Variables and Datatypes in JavaScript

Variables and Datatypes in JavaScript

JavaScript provides set of data types to hold data such as String values, Decimal values and Boolean values. The data types depend on the values which are hold by the variable. The JavaScript data types acts as dynamically means same variable can be used as different types.

JavaScript provides set of data types to hold data such as String values, Decimal values and Boolean values. The data types depend on the values which are hold by the variable. The JavaScript data types acts as dynamically means same variable can be used as different types.

Datatypes in JavaScript

There are majorly two types of languages. First, one is Statically typed language where each variable and expression type is already known at compile time.Once a variable is declared to be of a certain data type, it cannot hold values of other data types.Example: C, C++,Java.

// Java(Statically typed)

int x = 5 // variable x is of type int and it will not store any other type.

string y = ‘abc’ // type string and will only accept string values

Other, Dynamically typed languages: These languages can receive different data types over time. For example- Ruby, Python, JavaScript etc.

// Javascript(Dynamically typed)

var x = 5; // can store an integer

var name = ‘string’; // can also store a string.

JavaScript is dynamically typed (also called loosely typed) scripting language. That is, in javascript variables can receive different data types over time. Datatypes are basically typed of data that can be used and manipulated in a program.

The latest ECMAScript(ES6) standard defines seven data types: Out of which six data types are Primitive(predefined).
Numbers: 5, 6.5, 7 etc.String: “Hello GeeksforGeeks” etc.Boolean: Represent a logical entity and can have two values: true or false.Null: This type has only one value : *null.*Undefined: A variable that has not been assigned a value is *undefined.*Object: It is the most important data-type and forms the building blocks for modern JavaScript. We will learn about these data types in details in further articles.
Variables in JavaScript:

Variables in JavaScript are containers which hold reusable data. It is the basic unit of storage in a program.
The value stored in a variable can be changed during program execution.A variable is only a name given to a memory location, all the operations done on the variable effects that memory location.In JavaScript, all the variables must be declared before they can be used.
Before ES2015, JavaScript variables were solely declared using the var keyword followed by the name of the variable and semi-colon. Below is the syntax to create variables in JavaScript:

var var_name;
var x;

The var_name is the name of the variable which should be defined by the user and should be unique. These type of names are also known as identifiers. The rules for creating an identifier in JavaScript are, the name of the identifier should not be any pre-defined word(known as keywords), the first character must be a letter, an underscore (_), or a dollar sign ($). Subsequent characters may be any letter or digit or an underscore or dollar sign.

Notice in the above code sample, we didn’t assign any values to the variables.We are only saying they exist.If you were to look at the value of each variable in the above code sample,it would be undefined.

We can initialize the variables either at the time of declaration or also later when we want to use them. Below are some examples of declaring and initializing variables in JavaScript:

// declaring single variable
var name;

// declaring multiple variables
var name, title, num;

// initializng variables
var name = "Harsh";
name = "Rakesh";

Javascript is also known as untyped language. This means, that once a variable is created in javascript using the keyword var, we can store any type of value in this variable supported by javascript. Below is the example for this:

// creating variable to store a number
var num = 5;

// store string in the variable num
num = "GeeksforGeeks";

The above example executes well without any error in JavaScript unlike other programming languages.

Variables in JavaScript can also evaluate simple mathematical expressions and assume its value.

// storing a mathematical expression
var x = 5 + 10 + 1;
console.log(x); // 16

After ES2015,we now have two new variable containers : let and const. Now we shall look at both of them one by one. The variable type Let shares lots of similarities with var but unlike var it has scope constraints. To know more about them visit let vs var. Let’s make use of let variable:

// let variable
let x; // undefined
let name = 'Mukul';

// can also declare multiple vlaues
let a=1,b=2,c=3;

// assignment
let a = 3;
a = 4; // works same as var.

Const is another variable type assigned to data whose value cannot and will not change throught the script.

// const variable
const name = 'Mukul';
name = 'Mayank'; // will give Assignment to constant variable error.

Variable Scope in Javascript

Scope of a variable is the part of the program from where the variable may directly be accessible.

In JavaScript, there are two types of scopes:
Global Scope – Scope outside the outermost function attached to Window.Local Scope – Inside the function being executed.
Let’s look at the code below. We have a global variable defined in first line in global scope. Then we have a local variable defined inside the function fun().

let globalVar = "This is a global variable";
function fun() {
  let localVar = "This is a local variable";


When we execute the function fun(), the output shows that both global as well as local variables are accessible inside the function as we are able to console.log them. This shows that inside the function we have access to both global variables (declared outside the function) and local variables (declared inside the function).Let’s move the console.log statements outside the function and put them just after calling the function.

let globalVar = "This is a global variable";
function fun() { 
  let localVar = "This is a local variable"; 


We are still able to see the value of the global variable, but for local variable console.log throws an error. This is because now the console.log statements are present in global scope where they have access to global variables but cannot access the local variables.

Also, any variable defined in a function with the same name as a global variable takes precedence over the global variable, shadowing it.To understand variable scopes in details in JavaScript, please refer to the article on understanding variable scopes in Javascript.

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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

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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