Effective TypeScript
This is the code sample repo for Effective TypeScript: 62 Specific Ways to Improve Your TypeScript. The book is available through:
and others.
You can get the latest updates on the book at https://effectivetypescript.com.
Code Samples
Below you'll find a complete table of contents with links to standalone code samples for each item. All code samples should produce the expected errors (and no others) and expected types.
Unless otherwise noted in a comment at the top of the code sample, the samples are run with TypeScript's strict
setting. These were produced and checked using literate-ts and TypeScript 3.7.0-beta. You may want to copy/paste them into the TypeScript playground.
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Chapter 1: Getting to Know TypeScript
๐ Item 1: Understand the Relationship Between TypeScript and JavaScript๐ Item 2: Know Which TypeScript Options Youโre Using๐ Item 3: Understand That Code Generation Is Independent of Types๐ Item 4: Get Comfortable with Structural Typing๐ Item 5: Limit Use of the any Type
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Chapter 2: TypeScriptโs Type System
๐ Item 6: Use Your Editor to Interrogate and Explore the Type System๐ Item 7: Think of Types as Sets of Values๐ Item 8: Know How to Tell Whether a Symbol Is in the Type Space or Value Space๐ Item 9: Prefer Type Declarations to Type Assertions๐ Item 10: Avoid Object Wrapper Types (String, Number, Boolean, Symbol, BigInt)๐ Item 11: Recognize the Limits of Excess Property Checking๐ Item 12: Apply Types to Entire Function Expressions When Possible๐ Item 13: Know the Differences Between type and interface๐ Item 14: Use Type Operations and Generics to Avoid Repeating Yourself๐ Item 15: Use Index Signatures for Dynamic Data๐ Item 16: Prefer Arrays, Tuples, and ArrayLike to number Index Signatures๐ Item 17: Use readonly to Avoid Errors Associated with Mutation๐ Item 18: Use Mapped Types to Keep Values in Sync
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Chapter 3: Type Inference
๐ Item 19: Avoid Cluttering Your Code with Inferable Types๐ Item 20: Use Different Variables for Different Types๐ Item 21: Understand Type Widening๐ Item 22: Understand Type Narrowing๐ Item 23: Create Objects All at Once๐ Item 24: Be Consistent in Your Use of Aliases๐ Item 25: Use async Functions Instead of Callbacks for Asynchronous Code๐ Item 26: Understand How Context Is Used in Type Inference๐ Item 27: Use Functional Constructs and Libraries to Help Types Flow
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Chapter 4: Type Design
๐ Item 28: Prefer Types That Always Represent Valid States๐ Item 29: Be Liberal in What You Accept and Strict in What You Produce๐ Item 30: Donโt Repeat Type Information in Documentation๐ Item 31: Push Null Values to the Perimeter of Your Types๐ Item 32: Prefer Unions of Interfaces to Interfaces of Unions๐ Item 33: Prefer More Precise Alternatives to String Types๐ Item 34: Prefer Incomplete Types to Inaccurate Types๐ Item 35: Generate Types from APIs and Specs, Not Data๐ Item 36: Name Types Using the Language of Your Problem Domain๐ Item 37: Consider โBrandsโ for Nominal Typing
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Chapter 5: Working with any
๐ Item 38: Use the Narrowest Possible Scope for any Types๐ Item 39: Prefer More Precise Variants of any to Plain any๐ Item 40: Hide Unsafe Type Assertions in Well-Typed Functions๐ Item 41: Understand Evolving any๐ Item 42: Use unknown Instead of any for Values with an Unknown Type๐ Item 43: Prefer Type-Safe Approaches to Monkey Patching๐ Item 44: Track Your Type Coverage to Prevent Regressions in Type Safety
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Chapter 6: Types Declarations and @types
๐ Item 45: Put TypeScript and @types in devDependencies๐ Item 46: Understand the Three Versions Involved in Type Declarations๐ Item 47: Export All Types That Appear in Public APIs๐ Item 48: Use TSDoc for API Comments๐ Item 49: Provide a Type for this in Callbacks๐ Item 50: Prefer Conditional Types to Overloaded Declarations๐ Item 51: Mirror Types to Sever Dependencies๐ Item 52: Be Aware of the Pitfalls of Testing Types
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Chapter 7: Writing and Running Your Code
๐ Item 53: Prefer ECMAScript Features to TypeScript Features๐ Item 54: Know How to Iterate Over Objects๐ Item 55: Understand the DOM hierarchy๐ Item 56: Donโt Rely on Private to Hide Information๐ Item 57: Use Source Maps to Debug TypeScript
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Chapter 8. Migrating to TypeScript
๐ Item 58: Write Modern JavaScript๐ Item 59: Use @ts-check and JSDoc to Experiment with TypeScript๐ Item 60: Use allowJs to Mix TypeScript and JavaScript๐ Item 61: Convert Module by Module Up Your Dependency Graph๐ Item 62: Donโt Consider Migration Complete Until You Enable noImplicitAny