JavaScript function redefinition

Question

Is it possible to redefine a JavaScript function from within its own body. For example, could I do the following?

function never_called_again(args) {
  // Do some stuff
  never_called_again = function (new_args) {
    // Do some new stuff
  }
}

Is the above valid and does it have the correct semantics? I don't want to create a new global variable with the old function name, because I'm trying to do this kind of thing not in the global scope, but from various object scopes, and I don't want name clashes when I redefine the function within those local scopes.

Answer 1

It is indeed possible to redefine a function from its body. The technique is used in the so-called Lazy Function Definition Pattern.

It looks like this:

// Lazy Function Definition Pattern
var foo = function() {
    var t = new Date();
    foo = function() {
        return t;
    };
    return foo();
}

This function stores the Date of the first call, and returns this Date afterwards.

If you compare this with the module pattern, the difference is that the initialization only happens when it's first called, not when it's defined. For costly initializations it can be a huge benefit.

// Conditional Module Pattern
var foo = (function() {
    var t;
    return function() {
        if (t) {
            return t;
        }
        t = new Date();
        return t;
    }
})();

Answer 2

Yes, you can redefine a function that way.

Running This:

function never_called_again(args) {
    console.log('Func 1', args);

    never_called_again = function (new_args, x) {
        console.log('Func 2', new_args, x);
    }
}

never_called_again('arg A', 'arg B');

never_called_again('arg A', 'arg B');

---

Yields this:

Func 1 arg A
Func 2 arg A arg B