Modern TypeScript A Practical Guide to Accelerate Your Development Velocity Ben Beattie-Hood Modern TypeScript A Practical Guide to Accelerate Your Development Velocity Ben Beattie-Hood Modern TypeScript: A Practical Guide to Accelerate Your Development Velocity Ben Beattie-Hood Melbourne, VIC, Australia (pbk): 9 -9722-3 (electronic): 9 -9723-0 This work is subject to part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Trademarked names, logos, and images may appear in this book. 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Paper in this product is recyclable About the Author��ix About the Technical Reviewer��xi : How TypeScript Came to Be��1 Introduction��1 History��1 The Problem��5 Scalable Velocity��6 Testing Allows Scale��6 Docs Allow Scale��7 Types Provide Both Testing and Docs��7 Summary��8 : Getting Started with the Developer Experience��9 Introduction��9 Environment and IDE��10 No-Frills TypeScript Setup��10 Debugging TypeScript��11 TypeScript Within a More Realistic Setup��13 Honorable Mentions��15 Summary��16 iii : TypeScript Basics��19 Introduction��19 Structural Typing��20 JavaScript Types Are Structural Contracts��24 Adding Explicit Types to JavaScript��25 Optionality��31 All The Sugar��33 Array and Object Destructuring, Spread, and Rest��33 Async��37 Generators��38 Inferred Types��39 Types, Automagically��39 Type Widening and Narrowing��43 Auto-narrowing, but Not Too Much��45 Type Assertions��49 Compile-Time Assertions��50 Runtime Assertions��59 any and unknown Keywords��65 Caution: Handle with Care��68 Parameterized Values��69 Index Signatures��71 Function Signatures��72 Constructor Signatures��74 A New Mental Model��75 Summary��78 : Classes��81 Introduction��81 Classes��82 Constructors��82 Access Modifiers��82 Fields��83 Getters and Setters��86 Methods��87 Inheritance��87 Implements��89 Static Modifier��92 Warning 1: Classes Are Not Types��93 Warning 2: Classes Can Cause Scope Bleed��95 Summary��98 : Computed Types��101 Introduction��101 Type Aliases��102 Using Type Aliases��105 Union Types��107 Intersection Types��115 Generic Types��118 Type Parameters��118 const Modifier on Generic Parameters��122 Generic Constraints��123 Inferred Type Keywords��125 typeof Type Inference Keyword��126 keyof Type Inference Keyword��128 Where’s the valueof Type Inference Keyword?��131 Utility Types��132 Record��132 Pick��133 Omit��135 Partial��138 Required��139 Readonly��140 Exclude��144 Extract��147 Parameters��147 ConstructorParameters��149 ReturnType��150 Conditional Types��152 extends Keyword��153 infer Keyword��155 Extracting Inferred Types��159 Distributive and Nondistributive Conditional Types��161 Conditional Types via Property Accessors��163 Mapped Types��164 Changing the Type of Fields��164 Adding and Removing Fields��169 Renaming Fields��171 Recursive Types��172 Recursion Within an Object Type��172 Recursion over a Tuple Type��174 Template Literals��177 Summary��179 : Advanced Usage��181 Introduction��181 Expect and IsEqual��182 Compute��185 JsonOf��189 Flatten��196 UrlParameters��198 UrlParameters with Optional Params��201 Further Reading��208 Summary��209 : Performance��213 Introduction��213 Reducing Inline Types��214 Reducing Inferred Types��217 Inline Caching Using Conditional Types��221 Reduce Intersections��223 Reduce Union Types��225 Partition Using Packages and Projects��225 Partitioning Using Packages��225 Partitioning Using Projects��226 Other Performance Tweaks��228 Debugging Performance Issues��228 Using the @typescript/analyze-trace NPM Package��229 Using a Trace Viewer Within Your Browser��230 Summary��231 vii : Build��235 Introduction��235 Compiler Options��236 Recommended tsconfig.json Settings��238 Other Options��246 Linting��247 Installing ESLint��247 Ideal Ruleset��248 Additional Strict Errors��253 Additional Strict Warnings��257 Removed Rules��261 Further Rules��267 JSX/TSX��267 Modules��274 Module Types Explained��274 Exports and Imports��278 How TypeScript Resolves Modules��282 Summary��286 : Wrap Up��289 Index��291 viii About the Author Ben Beattie-Hood is a principal software engineer and professional mentor with over 20 years of industry experience. He currently specializes in front-end technology, technical strategy, system design, and development and training in TypeScript, React, and related technologies. Ben is passionate about evolvable systems, about creating learning organizations, and about how ideas are formed and communicated. He has given a wide range of talks covering product development, event sourcing microservices, event storming practice, modern database internals, functional programming, front-end design and build, as well as coaching in TypeScript as a velocity tool. Ben lives with his beautiful wife and two children in Melbourne, Australia, where he loves to hike and travel.