book

Effective TypeScript

by Dan Vanderkam

October 2019

Intermediate to advanced

261 pages

6h 14m

English

O'Reilly Media, Inc.

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Who This Book Is ForWhy I Wrote This BookHow This Book Is OrganizedConventions in TypeScript Code SamplesTypographical Conventions Used in This BookUsing Code ExamplesO’Reilly Online LearningHow to Contact UsAcknowledgments
Item 1: Understand the Relationship Between TypeScript and JavaScriptItem 2: Know Which TypeScript Options You’re UsingItem 3: Understand That Code Generation Is Independent of TypesItem 4: Get Comfortable with Structural TypingItem 5: Limit Use of the any Type
Item 6: Use Your Editor to Interrogate and Explore the Type SystemItem 7: Think of Types as Sets of ValuesItem 8: Know How to Tell Whether a Symbol Is in the Type Space or Value SpaceItem 9: Prefer Type Declarations to Type AssertionsItem 10: Avoid Object Wrapper Types (String, Number, Boolean, Symbol, BigInt)Item 11: Recognize the Limits of Excess Property CheckingItem 12: Apply Types to Entire Function Expressions When PossibleItem 13: Know the Differences Between type and interfaceItem 14: Use Type Operations and Generics to Avoid Repeating YourselfItem 15: Use Index Signatures for Dynamic DataItem 16: Prefer Arrays, Tuples, and ArrayLike to number Index SignaturesItem 17: Use readonly to Avoid Errors Associated with MutationItem 18: Use Mapped Types to Keep Values in Sync
Item 19: Avoid Cluttering Your Code with Inferable TypesItem 20: Use Different Variables for Different TypesItem 21: Understand Type WideningItem 22: Understand Type NarrowingItem 23: Create Objects All at OnceItem 24: Be Consistent in Your Use of AliasesItem 25: Use async Functions Instead of Callbacks for Asynchronous CodeItem 26: Understand How Context Is Used in Type InferenceItem 27: Use Functional Constructs and Libraries to Help Types Flow
Item 28: Prefer Types That Always Represent Valid StatesItem 29: Be Liberal in What You Accept and Strict in What You ProduceItem 30: Don’t Repeat Type Information in DocumentationItem 31: Push Null Values to the Perimeter of Your TypesItem 32: Prefer Unions of Interfaces to Interfaces of UnionsItem 33: Prefer More Precise Alternatives to String TypesItem 34: Prefer Incomplete Types to Inaccurate TypesItem 35: Generate Types from APIs and Specs, Not DataItem 36: Name Types Using the Language of Your Problem DomainItem 37: Consider “Brands” for Nominal Typing
Item 38: Use the Narrowest Possible Scope for any TypesItem 39: Prefer More Precise Variants of any to Plain anyItem 40: Hide Unsafe Type Assertions in Well-Typed FunctionsItem 41: Understand Evolving anyItem 42: Use unknown Instead of any for Values with an Unknown TypeItem 43: Prefer Type-Safe Approaches to Monkey PatchingItem 44: Track Your Type Coverage to Prevent Regressions in Type Safety
Item 45: Put TypeScript and @types in devDependenciesItem 46: Understand the Three Versions Involved in Type DeclarationsItem 47: Export All Types That Appear in Public APIsItem 48: Use TSDoc for API CommentsItem 49: Provide a Type for this in CallbacksItem 50: Prefer Conditional Types to Overloaded DeclarationsItem 51: Mirror Types to Sever DependenciesItem 52: Be Aware of the Pitfalls of Testing Types
Item 53: Prefer ECMAScript Features to TypeScript FeaturesItem 54: Know How to Iterate Over ObjectsItem 55: Understand the DOM hierarchyItem 56: Don’t Rely on Private to Hide InformationItem 57: Use Source Maps to Debug TypeScript
Item 58: Write Modern JavaScriptItem 59: Use @ts-check and JSDoc to Experiment with TypeScriptItem 60: Use allowJs to Mix TypeScript and JavaScriptItem 61: Convert Module by Module Up Your Dependency GraphItem 62: Don’t Consider Migration Complete Until You Enable noImplicitAny

Content preview from Effective TypeScript

Chapter 2. TypeScript’s Type System

TypeScript generates code (Item 3), but the type system is the main event. This is why you’re using the language!

This chapter walks you through the nuts and bolts of TypeScript’s type system: how to think about it, how to use it, choices you’ll need to make, and features you should avoid. TypeScript’s type system is surprisingly powerful and able to express things you might not expect a type system to be able to. The items in this chapter will give you a solid foundation to build upon as you write TypeScript and read the rest of this book.

Item 6: Use Your Editor to Interrogate and Explore the Type System

When you install TypeScript, you get two executables:

tsc, the TypeScript compiler
tsserver, the TypeScript standalone server

You’re much more likely to run the TypeScript compiler directly, but the server is every bit as important because it provides language services. These include autocomplete, inspection, navigation, and refactoring. You typically use these services through your editor. If yours isn’t configured to provide them, then you’re missing out! Services like autocomplete are one of the things that make TypeScript such a joy to use. But beyond convenience, your editor is the best place to build and test your knowledge of the type system. This will help you build an intuition for when TypeScript is able to infer types, which is key to writing compact, idiomatic code (see Item 19).

The details will vary from editor to editor, ...