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A beginner-friendly introduction to JavaScript covering what it is, how it works, and how to set up your JavaScript environment
Co-Founder, @CreoWis | Teacher, @tapaScript | Founder, @ReactPlay | YouTuber | Writer | Human
The async vs defer breakdown is one of those things experienced devs still get wrong. Really glad you covered it in Day 01 rather than burying it later.
One thing that tripped me up early on: the difference between how defer guarantees execution order (scripts run in document order) while async doesn't. That matters a lot when you have a utility library loaded before your app script. Would be great if a later day covers that execution ordering in depth.
Following the 40 Days series — love the structured approach!
Welcome to the 40 Days of JavaScript series! 🚀 There are 101 ways of learning something. But nothing can beat the “structured” and “progressive” learning methodologies. I have designed this playlist for you to stay structured and motivated. 💪🏼
Learn how JavaScript variables work, including let vs const, primitive and reference types, pass by value, and how JavaScript stores data in memory.
Traditional SSR is great, but it has a massive buffering problem. Let's look into the SSR Streaming and learn how it could be a game changer.
Learn how to build maintainable, reusable forms in React using design patterns like custom hooks, compound components, and context for scalable CRUD
Learn how JavaScript variables work, including let vs const, primitive and reference types, pass by value, and how JavaScript stores data in memory.
Learn how the promises are handled internally using event loop. This concept is essentials to debug and work with async programming.
GreenRoots Blog - A Blog by Tapas Adhikary
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If you’ve ever wondered how buttons become clickable, how animations react to user actions, or how websites feel alive, JavaScript is the answer.
This article is based on Day 01 of the 40 Days of JavaScript journey, where we lay the foundation for learning JavaScript in a structured, confidence-building way, from absolute basics to real-world usage.
By the end of this lesson, you’ll understand:
What JavaScript is and why it matters even today
How the 40 Days of JavaScript learning journey is structured
How JavaScript works on the browser and the server
How to set up your JavaScript development environment
Different ways to run JavaScript (browser, HTML, Node.js)
How script, async, and defer really work
Why script loading order matters for performance and correctness
If you prefer learning visually, this video guides you through every concept step by step, featuring live coding and demos.
JavaScript is the programming language that brings interactivity to the web.
It enables:
Button clicks and form interactions
Animations and dynamic UI updates
Real-time behaviour on web pages
Complex applications on boththe browser and server
Today, JavaScript is used to build:
Websites and web apps
Backend servers using Node.js
Mobile applications
Games and AI-powered products
In short, JavaScript is ubiquitous, and learning it opens up many doors.
This is not a daily-video challenge.
Instead:
You can take the sessions at your own pace
Each lesson includes tasks and assignments
Concepts start simple and gradually become advanced
Learners are encouraged to pause, practice, and reflect
JavaScript cannot be rushed.
You don’t “finish” it, you build confidence over time.
That’s why this journey focuses on:
Consistency over speed
Structure over randomness
Practice over passive watching
Many developers start strong but drop off midway due to a lack of structure.
To solve this, a free progress tracker is provided that you can add to your Notion workspace. With it, you can:
Track each day’s lesson
Add learning dates and video links
Record key takeaways
Maintain notes for revision
Track task completion and GitHub links
This tracker helps you build a habit, not just consume content.
JavaScript is a programming language that runs in two major places:
Adds interactivity to web pages
Works with HTML (structure) and CSS (styling)
Handles user actions like clicks, scrolls, and inputs
Handles data, APIs, authentication, and logic
Runs JavaScript outside the browser
This dual nature makes JavaScript extremely powerful and versatile.
JavaScript was created in 1995 in just 10 days, yet it continues to dominate web development.
Some major milestones:
ES5 standardised JavaScript features
ES6 (2015) introduced modern syntax like let, const, classes, and modules
Async/Await simplified asynchronous programming
JavaScript expanded into servers, frameworks, and AI tooling
JavaScript didn’t stagnate; it evolved continuously, and it’s still evolving today.
To begin coding, you need just three things:
Visual Studio Code is recommended for its simplicity and power.
Any modern browser works, but Chrome/Safari is commonly used. You’ll rely heavily on Developer Tools, especially the Console.
Node.js allows JavaScript to run outside the browser. You’ll use it more in later sessions.
You can instantly run JavaScript inside DevTools:
console.log("Hello JavaScript");
This confirms JavaScript is working in your browser.
A more realistic setup involves:
Writing HTML for structure
Writing JavaScript in a separate .js file
Linking them using the <script> tag
This introduces the principle of separation of concerns:
HTML for structure
CSS for styling
JavaScript for behaviour
Script loading order matters.
If JavaScript runs before HTML elements exist, it can cause errors.
async: Downloads in parallel and executes immediately
Best for independent third-party scripts (analytics, chat widgets).
defer: Downloads in parallel but executes after HTML parsing
Best for application code that interacts with the DOM.
Rule of thumb:
Use defer for your scripts.
Use async for external scripts.
JavaScript can also run outside the browser using Node.js:
node script.js
This allows JavaScript to power backend services and full-stack applications, a topic explored later in the journey.
All code demonstrated in this session is available in the GitHub repository linked below.
In Day 02, we’ll cover:
Variables (var, let, const)
Values and data types
Pass by value vs pass by reference
Concepts that directly impact React and modern frameworks
Let’s build JavaScript confidence. One solid step at a time 🚀
Liked it? Please let me know with your comments and likes. ❤️
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