TypeScript

Types on top of JavaScript, from annotations to the narrowing that makes the compiler useful rather than noisy.

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In this course

Types and inferenceWhat you annotate, what the compiler works out for itself, and why any is a hole.8 min
Unions and narrowingA value that is one of several things, and how to prove which one it is.9 min
Interfaces and type aliasesTwo ways to name a shape, and the one difference that decides which to use.9 min
GenericsWriting a function once without throwing away what it was given.9 min
Promises and asyncWork that finishes later, and the try/catch that never catches.9 min
Strict modeThe compiler flags that turn TypeScript from decoration into a safety net.9 min
Narrowing and type guardsThe checks the compiler understands, writing your own with `x is T`, and why a guard is only as honest as its body.9 min
unknown, any and neverThe top type you must prove, the escape hatch that spreads, and the empty type that makes exhaustiveness checks work.10 min
Structural typing, intersections and extra propertiesWhy a value with more properties than its type fits anyway, when excess property checks fire, and how intersections combine shapes.9 min
Enums, and what they compile toOne of the few TypeScript features that exists at runtime, the reverse mapping that surprises people, and the plain-object alternative.9 min
as const, satisfies and type assertionsKeeping literal types, checking a value without widening it, and why `as` is a promise rather than a conversion.9 min
Classes, access modifiers and abstractpublic, private, protected and readonly, how TypeScript privacy differs from a JavaScript10 min
Utility types in practicePartial, Readonly, Pick, Omit, Record and the types you derive from functions, with the gaps each one leaves at runtime.10 min
Mapped and conditional typesTypes computed from other types — mapping over keys, renaming them, branching with extends and pulling pieces out with infer.11 min
Modules, declaration files and declaration mergingWhat a .d.ts file is, how TypeScript finds the types for an import, why type-only imports vanish, and how merging lets you extend someone else's types.11 min
Typing async results and failuresWhat Promise<T> does and does not say about failure, narrowing allSettled results, return versus return await, and errors as values.11 min