IT DEPENDS

THE ANSWER IS IN THE TYPE.

type Answer<Q> =
  Q extends () => infer R
    ? R
    : "It depends";

IT
DEPENDS

TypeScript Principles, Debated

ELIRAZ KEDMI

Ask a senior developer any question about TypeScript, and you’ll get the same answer: ‘It depends.’ This book is about what it depends on.

A team of developers. Thirty arguments. Working code that puts their opinions to the test. From any versus unknown to reviewing AI-generated code, follow the debate as a neat rule meets a stubborn caller, a failing test or a requirement nobody mentioned. Sometimes the team reaches a verdict. Sometimes they still disagree. Either way, you see what changed their minds—and what didn’t.

Start reading: Chapter 1Paperback coming soon

Free to read online. Every example is checked against TypeScript 7.0.2.

How the arguments unfold

01

The Principle

A rule you can test, stated up front.

02

The Debate

The team argues it over working code.

03

The Turn

An example changes the conversation.

04

The Verdict / Debate Continues

A ruling—or a disagreement that remains.

05

Additional Takes

Other voices, costs and conditions.

Contents

Prologue

  1. Dedication
  2. Acknowledgments
  3. How to Read This Book
  4. Meet the Cast

Act I · The Type System

  1. Chapter 1: The any Problem
  2. Chapter 2: The Lies We Tell the Compiler
  3. Chapter 3: The Two Kingdoms
  4. Chapter 4: The Feature That Wasn't Just a Type
  5. Chapter 5: The Guard at the Gate
  6. Chapter 6: The Hole in the Function

Act II · Advanced TypeScript

  1. Chapter 7: The Fork in the Type
  2. Chapter 8: Who Owns the Fields?
  3. Chapter 9: Where Did This Event Come From?
  4. Chapter 10: The Call You Didn't Make
  5. Chapter 11: Some Assembly Required
  6. Chapter 12: The Same Object, Twice
  7. Chapter 13: Who Loaded It?

Act III · Patterns & Design

  1. Chapter 14: Two Seats or None
  2. Chapter 15: What the Number Means
  3. Chapter 16: What the Schema Does With the Rest
  4. Chapter 17: The Price You Were Given
  5. Chapter 18: The Ticket Hadn't Changed
  6. Chapter 19: Who Goes Next?
  7. Chapter 20: Before the First Line
  8. Chapter 21: The Message Kept Going
  9. Chapter 22: The Policy's Alibi
  10. Chapter 23: Still Waiting for No

Act IV · The Real World

  1. Chapter 24: Both Tests Were Green
  2. Chapter 25: The Other Money
  3. Chapter 26: It Can't Be Green
  4. Chapter 27: More Work, Less Time

Bonus · AI & TypeScript

  1. Bonus 1: Which File Says That?
  2. Bonus 2: Whose Refund Is It?
  3. Bonus 3: What Did the Fix Fix?

The cast

Fifteen people, one codebase, and very few agreements.

Prof. Eli TypeworthLanguage TheoristDaniel CompilerCompiler InternistGilad StacktraceSystems ArchitectLiron ClosureFP & Design MasterSahar FirstclassSimplicity ExpertNoam KipermanType Safety AbsolutistOded ShipleyVelocity AdvocateGil BenchmarkData-Driven AnalystEden LegacyMigration VeteranLinoy NightlyCutting-Edge AdvocateChen OverrideSocratic SkepticDafna FunctorFP PuristGuy SingletonOOP AdvocateDima BridgeBalanced PragmatistIdan GreenfieldParse-Don't-Validate Advocate