Skip to main content
This book is currently a work in progress. Content is being actively developed and refined.

​
You Don’t Know JS Yet: Types & Grammar - 2nd Edition

Welcome to the Types & Grammar book, part of the You Don’t Know JS Yet series. This book dives deep into JavaScript’s type system, exploring how values behave and interact in ways that many developers never fully understand.

​
What You’ll Learn

This book confronts the common misconception that “everything in JS is an object” and explores the core value types of JavaScript, specifically the non-object types called primitives.

Primitive Values

Explore the 7 primitive types: undefined, null, boolean, number, bigint, symbol, and string

Primitive Behaviors

Learn how primitives behave, their immutability, and type-specific operations

Object Values

Understand objects, arrays, functions, and the difference between primitives and objects

Coercing Values

Master type coercion, both explicit and implicit, and write type-aware code

​
Key Topics Covered

JavaScript doesn’t apply types to variables or properties (“container types”), but rather values themselves have types (“value types”). Learn the fundamental distinction and why it matters.
Master the typeof operator and understand its quirks, including why typeof null returns "object" (spoiler: it’s a historical bug that can’t be fixed).
Challenge the common belief that coercion is “bad” and learn when and how to use it effectively. Discover why == might be better than === in many cases.
Understand how JavaScript represents numbers using IEEE-754 double-precision floating-point format, and learn to handle precision issues.
Navigate the complexities of Unicode, including surrogate pairs, grapheme clusters, and why string length calculations aren’t always straightforward.

​
Why This Matters

Type-aware programming is always better than type-ignorant programming.Contrary to popular belief, embracing a dynamically-typed language does NOT mean being careless about types. Whether you use TypeScript or vanilla JavaScript, understanding how types actually work is essential.

​
About This Book

This is the second edition of “Types & Grammar,” completely rewritten to reflect modern JavaScript and deeper insights into the language’s type system. The book takes a controversial stance: coercion isn’t the enemy. With proper understanding, JavaScript’s type system becomes a powerful tool rather than a source of bugs.
This book is part of a larger series that covers JavaScript comprehensively:
  • Get Started - Your foundation
  • Scope & Closures - How variables work
  • Objects & Classes - Object-oriented programming
  • Types & Grammar - This book
  • Sync & Async - Asynchronous programming
  • ES.Next & Beyond - Future JavaScript

​
Getting Started

Ready to challenge your assumptions about JavaScript’s type system? Start with Chapter 1: Primitive Values and prepare to see JavaScript in a whole new light.

Read the Foreword

Start with the foreword to set the context

Jump to Chapter 1

Dive right into primitive values

Build docs developers (and LLMs) love

Get started for freeTalk to us