Question

Is it possible to extend dynamically javascript function scope? I tried without success the following:

function foo()
{
    var bar = 12;
    return function(x)
    {
        return eval(x);
    }
}

var e = foo();

console.log(e("bar"));           // 12
console.log(e("bar = 42;"));     // 42
console.log(e("bar"));           // 42
console.log(e("var baz = 99;")); // undefined
console.log(e("baz"));           // ReferenceError: baz is not defined

If however I remove var from the line baz = 99 then the baz variable becomes a global (this makes perfect sense to me):

...
console.log(e("baz = 99;"));     // 99
console.log(e("baz"));           // 99
console.log(baz);                // 99 (so baz is just a global)

Answer 1

Everytime you call e("var baz = 4"), it’s creating a variable on the stack of that function call, so it’s not going to be available the next time you call it.

If you need to dynamically add variables to the scope, I would use Rayno’s suggestions, use a map. http://jsfiddle.net/UVSrD/

function foo()
{
    var scope = {};
    return function(x)
    {
        return eval(x);
    }
}


var e = foo();

console.log(e("scope.bar = 12")); // 12
console.log(e("scope.bar")); // 12
console.log(e("scope.baz = 14")); // 14
console.log(e("scope.baz;")); // 14
// Not a global
console.log(typeof scope) // undefined