Introduction

Since JavaScript is the language of the web, there are some functions that by necessity are going to take a decent amount of time to complete, such as fetching data from a server to display on your site. For this reason, JavaScript includes support for asynchronous functions, or to put it another way, functions that can happen in the background while the rest of your code executes.

Lesson overview

This section contains a general overview of topics that you will learn in this lesson.

• Explain what a callback is.
• Explain what a promise is.
• Explain the circumstances under which promises are better than callbacks.
• Explain what the .then() function does.

Callbacks

In the recent past, the way that these were most commonly handled were with callbacks, and even now they are still used quite a lot in certain circumstances.

A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action. MDN

Callbacks are functions that get passed into other functions. For example:

myDiv.addEventListener("click", function(){
  // do something!
})

Here, the function addEventListener() takes a callback (the “do something” function) and then calls it when myDiv gets clicked.

You will likely recognize this pattern as something that happens all the time in JavaScript code. Unfortunately, though they are useful in situations like the above example, using callbacks can get out of hand, especially when you need to chain several of them together in a specific order. The rest of this lesson discusses patterns and functions that will help keep you out of Callback hell.

Take a moment to skim through this Callback article before moving on. Or, if you prefer to watch a video of Callback functions.

Promises

There are multiple ways that you can handle asynchronous code in JavaScript, and they all have their use cases. Promises are one such mechanism, and they’re one you will see somewhat often when using other libraries or frameworks. Knowing what they are and how to use them is quite useful.

Essentially, a promise is an object that might produce a value at some point in the future. Here’s an example:

Lets say getData() is a function that fetches some data from a server and returns it as an object that we can use in our code:

const getData = function() {
  // go fetch data from some API...
  // clean it up a bit and return it as an object:
  return data
}

The issue with this example is that it takes some time to fetch the data, but unless we tell our code that, it assumes that everything in the function happens essentially instantly. So, if we try to do this:

const myData = getData()
const pieceOfData = myData['whatever']

We’re going to run into trouble because when we try to extract pieceOfData out of the returned data, the function getData() will most likely still be fetching, so myData will not be the expected data, but will be undefined. Sad.

We need some way to solve this problem, and tell our code to wait until the data is done fetching to continue. Promises solve this issue. We’ll leave learning the specific syntax for the articles you’re about to read, but essentially Promises allow you to do this:

const myData = getData() // if this is refactored to return a Promise...

myData.then(function(data){ // .then() tells it to wait until the promise is resolved
  const pieceOfData = data['whatever'] // and THEN run the function inside
})

Of course, there are many more occasions where one would want to use Promises beyond fetching data, so learning these things now will be very useful to you.

Assignment

Knowledge check

This section contains questions for you to check your understanding of this lesson. If you’re having trouble answering the questions below on your own, review the material above to find the answer.

• What is a callback?
• What is a promise?
• When should you use promises over callbacks?
• What does the .then() function do?

Additional resources

This section contains helpful links to other content. It isn’t required, so consider it supplemental.

1. The Promises by ColorCode is a nice introduction to Promises if you need additional information.
2. For a more interactive explanation and example, try a Scrim on asynchronous coding.
3. Read Chapter 2: Callbacks and Chapter 3: Promises from “You Don’t Know JavaScript”. Both chapters dive deep into their respective topics, which may make for a difficult - but worthwhile - read.