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 (Lu

Answer 1

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) {};

Answer 2

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

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