Question

The following code prints “true”. I can understand why ff.f is equal to undefined, but I don’t understand why console.log(“Hi”) is not executed inside ff.f when checking this value. Isn’t f immediately executed upon definition?

var ff = function(){
    var f = function(){
        console.log("Hi");
    }();
};

console.log(ff.f === undefined);

[Edit] I guess a better way to ask this question is “When does this f function inside ff get executed?”. I think it’s weird that ff.f’s value is “undefined” if it doesn’t get executed until ff gets executed. Shouldn’t it be the function instead?

Answer 1

No, it is not – it is executed once it is evaluated – which in this case is when the parent function (ff) is executed.

When is f executed

Any IIFE will be executed once it is evaluated – any such expressions included inside of other functions will only evaluated once the function it is scoped to (wrapped in) executes:

// Executed when the execution flow reaches this point
// i.e. immediately after the script starts executing
var outer = function() {
    console.log("Hello from outer");
}();

var wrapper = function() {
    // Executed when the flow reaches this point
    // which is only when the function `wrapper`
    // is executed - which it isn't, so this never fires.
    var inner = function() {
        console.log("Hello from inner");
    }();
};

Why ff.f is not a function

JavaScript is function scoped but JavaScript does not provide any way to access a function’s inner scope (at least not, as far as I am aware). So when you try to access ff.f you are looking for a property named f on the function ff – and by default there is no such property. Even if you did:

var ff = function () {
    ff.f = function() {
        console.log("Hello from f");
    }();
};

ff.f would still be undefined (because the IIFE does not return anything).