Using eval on a function string

Question

I am doing :

eval(\'function(){ console.log(\"Hello World\"); }\')();

But that gives error :

Uncaught SyntaxError: Unexpected t         


        

Answer 1

The eval operator expects a Program as input, and the grammar of JavaScript requires that all top-level program elements are either declarations or statements.

The spec says:

Let prog be the ECMAScript code that is the result of parsing x as a Program.

function can't start a top-level statement, but it can start a function declaration, but only when it has a name.

That's why you get "Unexpected token ("; it expects a function name before the parenthesis that opens the argument list.

As others have noted, to eval a function, you need to trick the JavaScript parser into finding an expression where it expects a statement. Wrapping the body in parentheses is one way to do this.