What are Higher-Order Functions in JavaScript?

What are Higher-Order Functions in JavaScript?

Have you heard your peers talk about Higher-Order functions in JavaScript? Don’t worry if you are lost in the conversation and are unsure about what it is. In this post, we will learn about Higher-Order functions, and why they are useful.

Have you heard your peers talk about Higher-Order functions in JavaScript? Don’t worry if you are lost in the conversation and are unsure about what it is. In this post, we will learn about Higher-Order functions, and why they are useful.

Let’s get the definition out of the way:

A function that accepts and/or returns another function is called a higher-order function.
It’s higher-order because instead of strings, numbers, or booleans, it goes higher to operate on functions.

A Higher-Order function is a function that receives a function as an input argument or returns a function as output. This is a concept that was born out of functional programming. This maybe an alien concepts for those who are more used to the Object Oriented Programming world. JavaScript, is a language that has started to use a lot of the functional programming concepts lately. Higher-Order functions is one of them.

Why Higher Order Functions?

Before we get started to learn something, we need to understand why that is important. You maybe curious, what purpose do higher-order functions serve?

Simple and Elegant Code

Higher-Order functions allow you to write simple and clean code. It allows you to write smaller functions, that do only on thing. This kind of composition results in simple, readable code.

Fewer Bugs

With simple and elegant code you end up with fewer bugs. Trust me on this one.

Easy to Test and Debug

With functions that do only one thing, you end up with code that is easy to test. Testable code, results in fewer bugs. Therefore debugging these simple functional units is also easy.

Built-in Higher Order Functions in JavaScript

JavaScript comes with some built-in higher-order functions. You may already be using them, without realising that they are higher-order functions. Let’s take a look at some of them, to understand how they work.


The map*() *method creates a new array with the results of calling a provided function on every element in the calling array. What this means is map() calls a provided callback function once for each element in an array, in order, and constructs a new array from the results.

The callback accepts three arguments:

  • value of the element
  • index of the element
  • array object

You may have used the *map() *function before. It qualifies as a higher-order function, because it takes in a *callback *function as an input argument.

var numbers = [1, 4, 9];

var doubles = numbers.map(function(num) {

  return num * 2;



// doubles is now [2, 8, 18]

// numbers is still [1, 4, 9]

In the example above, we have an array of numbers and creating a new array using the *map(). *The *map() *takes a function as an argument. The argument num within the function will automatically be assigned from each element of the array as map() loops through the original array.


The *filter() *method is another example of an in-built higher-order function. It creates a new array with all the elements that pass the test provided by a callback function. It also takes in a function as an argument, hence making it a higher-order function. The callback function passed to the *filter() *method accepts three arguments:

  • value of the element
  • index of the element
  • array object

Array elements which do not pass the callback test are simply skipped, and are not included in the new array.

Let’s take a look at an example that shows filter() in action.

function isAboveMyRange(value) {
  return value >= 25;
var filtered = [12, 5, 8, 130, 44].filter(isAboveMyRange);
// filtered is [130, 44]

The example is used to find values greater than 25 and filter the array. The values that don’t pass this test, will not be a part of the filtered array. The* filter()* function takes the isAboveMyRange function as an input parameter.


Another built-in higher-order function in JavaScript is the reduce() method. It executes the callback function on each member of the calling array, and results in a single output value. The *reduce() *method takes in two input parameters:

  • The reducer *callback *function (making this method a higher-order function)
  • Optional initial value
arr.reduce(callback[, initialValue])

The reducer function (callback) accepts four parameters:

  • accumulator
  • currentValue
  • currentIndex
  • sourceArray

If an *initialValue *is provided, then the accumulator will be equal to the *initialValue, *and the currentValue will be equal to the first element in the array. Suppose no initialValue is provided, then the accumulator will be equal to the first element in the array and the *currentValue *will be equal to the second element in the array. Let’s try to understand this better with a simple example.

var sum = [0, 1, 2, 3].reduce(function (accumulator, currentValue) {
  return accumulator + currentValue;
}, 0);
// sum is 6

In this example, we have passed an initialValue of zero, this is assigned to the *accumulator *in the beginning. Every time the reduce() function is called on each value in the array, the accumulator keeps the result of previous operation returned from the function, and the currentValue is set to the current value of the array. In the end the result is stored in the sum variable.

You can write your own higher-order functions

We saw some examples of in-built higher-order functions that come with JavaScript. But that’s not all. You can always create your own higher-order functions based on your needs.


In this article, we learned what higher-order functions are and we also learned some in-built higher-order functions in JavaScript. You may have already been writing higher-order functions or using them, without realising their significance.

Higher-order functions are just like regular functions, but they can accept a function as an argument and/or return a function as an output.

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JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at https://medium.com

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:


First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?


I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI

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