JavaScript® (often shortened to JS) is a lightweight, interpreted, object-oriented language with first-class functions, most known as the scripting language for Web pages, but used in many non-browser environments as well such as node.js or Apache CouchDB. It is a prototype-based, multi-paradigm scripting language that is dynamic, and supports object-oriented, imperative, and functional programming styles.

javascript

Before we dive deep into the pattern of composing callbacks and closures, we need to take start with the definitions and basic building blocks that we will be using for the rest of the course. 

You'll be able to answer questions like:

- What is a Closure?
- What is a Callback?
- Why does Callback Hell happen and how can we handle it?

We will see that by establishing a contract between functions and holding to the pattern that is established, you will have gained a lot of power and flexibility in writing reusable, robust functions.

**You're in Section 01 of [Composing Closures and Callbacks in JavaScript](https://egghead.io/playlists/composing-closures-and-callbacks-in-javascript-1223).**

When working with closures you'll often hear the words "capture", "wrap", or "enclose". The entire idea behind a closure is to take a variable outside of the function and use that variable inside the function. A closure enables patterns for passing around functions that can manage buffers, caches, and values that can travel with the function.

What Is a Closure in JavaScript


If you've used `addEventListener`, `Array.filter`, or many other methods in JavaScript, then you've already used a callback. Callbacks are often invoked inside of APIs of libraries where we don't get to see them called. This course explores internals and patterns where we have control of invoking the callback and discovering the features callbacks enable.

What Is a Callback in JavaScript


The short answer: "yes". Functions have a lot of names: predicates, lambdas, reducers, etc. It all depends on the usage of the function or if its inputs/outputs match certain rules. This lesson shows how closures and callbacks are both types of functions.

Can a Function Be a Closure and a Callback?


Composition is all about passing one function to another to create a new function. Many libraries have utility functions (which you can write on your own) that allow you to express composition as a list of functions instead of a messy function inside of a function inside of a function. This lesson shows how to compose closures and callbacks and use the utility functions like pipe and compose to make it more readable.

Compose Closures and Callbacks to Create New Functions

It's time to set up the core of the pattern we'll be using throughout the entire course. Establishing a contract between functions and rules that we'll follow will allow us to use and define many different functions interchangeably. For many people, "pass a function to a function" is a difficult new concept, but with repetition and using this pattern in many different scenarios, we hope to make it very comfortable so it can be a useful pattern in your JavaScript tool belt.

Defining the Broadcaster and Listener Relationship

Time causes bugs. This is because we often don't know how to handle the many different scenarios that emerge when dealing with asynchronous code. A main goal of this course is to help your feel comfortable attacking bugs due to time so you can build the advanced features that main your app shine.

Time is a Hidden Variable in JavaScript

Many APIs in JavaScript rely on callbacks, so when using APIs together, we end up in deeply nested callbacks AKA "Callback Hell". This course solves callback hell through composition by moving all of the nesting into the internals of our helper functions which can expose a clean API that becomes a list of functions in a pipe.

Solve Callback Hell with Composition

Callback Hell is made up of nested functions. If we compose our functions instead of nesting them, we need to remember to pass values down to the next API that takes a callback. We can do this by setting up a "mapping" behavior in our outer/inner relationship so that the value from the outer can be used to create the broadcaster of the inner.

  ## Congrats on finishing Section 01: Introduction to Callbacks, Broadcasters, and Listeners!🎉🎉

  👉  **[CLICK HERE to continue to the next section, Establish Callback and Closure patterns Around Async Browser Behaviors.](https://egghead.io/playlists/establish-callback-and-closure-patterns-around-async-browser-behaviors-c813)** 👈 

Introduction to Callbacks, Broadcasters, and Listeners

John Lindquist

Course Content