JavaScript eval() "syntax error" on parsing a function string

Question

I have a bit of JavaScript code that is specified in a configuration file on the server-side. Since I can't specify a JavaScript function in the configuration language (Lua), I have it as a string. The server returns the string in some JSON and I have the client interpret it using a clean-up function:

parse_fields = function(fields) {
    for (var i = 0; i < fields.length; ++i) {
        if (fields[i].sortType) {
            sort_string = fields[i].sortType;
            fields[i].sortType = eval(sort_string);
        }
        return fields;
    }
};

So basically it just evaluates sortType if it exists. The problem is that Firebug is reporting a "Syntax error" on the eval() line. When I run the same steps on the Firebug console, it works with no problems and I can execute the function as I expect. I've tried some different variations: window.eval instead of plain eval, storing the sortType as I've done above, and trying small variations to the string.

A sample value of fields[i].sortType is "function(value) { return Math.abs(value); }". Here's the testing I did in Firebug console:

>>> sort_string
"function(value) { return Math.abs(value); }"
>>> eval(sort_string)
function()
>>> eval(sort_string)(-1)
1

and the error itself in Firebug:

syntax error
[Break on this error] function(value) { return Math.abs(value); }

The last bit that may be relevant is that this is all wrapped in an Ext JS onReady() function, with an Ext.ns namespace change at the top. But I assumed the window.eval would call the global eval, regardless of any possible eval in more specific namespaces.

Any ideas are appreciated.

@SLaks: eval works just fine for me in IE for the above string, no errors... as does eval("a = function () { return 'b'; }") -- can you clarify what you meant please? — Andy E
– Andy E, Commented May 3, 2010 at 20:36
@Andy: Paste javascript:alert(eval("(function() { return 4; })")) into the address bar in FF and IE. — SLaks
– SLaks, Commented May 3, 2010 at 21:07
To eval functions in all browsers, use eval("[function(){return 1}][0]"); See my comment on stackoverflow.com/questions/4599857/… — Ruan Mendes
– Ruan Mendes, Commented May 18, 2011 at 22:06

jhurshman · Accepted Answer · 2010-05-03 20:27:31Z

43

To do what you want, wrap your string in parentheses:

a = "function(value) { return Math.abs(value);}";
b = eval("("+a+")");
b(-1);

answered May 3, 2010 at 20:27

jhurshman

6,0672 gold badges29 silver badges17 bronze badges

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

6 Comments

Kenny Peng Over a year ago

Yep, that worked. Why are the parentheses required when Firefox executes the code outside of the Firebug interface, but not required when using the Firebug console?

Jasper Over a year ago

I, for one, can't get it to work without the braces even in the Firebug console. I don't know why it would work for you. What version of Firebug are you using?

Andy E Over a year ago

@Kenny Peng: I remember wondering this myself once and found this answer to a similar question: stackoverflow.com/questions/964397/…. This was my first thought when I saw the question, +1 to @jhurshman for his answer.

Kenny Peng Over a year ago

@Jasper: I was using Firebug 1.5.3.

Jasper Over a year ago

@Kenny: I am using 1.5.4, so perhaps something has change. I can but guess about why it is working in your commandline, but it is probably a tricky matter anyway, seeing that the commandline is doing an eval (or something similar) on what you typed already...

|

Aliaksei Kliuchnikau · Accepted Answer · 2012-01-31 08:19:42Z

5

The parentheses are required because they force the thing inside them to be evaluated in an expression context, where it must be a function-expression.

Without the parentheses, it could instead be a function declaration, and it seems as if it is sometimes being parsed that way - this could be the source of the odd/inconsistent behaviour you're describing.

Compare this function declaration:

function foo(arg) {}

with this function-expression:

var funcExpr = function foo(arg) {};

It also has to be a function-expression if it doesn't have a name. Function declarations require names.

So this is not a valid declaration, because it's missing its name:

function (arg) {}

but this is a valid, anonymous function-expression:

var funcExpr = function(arg) {};

edited Jan 31, 2012 at 8:19

Aliaksei Kliuchnikau

13.7k4 gold badges63 silver badges74 bronze badges

answered Jan 31, 2012 at 8:17

whatever

511 silver badge1 bronze badge

Collectives™ on Stack Overflow

JavaScript eval() "syntax error" on parsing a function string

2 Answers 2

6 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related