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Guide · Open source

How to use 33-js-concepts: a guide to 33 core JavaScript concepts

Updated · written with AI from the project's own documentation and checked against our catalogue data

33-js-concepts is an MIT-licensed open-source repository that lists 33 core JavaScript concepts, from primitive types and closures to the event loop, Promises and async/await. Each entry links to an explanation and examples on the companion site, 33jsconcepts.com. This guide shows you how to get the repository, how to read the material in a sensible order, and which topics it covers, so you can fill gaps in how the language works beneath everyday code.

33 JS Conceptsleonardomso/33-js-concepts · ★ 66.5k · MIT

Quick start

  1. 1Check your fit. Confirm you already know basic JavaScript, since the entries assume some familiarity with the language.
  2. 2Clone the repository. Run the git clone command from the README to get a local copy of the project.
  3. 3Open the concept list. Read the Concepts section of the README to see all 33 topics grouped by theme.
  4. 4Pick a starting section. Start with Fundamentals, or jump to Scope and Closures and the Event Loop if those are your gaps.
  5. 5Read the full explanation. Follow the linked entry to 33jsconcepts.com for the explanation and examples.
  6. 6Cross-check with a reference. Compare difficult topics with javascript.info or MDN Web Docs for a second explanation.

What is 33-js-concepts and who is it for?

The repository is a curated list of JavaScript concepts. Its README describes it as a way for developers to master core JavaScript concepts, with clear explanations, practical code examples and curated resources for each one.

It suits developers who already know basic JavaScript and want to fill gaps in fundamentals, and students preparing for interviews or courses. It is less suited to complete beginners, because the entries assume some familiarity with the language. It also does not cover React or Angular, since the topics are general JavaScript.

  • Good fit: developers who want to understand the language beneath everyday usage
  • Good fit: readers who prefer short explanations with links to deeper material
  • Not a fit: anyone without programming background
  • Not a fit: readers who want exercises, a hands-on project or an interactive course

How do you get 33-js-concepts on your computer?

The project is read online. The README gives one command to get a local copy of the repository. Prerequisites are not listed in the source.

Cloning gives you the repository files, but the explanations themselves live on the companion website, so you will usually read them there.

git clone https://github.com/leonardomso/33-js-concepts

How should you read the 33 concepts?

The README groups the concepts into sections: Fundamentals, Functions & Execution, Web Platform, Object-Oriented JavaScript, Async JavaScript, Functional Programming and Advanced Topics. Reading in that order moves from primitive types and scope towards the event loop, Promises and async/await.

If you are short on time, focus on the topics most interviews and courses cover: scope and closures, this, the event loop, Promises and async/await. Each entry has a short description on the README, so you can skim the list and jump to the concepts you are unsure about.

  • Fundamentals: Primitive Types, Primitives vs Objects, Type Coercion, Equality Operators, Scope and Closures, Call Stack
  • Functions & Execution: Event Loop, IIFE, Modules & Namespaces
  • Web Platform: DOM, Fetch API, Web Workers
  • Object-Oriented JavaScript: Factories and Classes, this, call, apply, bind, Object Creation & Prototypes, Inheritance & Polymorphism
  • Async JavaScript: Callbacks, Promises, async/await, Generators & Iterators
  • Functional Programming: Higher-Order Functions, Pure Functions, map reduce filter, Recursion, Currying & Composition
  • Advanced Topics: JavaScript Engines, Error Handling, Regular Expressions, Modern JS Syntax, ES Modules, Data Structures, Algorithms & Big O, Design Patterns, and more

What do the entries cover?

Each entry lists the points it explains. For example, the Event Loop entry covers the call stack, task queue and microtasks, and why Promises run before setTimeout(). The Scope and Closures entry covers the three types of scope, var vs let vs const, and the scope chain.

Use these summaries to check whether a topic answers your question before you open the full explanation.

  • Event Loop: call stack, task queue, microtasks, Promises before setTimeout()
  • Equality Operators: == vs ===, Object.is(), why NaN !== NaN
  • this, call, apply, bind: the 5 binding rules and arrow functions
  • Promises: creating, chaining and combining Promises, handling errors
  • async/await: try/catch error handling and parallel execution with Promise.all

Which alternatives should you compare?

The repository is one of several resources for learning JavaScript fundamentals. The source names You Don't Know JS, javascript.info and MDN Web Docs as comparable projects. Read a concept here, then check the same topic in one of these if you want a second explanation or more depth.

Choose based on how you learn: a short concept list with links suits a quick review, while a longer book or reference suits a deep dive.

What people use it for

  • Filling gaps in closures, this and the event loop before a code review
  • Preparing for a JavaScript interview by skimming the 33 topics
  • Working through a course in a structured order from primitives to async code
  • Refreshing how Promises and async/await behave before writing asynchronous code

Frequently asked questions

How do I install 33-js-concepts?

The project is read online. The README gives the command git clone https://github.com/leonardomso/33-js-concepts to get a local copy, and the full explanations are on 33jsconcepts.com.

Is 33-js-concepts free to use?

The repository is licensed under the MIT licence. The source does not mention any paid tier for the companion website.

What are the prerequisites for 33-js-concepts?

The source lists no prerequisites. The entries assume you already know some JavaScript, so basic familiarity is needed to get value from them.

Is 33-js-concepts good for beginners?

Not really. The project is aimed at developers who know the basics and want to understand core concepts more deeply. Complete beginners may find it hard to follow.

What are alternatives to 33-js-concepts?

The source names You Don't Know JS, javascript.info and MDN Web Docs as comparable projects. These cover similar fundamentals in more depth or as reference material.

Does 33-js-concepts include exercises or framework guidance?

No. The README points to a website for the content rather than exercises, and the topics are general JavaScript rather than React or Angular.

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Our verdict

33-js-concepts is a good fit for developers who know basic JavaScript and want a structured, short-form review of fundamentals like scope, this, the event loop and async/await. Skip it if you are a complete beginner, want hands-on exercises, or need framework-specific guidance, and consider javascript.info, MDN Web Docs or You Don't Know JS for more depth.

Install options & downloads