Functions, callbacks, & async JavaScript

Learn how functions work, and how to manage asynchronous JavaScript code using callbacks.

  • js
  • functions
  • async
  • timers
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In JavaScript functions are treated like any other variable. This concept is sometimes referred to as “first-class functions”. It lets us use functions in some interesting ways, and helps us manage “asynchronous” code.

Functions are variables

When you create a function in JS you are creating a normal variable:

function returnsOne() {
return 1;

This code creates a new variable named returnsOne.

This is still true (and maybe more obvious) for arrow functions:

const returnsOne = () => 1;

This code also creates a new variable named returnsOne. Notice how this is similar to defining a different type of variable:

const num = 1;

You can reference this variable the same way you would any other (by using its name):

// logs something like: `function returnsOne()`

Since functions are normal variables you can even pass them as arguments to other functions.

Mini-challenge 1

Lets try passing a function as an argument to another function. Since we’re just playing around to see what happens we can write this code in the console on this page. Open up the console and try this:

  1. Write a function named logger
  2. It should take one argument, then log that argument
  3. Call logger with the returnsOne function as an argument

What does the browser print?

Toggle answer
function logger(thing) {


In Chrome this logs something like:

ƒ returnsOne() {
  return 1;

If you defined returnsOne with an arrow function it might log something like this instead:

() => 1;

The browser is showing the value stored in the variable. It’s the same as if we ran:

const num = 1;

This would log 1 to the console.

Functions are callable

The main distinction between a function and other types of variable is that you can call a function. You call a function by putting parentheses (round brackets) after it:


Calling a function will run the lines of code inside of it. This is useful for two reasons:

  1. Functions let us reuse code without copy/pasting it.
  2. Functions let us delay running code until we’re ready.

Returning values

Functions need to be able to talk to each other. This is how you create a more complex program. You compose together a bunch of functions, passing the output of one into another.

The return keyword lets us control what value we get after calling a function. Our returnsOne function always returns a value of 1. When you call a function the lines of code inside are run, and the function spits out its return value in place. You can then use this returned value however you like.

You can save it as a new variable:

const answer = returnsOne();
// This will log `1`

Here you can imagine that returnsOne() replaces itself with its return value. It’s the same as if we’d written const answer = 1 directly.

You can also use the called function directly without an intermediary variable:

// This will also log `1`

Here the same thing happens. returnsOne() replaces itself with its return value. It’s the same as if we’d written console.log(1) directly.

If the function doesn’t have a return statement you’ll get undefined:

function returnsNothing() {
const x = 3 + 5;
// doesn't have a return statement
// the x variable is basically thrown away
const answer = returnsNothing();
// This will log `undefined`

Calling or not calling a function is often a source of confusion when passing functions as arguments to other functions.

Mini-challenge 2

  1. Open your console and recreate your logger function from above

  2. Call logger, but this time pass in returnsOne() (don’t forget the parentheses)

  3. Why do we see a different value logged than before?

  4. Edit logger to log the type of the value using the typeof operator

Toggle answer
function logger(thing) {

If we call logger with the function itself:


we see the function logged as before:

ƒ returnsOne() {
  return 1;

However when we call the function and log that:


we see the the function’s return value logged:


If we amend logger to log the type of its argument:

function logger(thing) {
console.log(typeof thing);

we can see that in the first case thing is a function, whereas in the second case its a number:

// Logs: "function"

// Logs: "number"

Inline functions

You can also define functions inline: i.e. directly as you’re using them. This is a common pattern for passing functions as arguments to other functions. For example we could re-write our logger example:

logger(function () {
return 1;

Here we’re defining a new function inline at the same time that we’re passing it to logger. This is a little hard to read, which is why most developers use arrow functions for inline functions like this:

logger(() => 1);

This has the same result as before, when we defined a separate returnsOne variable and passed it by name. The main difference here is we can’t re-use the function, since it only exists as an argument to logger.

Mini-challenge 3

Inline functions are often used for event listeners in the DOM. For example this code will log wherever the user clicks on the window:

window.addEventListener("click", (event) => {
console.log(event.clientX, event.clientY);
  1. Open your console and enter the event listener above
  2. Extract the inline function and assign it to a named variable
  3. Pass your extracted function variable to addEventListener instead

The event listener should work the same whether your function is inlined or defined as a separate variable.

Toggle answer
const handleClick = (event) => {
console.log(event.clientX, event.clientY);

window.addEventListener("click", handleClick);

We have defined a named variable handleClick and assigned the previously inlined function as its value. We then pass this variable to addEventListener (since it expects a function as its second argument).

You may have done something like this:

window.addEventListener("click", (event) => handleClick(event));

This will achieve the same result, and is not incorrect. However it adds an unnecessary extra function: we’re defining an inline arrow function that takes an event argument, then passes that on to the handleClick function.

Since addEventListener will call whatever function is passed in with the event argument we don’t need this extra function.

It’s important to note that we don’t want to call our function when we pass it. This won’t work since addEventListener expects to be passed a function. Remember if we put parentheses after the function name then we’re effectively passing its return value instead.

window.addEventListener("click", handleClick());

Since handleClick doesn’t return anything this is equivalent to:

window.addEventListener("click", undefined);


“Callback” is a scary word, but you’ve actually been using them the whole time. A callback is a function passed to another function as an argument. The name refers to what callbacks are usually used for: “calling you back” with a value when it’s ready.

For example the addEventListener above takes a function that it will call when the "click" event happens. We’re telling the browser “hey, call us back with the event info when that event happens”.

Functions are a way to delay a block of code. Without them all our statements would run in order all in one go, and we’d never be able to wait for anything or react to user input.

Mini-challenge 4

  1. Write a function named one that takes a function as a parameter
  2. It should call that function with 1
  3. Call your one function and pass in a function that logs its argument
Toggle answer
function one(callback) {

The one function has a single parameter: callback. Since we know callback is going to be a function we can call it using parentheses, and pass in 1.

To use our one function we need to pass in a function (the callback):

one((x) => console.log(x));

We could also define this callback as a separate named variable instead of inlining it, just like in our event listener example above:

const log = (x) => console.log(x);

Asynchronous callbacks

The callback above might feel a bit convoluted: why pass in a callback to access a variable from inside when we could make the one function return 1 directly?

Callbacks make more sense when dealing with asynchronous code. Sometimes we don’t have a value to return straight away.

What is “asynchronous” code?

JavaScript is a “single-threaded” language. This means things generally happen one at a time, in the order you wrote the code.

// This logs 1, then 2, then 3

When something needs to happen out of this order, we call it “asynchronous” (“async” for short). JavaScript handles this using a “queue”. Anything async gets pushed out of the main running order and into the queue. Once JS finishes what it was doing it moves on to the first thing in the queue.

window.setTimeout(() => console.log(2), 1000);
// This logs 1, then 3, then (after about 1 second) logs 2

setTimeout is a built-in function that lets you run some code after a specified wait time.

It’s intuitive that the above example logs 2 last, because JS has to wait a whole second before running the function passed to setTimeout.

What’s less intuitive is that the order is the same even with a timeout of 0ms.

window.setTimeout(() => console.log(2), 0);
// This logs 1, then 3, then (as soon as possible) logs 2

This is because setTimeout always gets pushed to the back of the queue—the specified wait time just tells JS the minimum time that has to pass before that code is allowed to run.

How do callbacks help?

Callbacks let us access values that may not be ready yet. Imagine ordering food in a takeaway. If you just get a pre-packaged sandwich they might be able to hand it to you straight away. This is “synchronous”—they can give you what you need then move on to the next person in the queue.

However if your food needs to be cooked you might give them your phone number, so they can text you when it’s ready. This is “asynchronous”—they can move on to the next person in the queue, and “call you back” to collect your food later.

Our addEventListener example from above can’t return the click event, since it hasn’t happened yet. The browser won’t know where the user clicked until the click happens. So instead we pass a callback that the browser will run for us when the user clicks somewhere. It calls this callback with the event object containing the info we need.

Mini-challenge 5

  1. Write a function asyncDouble that takes 2 arguments: a number and a callback
  2. It should use setTimeout to wait one second
  3. Then it should call the callback argument with the number argument multiplied by 2
  4. Call asyncDouble with 10 and a callback that logs whatever it is passed. You should see 20 logged after 1 second.
  5. Can you see why asyncDouble can’t just return the doubled value?
Toggle answer
function asyncDouble(num, callback) {
window.setTimeout(() => callback(num * 2), 1000);

asyncDouble(10, (x) => console.log(x));
// (after one second) logs `20`

asyncDouble takes two arguments: num (the number to be doubled) and callback, the function it should call with the doubled number once it’s ready.

It uses setTimeout to queue up a function to run in 1000ms. This function calls the callback argument and passes in the doubled number.

It can’t return the doubled number since JS code executes synchronously in order. We don’t have the result ready to return straight away—only after 1000ms passes.

(Obviously we could double a number synchronously—the timeout is for example’s sake. Imagine we’re sending a request over a slow Wi-Fi network and it takes a whole second to get the result back)


Let’s use callbacks to make some traffic lights. Download the starter files using the command at the top of this workshop. Open challenge/index.html in your editor.

  1. Inside the script tag write a function light that takes two parameters: a string and a callback
  2. It should wait 1 second, log the string and then call its callback parameter
  3. Use light to log each colour of a traffic light sequence, in order, followed by "finished"
    • e.g. It should log:
    with a 1 second pause before each colour

Hint: to runs async tasks one after the other remember that you can nest function calls.