Learn about higher-order functions in JavaScript.

Higher-Order Functions

Great job! By thinking about functions as data, and learning about higher-order functions, you've taken important steps in learning  to write clean, modular code that takes advantage of JavaScript's flexibility. 
 
Let's review what we learned in this lesson:

- Abstraction allows us to write complicated code in a way that's easy to reuse, debug, and understand for human readers.

- We can work with functions the same way we work with any other type of data, including reassigning them to new variables.

- JavaScript functions are first-class objects, so they have properties and methods like any other object.

- Functions can be passed into other functions as parameters. 

- A higher-order function is a function that either accepts functions as parameters, returns a function, or both.


Review

We are often unaware of the number of assumptions we make when communicating in our native languages. If we told you to "count to three," we would expect you to say the numbers "one", "two", and "three". We assumed you would know to start with "one" and end with "three". With programming, we need to be more explicit with our directions to the computer. Here's how we might tell the computer to "count to three":

```js
for (let i = 1; i <= 3; i++) {
  console.log(i);
}
```

When we speak to other humans, we share a vocabulary that provides quick ways to communicate complicated concepts. When we say the word "bake", it calls to mind a familiar subroutine&mdash; preheating an oven, putting something into an oven for a set amount of time, and finally removing it. This allows us to _abstract_ away a lot of the details and communicate key concepts more concisely. Instead of listing all those details, we can say, "We baked a cake," and still impart all that meaning to you. 

In this lesson, we'll learn how to use “abstraction” in programming by writing functions. In addition to allowing us to reuse our code, functions help to make clear, readable programs. If you encounter `countToThree()` in a program, you might be able to quickly guess what the function does without having to stop and read the function's body. 

We're also going to learn about a way to add another level of abstraction to our programming: _higher-order functions_.  _Higher-order functions_ are functions that accept other functions as arguments and/or return functions as output. This enables us to build abstractions on other abstractions, just like "We hosted a birthday party" is an abstraction that may build on the abstraction "We made a cake."

Introduction

As you know, a [parameter](https://www.codecademy.com/courses/introduction-to-javascript/lessons/functions/exercises/parameters) is a placeholder for the data that gets passed into a function. Since functions can behave like any other type of data in JavaScript, it might not surprise you to learn that functions can accept other functions as parameters. A _higher-order function_ is a function that either accepts functions as parameters, returns a function, or both! We call functions that get passed in as parameters _callback functions_. Callback functions get invoked during the execution of the higher-order function.

When we invoke a higher-order function, and pass another function in as an argument, we don't invoke the argument function. Invoking it would evaluate to passing in the return value of that function call. With callback functions, we pass in the function itself by typing the function name _without_ the parentheses:



```js
const higherOrderFunc = param => {
  param();
  return `I just invoked ${param.name} as a callback function!`
}
 
const anotherFunc = () => {
  return 'I\'m being invoked by the higher-order function!';
}

higherOrderFunc(anotherFunc);
```

We wrote a higher-order function `higherOrderFunc` that accepts a single parameter, `param`. Inside the body, `param` gets invoked using parentheses. And finally, a string is returned, telling us the name of the callback function that was passed in.

Below the higher-order function, we have another function aptly named `anotherFunc`. This function aspires to be called inside the higher-order function.

Lastly, we invoke `higherOrderFunc()`, passing in `anotherFunc` as its argument, thus fulfilling its dreams of being called by the higher-order function.

```js
higherOrderFunc(() => {
  for (let i = 0; i <= 10; i++){
    console.log(i);
  }
});
```

In this example, we invoked `higherOrderFunc()` with an [anonymous function](https://www.codecademy.com/courses/introduction-to-javascript/lessons/functions/exercises/function-expressions) (a function without a name) that counts to 10. Anonymous functions can be arguments too!

Let’s get some practice writing higher-order functions.

Functions as Parameters

JavaScript functions behave like any other data type in the language; we can assign functions to variables, and we can reassign them to new variables. 

Below, we have an annoyingly long function name that hurts the readability of any code in which it's used. 
Note: If the below function's syntax feels unfamiliar, revisit the [arrow functions exercise](https://www.codecademy.com/courses/introduction-to-javascript/lessons/functions/exercises/arrow-functions) to refresh your knowledge on ES6 arrow notation.

```js
const announceThatIAmDoingImportantWork = () => {
    console.log("I’m doing very important work!");
};
```

Let's pretend this function does important work and needs to be called repeatedly. To  rename this function without sacrificing the source code, we can re-assign the function to a variable with a suitably short name:

```js
const busy = announceThatIAmDoingImportantWork;

busy(); // This function call barely takes any space!
```

`busy` is a variable that holds a _reference_ to our original function. If we could look up the address in memory of `busy` and the address in memory of `announceThatIAmDoingImportantWork` they would point to the same place. Our new `busy()` function can be invoked with parentheses as if that was the name we originally gave our function. 

Notice how we assign `announceThatIAmDoingImportantWork` without parentheses as the value to the `busy` variable. We want to assign the value of the function itself, not the value it returns when invoked. 

In JavaScript, functions are _first class objects_. This means that, like other objects you've encountered, JavaScript functions can have properties and methods. 

Since functions are a type of object, they have properties such as `.length` and `.name`, and methods such as `.toString()`. You can see more about the methods and properties of functions 
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" target="_blank" rel="noopener noreferer">in the documentation</a>.

Functions are special because we can invoke them, but we can still treat them like any other type of data. Let's get some practice doing that!