec for you, section 12.4 of ECMA-262 3rd Edition:

Note that an ExpressionStatement cannot start with an opening curly brace because that might make it ambiguous with a Block. Also, an ExpressionStatement cannot start with the function keyword because that might make it ambiguous with a FunctionDeclaration.

Emphasis mine.

the first example will work if you'll do your eval like this

var callbackStr = "function(){alert('asdf');}";  
var callback = eval("(" + callbackStr + ")");  
callback();

eval does not return the function like that. Instead it executes a piece of code in the global scope, thats why the second example works - the callback var has been created in the global scope.

I'm sure these are just extremely simplified examples but you can almost always avoid using eval in the first place, which is remended.

The first case simply evaluates your string and executes the function. It does not return a pointer to that particular function, that's why you cannot execute it by calling callback.

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