33 Concepts Every JavaScript Developer Should Know

Read the Full Guide • Concepts • Translations • Contributing

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This repository helps developers master core JavaScript concepts. Each concept includes clear explanations, practical code examples, and curated resources.

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Concepts

Fundamentals

• Primitive Types
  Learn JavaScript's 7 primitive types: string, number, bigint, boolean, undefined, null, and symbol. Understand immutability, typeof quirks, and autoboxing.

• Primitives vs Objects
  Learn how JavaScript primitives and objects differ in behavior. Understand immutability, call-by-sharing semantics, why mutation works but reassignment doesn't, and how V8 actually stores values.

• Type Coercion
  Learn JavaScript type coercion and implicit conversion. Understand how values convert to strings, numbers, and booleans, the 8 falsy values, and how to avoid common coercion bugs.

• Equality Operators
  Learn JavaScript equality operators == vs ===, typeof quirks, and Object.is(). Understand type coercion, why NaN !== NaN, and why typeof null returns 'object'.

• Scope and Closures
  Learn JavaScript scope and closures. Understand the three types of scope, var vs let vs const, lexical scoping, the scope chain, and closure patterns for data privacy.

• Call Stack
  Learn how the JavaScript call stack tracks function execution. Understand stack frames, LIFO ordering, execution contexts, stack overflow errors, and debugging with stack traces.

Functions & Execution

• Event Loop
  Learn how the JavaScript event loop handles async code. Understand the call stack, task queue, microtasks, and why Promises always run before setTimeout().

• IIFE, Modules & Namespaces
  Learn how to organize JavaScript code with IIFEs, namespaces, and ES6 modules. Understand private scope, exports, dynamic imports, and common module mistakes.

Web Platform

• DOM
  Learn how the DOM works in JavaScript. Understand how browsers represent HTML as a tree, select and manipulate elements, traverse nodes, and optimize rendering performance.

• Fetch API
  Learn how to make HTTP requests with the JavaScript Fetch API. Understand GET, POST, response handling, JSON parsing, error patterns, and AbortController for cancellation.

• Web Workers
  Learn Web Workers in JavaScript for running code in background threads. Understand postMessage, Dedicated and Shared Workers, and transferable objects.

Object-Oriented JavaScript

• Factories and Classes
  Learn JavaScript factory functions and ES6 classes. Understand constructors, prototypes, private fields, inheritance, and when to use each pattern.

• this, call, apply, bind
  Learn how JavaScript's 'this' keyword works and how to control context binding. Understand the 5 binding rules, call/apply/bind methods, arrow functions, and common pitfalls.

• Object Creation & Prototypes
  Learn JavaScript's prototype chain and object creation. Understand how inheritance works, the new operator's 4 steps, Object.create(), Object.assign(), and prototype methods.

• Inheritance & Polymorphism
  Learn inheritance and polymorphism in JavaScript — extending classes, prototype chains, method overriding, and code reuse patterns.

Async JavaScript

• Callbacks
  Learn JavaScript callbacks, functions passed to other functions to be called later. Understand sync vs async callbacks, error-first patterns

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