Getting Started with ES6 Arrow Functions in JavaScript

Getting Started with ES6 Arrow Functions in JavaScript

In this article, we will walk through the basics of ES6 arrow functions in javascript and discuss their benefits.

With ES6 JavaScript came many updates to the language including the spread operator, object destructuring, new type of variables, and more. On top of all those amazing features came arrow functions, a new and concise way to write functions.

Table of Contents

  • ES5 Functions
  • Your First ES6 Arrow Function
  • Removing Unnecessary Parenthesis
  • Implicit Return
  • Using Arrow Functions in Map and Filter
  • 'This' Binding with Arrow Functions
  • Wrap Up
ES5 Functions

Let's start with looking at how we defined functions with ES5 JavaScript. To do define a function, it required the function keyword. For example, if we wanted to define a function that would multiply a number by two, it would look something like this.

function multiplyByTwo(num){
    return num * 2;
}

We could also define the function and assign it to a variable if we wanted to.

const multiplyByTwo = function(num){
    return num * 2;
}

Regardless of which way you do it, the keyword function has to be included.

Your First ES6 Arrow Function

To create an arrow function, you don't need the keyword function. In fact, you basically remove that keyword and add an arrow right after the parameters but before the open curly bracket. It would look like this.

const multiplyByTwo = (num) => {
    return num * 2;
}

At this point, it doesn't look substantially different than the "old" way to do it, but we can make a few enhancements.

Removing Unnecessary Parenthesis

The parenthesis around the parameters are required if there are no parameters or more than one parameter. However, when your arrow function only has one parameter, you can leave out the parenthesis to simplify it a bit like so.

const multiplyByTwo = num => {
    return num * 2;
}
Implicit Return

Often times, we write functions that return after just one line of code. With the "old" way of writing functions, the number of lines in the function didn't affect how you defined the function. With arrow functions, it can.

If the only thing you want to do in a function is a one-line return, you can use *_implicit return *_to greatly simplify your function. While using implicit return, you don't need the curly braces or the return keyword. It would look like this.

const multiplyByTwo = num => num * 2;

One thing to think about it is that you can still use the implicit return syntax even if you don't necessarily need to return anything. In other words, if the callers of your function are not expecting it to return anything, then having it return something doesn't matter.

For example, if I just wanted to print something to the console, I could use implicit return to shorten the length of the function.

const printName = (first, last) => console.log(`${first} ${last}`);
Using Arrow Functions in Map and Filter

One of the most common places you'll see arrow functions used are with JavaScript Array methods like map, reduce, filter, etc. By use arrow functions with these methods, you can make complete array transformations in just one line.

Let's look at two examples, one with map and one with filter. For the map version, let's say we want to convert an array by multiplying each number by two. It would look something like this.

const twodArray = [1,2,3,4].map( num => num * 2);


Notice with this arrow function, I left off the parenthesis (because there's only one parameter) and used implicit return. This kept the entire transformation to one line!

Now, let's do another with filter. Let's say we want to filter all numbers that are not even.

const filteredArray = [1,2,3,4].filter( num => num % 2 == 0);


Again, no parenthesis and implicit return. Super quick to make array transformations with just one!

'This' Binding with Arrow Functions

The conversation around the _this _keyword is definitely intermediate JavaScript, so you might need to do a little bit of additional research for this section. Regardless, let's start with an example using an ES5 function definition inside of a person object.

const person = {
    first: "James",
    last: "Quick",
    getName: function() {
        this.first + " " + this.last
    }
}

In this case, we created a person object with a first and last name as well as a getName() function that returns the full name of the person. Inside of the function, we are trying to reference the first and last properties, by calling this.first and this.last.

When ES5 functions are defined in an object, 'this' refers to the object itself.

The reason we are able to access those properties through the this keyword, is that when those functions are defined inside of an object, it is automatically bound to the object itself. Therefore, with ES5 functions, we can still reference the object propreties by using 'this'.

Arrow functions don't bind anything to the keyword 'this'.

However, when you use arrow functions, things change a bit. Arrow functions don't do any binding for the keyword this. Therefore, if we were to change the function definition to be an arrow functions, things wouldn't work.

const person = {
    first: "James",
    last: "Quick",
    getName: () => {
        return this.first + " " + this.last
    }
}

In this case, undefined would be printed for both the first and last property. The reason is since the keyword 'this' is not bound to the person object, it doesn't have a first and last variable to refer to.

Understanding the difference between using this in arrow functions is really important!

Wrap Up

Arrow functions are one of many nifty little features of ES6 JavaScript. You will see them used more and more in examples and documentation, so it's worth learning how they work. Not to mention, they can significantly improve the conciseness and readability of your code!

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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:

Hmmm.

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?

No.

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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