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1. Practice writing inline functions
2. Practice writing functions to use as callbacks
3. Explain what a "first-class function" is

Sometimes in life, we need to take a first step. Since life isn't scripted like a reality TV show, anything can happen after that initial step. We need to be able to adjust accordingly.

Imagine an exercise routine: every morning, we run 5 miles. But afterwards — depending on the day — we might lift weights, go for a swim, or run an extra 5 miles.

In programming-speak, we could write out a function for every day (follow along!):

function Monday() {
  console.log('Go for a five-mile run');
  console.log('Pump iron');
}

function Tuesday() {
  console.log('Go for a five-mile run');
  console.log('Swim 40 laps');
}

function Wednesday() {
  console.log('Go for a five-mile run');
  console.log('Go for a five-mile run');
}

function Thursday() {
  console.log('Go for a five-mile run');
  console.log('Pump iron');
}

function Friday() {
  console.log('Go for a five-mile run');
  console.log('Swim 40 laps');
}

But that's pretty tedious. And we already know that functions are supposed to help us reduce this kind of repetition.

What if we pull all of our five-mile runs into their own function?

function runFiveMiles() {
  console.log('Go for a five-mile run');
}

Okay, that cuts down slightly on how much code we need to write. Let's do the same thing for lifting weights and swimming:

function liftWeights() {
  console.log('Pump iron');
}

function swimFortyLaps() {
  console.log('Swim 40 laps');
}

Awesome! We've cut down a little bit more: Monday() could now look like

function Monday() {
  runFiveMiles();
  liftWeights();
}

While it is a tiny bit shorter than before, there is definitely still room for improvement. We know that every day, our routine includes two activities. We also know that the first activity is always a run. That means that the second activity can be variable. What if we created a function that took the second activity as a parameter?

function exerciseRoutine(postRunActivity) {
  runFiveMiles();
  postRunActivity();
}

Notice that, in exerciseRoutine(), the postRunActivity parameter is actually a function — we call it after we call runFiveMiles(). Now let's try to use this new function we created in our Monday() function:

function Monday() {
  exerciseRoutine(liftWeights);
}

Notice that we aren't calling liftWeights. When we want to pass a function as a value, we pass it by reference. We do this by omitting the parentheses. We're not running the function at this point. It's up to exerciseRoutine() to call the function when it is needed.

If we call Monday(), we'll see that we run five miles, and then we lift weights — awesome!

Functions in JavaScript are first-class functions. Among other things, this means that we can pass them as values to other functions, just like we did above. They're super useful, as you can see — they even help us exercise in the mornings!

Note: you'll often see functions used in this way referred to as "callbacks." That's because they're called back after the body of the function they're passed to completes! Callbacks are mostly used for asynchronous operations, like requesting a JSON file from a server, or in the case of Node.js, accessing the file system, a database, etc.

What if, though, we want to have a one-off day of Pilates in our exercise routine? Keep in mind that our exerciseRoutine() function requires a function as its first (and only) parameter. However, that function doesn't have to be defined beforehand! We can pass what's called an anonymous function to exerciseRoutine().

To start with, let's use the full function syntax we've come to know and love:

exerciseRoutine(function() {
  console.log('Stretch! Work that core!');
});

// "Go for a five-mile run"
// "Stretch! Work that core!"

We can rewrite this to be more concise by using an arrow function:

exerciseRoutine(() => {
  console.log('Stretch! Work that core!');
});

// Or even shorter:
exerciseRoutine(() => console.log('Stretch! Work that core!'));

Notice how neither of these functions have a name — we can't refer to it elsewhere, we just pass it in as an argument to exerciseRoutine(). Functions that don't have names are, for obvious reasons, known as anonymous functions.

Functions can also return other functions. This is useful when we want to package up a function and its environment, but when we don't want to call it just yet.

For example, let's say our morning routine involves drinking a cup of coffee, exercising immediately, and then at some point later (depending on how we feel), eating breakfast. What we'll have for breakfast depends on what kind of exercise we're doing.

Let's translate this to a function:

function morningRoutine(exercise) {
  var breakfast = null;

  if (exercise === liftWeights) {
    breakfast = 'protein bar';
  } else if (exercise === swimFortyLaps) {
    breakfast = 'kale smoothie';
  } else {
    breakfast = 'granola';
  }

  exerciseRoutine(exercise);

  // we could give this function a name if we wanted to,
  // but since it's only available _inside_ morningRoutine(),
  // we don't need to
  return function() {
    console.log(`Nom nom nom, this ${breakfast} is delicious!`);
  }
}

Now when we call morningRoutine(), we'll get a function back:

var afterExercise = morningRoutine(liftWeights);

And we can call that function later:

afterExercise();

If you haven't been following along, it's vitally important that you go back and do so. First-class functions are one of JavaScript's most powerful features, but it takes some practice for them to sink in.

• Wikipedia: First-class function
• JavaScript is Sexy: Higher-order functions

View First Class Functions on Learn.co and start learning to code for free.