Question

Yes I know eval is evil but …

Few times I saw a function being created from a string in few different ways:

var s = "function fname() { this.m = function(s){alert(s);}}";

//1
var x = new( eval("("+s+")") );
x.m("aaa")

//2
var x = new( eval('[' + s + ']')[0] );
x.m("bbb")

//3
var x = new ( eval(s + " fname;") );
x.m("ccc")

The first 2 are clear to me but I wonder about the third one.
Could someone explain how adding a name of the function after it’s definition helps eval do the job ?
Also do you know any other ways of using eval to create funcions ?

Thanks
Szymon

Answer 1

First, with the function declaration (which is not an expression), you create the function and put it into the current scope:

function fname() { ... }

The closing brace finishes the function statement, and an expression follows:

fname

As the function is already in the scope, the expression fname simply refers to the function. And because the expression is the last thing in your eval’ed code, eval returns exactly that function reference.

Instead of fname, you could write the name of any function in the current scope, e.g. alert, which would then be returned by eval.