Adding a local scope to functions

The answer is no, you can't implicitly add a variable declaration and initialize it in every function scope (thank goodness). You need to manage your variable scopes manually, employing analysis tools when/if desired.

Imagine for a moment if this was possible. Any code you load could potentially corrupt any other variable scope, effectively defeating the purpose of scoped variables.

The Function.prototype will not help because prototypal inheritance has nothing to do with variable scope.

Your trick to protect i by putting it in a local object doesn't solve anything. It just moves the problem from the i variable to the local variable, and still doesn't keep you from using i instead of local.i.

While including "use strict"; can help by providing helpful error messages, you need to keep in mind that it changes behavior in some situations that can break code.

No, you can't do this. You can't inject code into arbitrary functions and mess around with their local scope by changing the Function.prototype, and that's a good thing.

Besides, how would you enforce using the local object for local variables anyway? And how would you allow variable lookups through parent scopes, such as in:

function f() {
    var a = 5;
    return function g(b) {
        return a + b;
    };
 }

In your solution, you'd have a local object in the f and g functions, but the local variable in g would hide the parent's variable...

function f() {
    var local = { a : 5 };
    return function g(b) {
        // This declaration hides the parent's declaration
        var local = { b : b };
        // How would local.a be found?
        return local.a + local.b;
    };
 }

There's a much simpler solution to your original problem though: just enable strict mode with "use strict". That way, you get an error when you accidentally leak a variable into the global scope. For example, your second sample would throw a ReferenceError when it tries to access the undeclared i variable (instead of creating a new global variable).

Do note that strict mode goes further than just preventing accidental global variables, read up on it so you understand the consequences!