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.

Answer 1

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

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

Answer 2

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) {};