Javascript ES6: Let vs. Var vs. Const

Javascript ES6: Let vs. Var vs. Const

Javascript ES6: Let vs Var vs Const. In this post you’ll learn three new ways to create variables in JavaScript (ES6), let, var and const. Along the way we’ll look at the differences between var, let, and const as well as cover topics like function vs block scope, variable hoisting, and immutability. `const` is a signal that the identifier won't be reassigned. `let` is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable will be used only in the block it's defined in, which is not always the entire containing function.

Javascript ES6

JavaScript ES6 (ES2015) brings new syntax and new awesome features to make your code more modern and more readable. It allows you to write less code and do more. ES6 introduces us to many great features like arrow functions, template strings, class destruction, Modules… and more. Let’s take a look.

The reason why ES6 is so important because it brought a radical change to the Javascript community and meaningful changes and features that allow millions of developers to improve their code.

You can check all the new features brought by ES6 at http://es6-features.org/.

Every year there is a new Javascript or ECMAScript version gets released with awesome new features and functionalities you could try or even use it on your ongoing projects. You can check more about how to work with the new versions (ESNext) using Babel take a closer look at their Docs.

Var Keyword

Var Keyword is the original keyword used to declare variables in javascript since the early versions it has some downsides which may cause bugs on your code that's why ES6 brought new keywords to the javascript word for better variables context control.

  • Can be re-declared.
  • Global Scope (any variable declared out of function scop can be accessed in any function on the global window).

This code is completely valid using the Var Keyword

var someVar = 1337;
//This will not give you an error cause Var supports redeclarations 
var someVar = 2000;

console.log(someVar); ///< 2000

Var Hoisting:

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution.

console.log("Hoisting: ", someVariable); ///< Hoisting: undefined
var someVariable;
Let Keyword

The Let keyword has been introduced in ES6 as a new way to declare variables which brought a better scope constraint and safe declarations of variables which is much safer than var.

  • Used Scope Variable Declaration
  • Recommended since ES2015 (ES6)
  • Two variables with the same identifier can be declared in different scopes
  • Cannot redeclare variables
  • Variables can be updated but not re-declared

This fact makes let a better choice than var. When using let, you don't have to bother if you have used a name for a variable before as a variable exists only within its scope. Also, since a variable cannot be declared more than once within scope, then the problem discussed earlier that occurs with var does not occur.

let someVar = 1337;
/*This will throw an exception since you can declare two variables using let with the same identifier at the same scope level */
let someVar = 2000;

console.log(someVar); ///< this code won't be reached

When using let you declaring two variables at different scope levels with the same identifier is not a redeclaration cause let operates at scope level.

let globalVar = 1500;

function localFunction() {
  let globalVar = 2000; ///< Completely different variable
  console.log("Inside: ", globalVar); ///< 2000
}

localFunction();

console.log("Outside: ", globalVar); ///< 1500

Let Hoisting:

Unlike var which is initialized as undefined, the **Let **keyword is not initialized. So if you try to use let variable before declaration you will get a reference error (variable not defined).

/* Unlike Var undefined referencing is not available using Let */
console.log("Hoisting: ", someVariable); ///< Throw reference exception error
var someVariable;
Const Keyword

Const Declaration Keyword has been introduced in ES6 to bring the ability to declare variables with constant values which can be set only within the declaration.

  • Const are constant values (which cannot be changed)
  • Can only be initialized in the declaration
  • Share some similarities with Let
  • Block Scoped (can only be accessed through the block it was declared on)
  • Cannot be re-declared or updated
  • Used for immutable pattern and constructing objects
const globalVar = 1500;

function localFunction() {
  const globalVar = 2000; ///< Different variable cause it is in a different scope 
  console.log("Inside: ", globalVar); ///< 2000
}

localFunction();

console.log("Outside: ", globalVar); ///< 1500

Let and Const are the same the only difference is Const declares variables with constant values which cannot be changed once declared, if you try to update const variable value you will get "TypeError: Assignment to constant variable".

const value = 1337;
value = 1500; ///< TypeError: Assignment to constant variable

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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