ES6 Cool stuffs — A big fat Arrow

Arrow function is considered as one of most popular ES6 features. Have you ever wonder why and how to use it? If so, you got to the right place. Let our journey of understanding arrow function expression begin.

As usual, let’s do a quick brief on old concepts about function in JS for the start.


Function in JS

Every function in JS (before ES6) can be declared with function operator syntax

function HelloWorld(){ console.log('Hello World');}
HelloWorld(); //'Hello World'

Or can be declared as non-method functions using function key word inside an expression

let sayHello = function() { console.log('Hi there, do you come here often? ')

Or by using Function constructor directly with new syntax (can’t do without)

let sayHello = new Function(`console.log('Hello World');`);
sayHello(); //'Hello World';

Reminder: Function is global-scoped object. In reality, using Function constructor to define function is totally not recommended, due to performance issues similar to eval Why so? Because

Each time eval or the Function constructor is called on a string representing source code, the script engine must start the machinery that converts the source code to executable code

Source: Opera Team

And that costs heavily! After all, we all know eval equals to evil, don’t we?

Moreover, function can be used as constructors to achieve object-oriented prototypical inheritance. Sound familiar? Indeed, that was how we did it until class is introduced.

In short, the classical way of defining/declaring function is quite straightforward. Hence comes the question — so why do we need another additional way to achieve this? Is it because we want to make JS more complicated to scare off the newbies?

😆 Well, absolutely not. In order to answer the question, let’s have a closer look on the new feature itself, shall we?


Arrow function expression

Basic

=> , or “fat” arrow expression, is a feature inspired from CoffeeScript’s arrow . It is a new, shorter and more concise syntax (less verbose) than the classical function expression, using the following basic format:

//Several parameters
(param1, param2, ..., paramN) => { //statements }
//No parameter
() => { //statements }

So our example above can be re-written as:

let HelloWorld = () => { console.log(‘Hello World’) }

Note:

  • No line break between parameter definition () and =>
let sumUp = (a, b) => a + b //OK
let sumUp = (a, b)
=> a + b                    //NOT OK
let sumUp = (a, b) =>
a + b                       //OK
  • If the function contains not more than one code line — we can make it even shorter by eliminating {}:
let HelloWorld = () => console.log('Hello World')
  • If it contains only return statement, we can get rid of return syntax also
let sayMyName = () => `Maya Shavin`
sayMyName() //"Maya Shavin"

BUT wait — there is an exception here, returning object literals need to be wrapped in parenthesis in order to indicate the return object as function body expression, not a block of code.

let getObject = () => { greet: function(){ console.log('hi') }}
getObject() //SyntaxError: Unexpected token '('
let getObject2 = () =>({ greet: function(){ console.log('hi') }})
getObject2().greet() //hi
  • If there is ONLY ONE parameter, we can absolutely say goodbye to ()
let printName = name => console.log(name)

Awesome. Now what else is interesting?


Binding 'this'

Let’s look at the following example:

let WaitAndSee = {
    counter: 1,
    wait: function(){
        let numbers = [1,2,3,4]
        numbers.map(function(x) {
             this.counter++
        })
    },
    see: function(){
        console.log(`Counter: ${this.counter}`)
        console.log(`Window counter: ${window.counter}`}
}

What will happen in the following code?

window.counter = 1
WaitAndSee.wait()
WaitAndSee.see()

Classic, isn’t it? In the wait call, this referred to either window global object (non-strict mode) or undefined (strict mode), but not to WaitAndSee. Hence in strict mode, there will be an TypeError thrown:

while in non-strict mode, only the property counter of window will be updated, which means:


Confusing ? Indeed. Because in classical function, this is determined dynamically by how the function is called. To overcome this problem requires kind-of-a trick — assign this to a variable, normally is named as self, that, etc… . Or we can also use Function.prototype.bind() to ensure this is bind correctly. However, neither way is pretty enough to make our developers’ life easier 😫.

Fortunately, arrow function doesn’t define its own this. The value of this is determined by the outer scope in which arrow functions are used. Thus if you wish to get the right this, just need to make sure:

  • Trigger non-arrow function by proper syntax in order to receive a meaningful this value from their caller.
  • Use arrow function for everything else

So the above example can be written as below and will yield the correct result:

let WaitAndSee = {
    counter: 1,
    wait: function(){
        let numbers = [1,2,3,4]
        numbers.map(x => this.counter++)
    },
    see: function(){
        console.log(this.counter)
    }
}
WaitAndSee.wait()
WaitAndSee.see() //5

Cool? Wait, there’s more


No ‘arguments’ object

Remember the almighty arguments parameter in function?


However, unfortunately (or fortunately?) there is no arguments defined explicitly for arrow function. That says

let showArguments = () => console.log(arguments)
showArguments() //ReferenceError: arguments is not defined

Instead, if you still wish achieve the same effect like arguments, using ... syntax as rest parameters is a good alternative.

let showArguments = (...arguments) => console.log(arguments)
showArguments() //[]

Important note: Rest parameters are not the same as arguments object, as it is an instance of Array with full array functionalities, while arguments is just array-like object.

OK, that’s clear. How about..


Using as constructor?

Arrow function is, by definition, type of function expression. Therefore, it can’t be used as function declaration:

function WaitAndSee(){...} //OK
let WaitAndSee = () => {} //OK
WaitAndSee() => {} //Wrong syntax

And it can’t be used as constructor for object-oriented implementation — yes, it also mean new syntax can not be used together with it.


And certainly, there is no prototype !

let WaitAndSee = function(){}
console.log(WaitAndSee.prototype) //{constructor: f}
let WaitAndSee2 = () => {}
console.log(WaitAndSee2.prototype) //undefined

Oh, and don’t even think of using super() inside arrow function. It simply won’t work as you expected 🙀!


Non-method function

Theoretically arrow function can be used as method function. However, it’s totally not recommended, due to the fact that it doesn’t have its own this. So don’t be surprised if using it as method function can result in unexpected behavior and cause unwanted 🐛, for instance:

let WaitAndSee = {
    counter: 1,
    wait: () => { this.counter++ },
    see(){
        console.log(this.counter)
        console.log(window.counter)
    }
}
window.counter = 1
WaitAndSee.wait()
WaitAndSee.see() // 1, 2 - window.counter is updated instead!

So far so good? Great. Lets get back to our original question — what is the benefits of using arrow function?


Pros

One of the most obvious benefits of using arrow function is shorter and less verbose syntax. Tell me honestly, did you ever mistype function to fucntion or even fucntoin as a result of fast typing? For me, I did 🐼. Hence arrow function is definitely a savior for me when I need to type a lot of code lines.

For example, instead of

let request = fetch("")
              .then(function(response){...})
              .then(function(result){...})
              .then(function(data){...})
              .catch(function(error){...})

it can be written as

let request = fetch("")
             .then(response => {...})
             .then(result => {...})
             .then(data => {...})
             .catch(error => {...})

Less code, less potential 🐛!

In addition, an important benefit is the lexical binding of this — value is decided depends on its surrounding scope, not by how it is called.

This improvement surely simplifies function scope, prevents a lot of unwanted bugs 🐛 and again, hereby reduces the amount of code needed for a workaround fix 😆.

After all, who wants to write Object.method.bind(this) while you have a better alternative?

OK. That’s pretty much the main benefits of ES6 Arrow function. What about the disadvantages/pitfalls? Is there any? Let’s find out.


Cons

As some developers may argue, sometimes less code doesn’t mean more readable. Thus being a shorter syntax is also, arguably, its disadvantage. Too short can cause confusing, for instance

let getMyName = () => `Maya Shavin`

In the first impression, it’s a bit difficult for developer who is not familiar with CoffeeScript to understand that this function will return a string. We are all used to return syntax when the function supposes to return something, aren’t we?

Secondly, arrow => expression is pretty loosely-bind — aka in case there is a conflict with other operators, normally => will lose first. Hence it requires developers to be more careful in using it.

Lastly, arrow => function can’t be used as constructor. Instead, we have class as proper replacement for classic function constructor.


Conclusion

As stated in my other articles, most of ES6 new features are just syntactic sugar for function expression. Arrow function is no exception and has mostly no significant improvement towards performance compared to classical function.

To my opinion, besides the short and more concise syntax, its most significant benefit is to provide a better solution regarding this scope binding. Finally developers can enjoy cleaner, more elegant and less workaround code 😄.

But again, don’t use it just because of its coolness or benefits. You always need to understand also what use cases and feature’s pitfalls are in order to write good code 😉. Happy coding and always remember KISS, everyone!

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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