Immediately invoked functional expressions

- [Narrator] In the previous example, I treat the variable containing the anonymous function as if it were a function itself. You see it right here, I'm calling the variable name, then adding parenthesis after it, pretending its a function, and then passing arguments into that parenthesis. These arguments are then passed to the anonymous function inside the variable, the function runs, some result is returned, and then we're console logging the result as if everything here was a function. It begs the question, what happens if we just console log out the variable itself? Well, let's try it.

Save that, and you see we actually get the function in the console. This is important, because later we'll hook up anonymous functions to events, and in that case, we actually want the function to be returned, not the result of the function. More on that later in the course. This scenario brings up an important question: How do I populate the variable with the result of an anonymous function? The answer is you need to use an immediately invoked function expression that runs as soon as the browser encounters it.

Here's what that looks like. First, we wrap our entire anonymous function in parenthesis. Then, we invoke the function by adding a set of parenthesis at the end. And because we have arguments here, we need to place those arguments inside this new set of parenthesis. So I'll cut them out, and paste them in here. Now when the script is read from the top down, the browser says, "Hey, here is a variable, "and inside the variable we have an "immediately invoked function expression, "so I'm going to run the function right now, "with these arguments, and return "the result back into the variable." That means, now we can console log out just theBiggest, and get the result.

This is a radically different way of doing things from what I showed you with both named functions and anonymous functions. And it has some significant consequences. Remember the example we used previously where I set up two variables, firstFraction seven/nine, and secondFraction... 13/25, and then used these two variables as the arguments sent to the function? Well, guess what. If I do that here, and save it, well guess what, if I do that here, and save, I end up in trouble.

In my console, I get B and undefined, and that's because I'm running the ternary operator here and I'm saying if A is bigger than B, then give us A and the result of A, otherwise give us B and the result of B, and in this case, we have otherwise meaning we don't have any values, so we'll always get the letter B and then undefined, which is nothing. Why is this happening? Well, it's because this isn't an immediately invoked function expression. The browser runs the function when it's encountered, so you'll remember previously when we made our named function, we placed it at the top, and then we called the function later on after we defined the variables? Well now, if we want to use external variables, those variables have to be defined before we list out our immediately invoked function expression.

So, cut the variables out, move them to the top, save, and everything works again. So why would you want to use an immediately invoked function expression? Good question. The benefit of this function is that it runs immediately where it's located in the code, and produces a direct output. That means, on first run, it is unaffected by code which appears further down in the script, which can be useful. These functions are great for quickly populating a variable or argument in a larger function or a property in an object, and are often hooked to event listeners for an immediate output.

However, when you use these, you need to be very careful about how you structure your script, because as you see, they are invoked immediately, they don't wait for you to call the biggest variable, they just run as soon as the browser encounters them.