TypeScript, by example

TypeScript through annotated examples - the type system made practical.

30 lessons · Updated 2026-07-08

01
Hello, WorldRunning TypeScript with tsx and compiling with tsc - the two modes every TypeScript project uses.
20
Basic TypesTypeScript's scalar vocabulary: string, number, boolean, null, undefined, symbol, bigint, any, and unknown.
30
The never TypeUsing never for unreachable code, exhaustive switches, and filtering impossible union branches.
35
Filtering Unions with neverReturning never from a conditional type to remove members from a union.
40
Type InferenceHow TypeScript infers types from assignments: const vs let widening, as const, and function return types.
50
InterfacesDeclaring object shapes with required, optional, readonly, and extended interface fields.
60
Type AliasesNaming primitive, object, union, and intersection shapes with the type keyword.
70
Advanced Type AliasesGeneric, recursive, and template literal type aliases for reusable type-level shapes.
80
Union TypesUnion, intersection, and literal unions, the building blocks for TypeScript variant modeling.
90
Discriminated UnionsModeling object variants with one shared literal field that TypeScript can narrow.
100
Designing Tagged ShapesWhy loose object shapes do not narrow, and how to parse unknown JSON into strict tagged unions.
110
GenericsType parameters for functions, interfaces, and classes that preserve the caller's exact type.
120
Generic ConstraintsConstraining type parameters with extends and adding default type parameters for ergonomic APIs.
130
FunctionsTyped function parameters, return annotations, optional parameters, defaults, and rest parameters.
140
Function Types and OverloadsTyping callbacks, higher-order functions, stored function references, and overload signatures.
150
ClassesConstructor parameter shorthand, access modifiers, readonly fields, and abstract classes.
160
Class ContractsHaving classes implement interfaces and checking that required methods are present.
170
Private FieldsThe difference between TypeScript private fields and ECMAScript
180
EnumsNumeric and string enums, reverse mapping, runtime emit, and why string values are safer.
190
Const Enums and AlternativesConst enum inlining, package-boundary hazards, and string literal unions as the usual alternative.
200
Mapped TypesBuilding new object types by transforming every key of an existing type.
210
Key RemappingRenaming and filtering keys inside mapped types with as, template literals, and never.
220
Utility TypesBuilt-in helpers like Partial, Required, Readonly, Record, Pick, Omit, Exclude, and Extract.
230
Function Utility TypesDeriving function, constructor, and async payload types with ReturnType, Parameters, InstanceType, and Awaited.
240
NarrowingControl-flow analysis with typeof, instanceof, in, equality, truthiness, and assignment narrowing.
250
Type Guards and AssertionsUser-defined type guards, runtime validation for unknown data, assertion functions, and exhaustive never checks.
260
ModulesES module import/export in TypeScript, import type, and barrel re-exports.
270
Module ResolutionmoduleResolution, path aliases, node16/nodenext imports, and isolatedModules for single-file transpilers.
280
Declaration FilesWriting .d.ts files for untyped JavaScript modules, globals, @types packages, and augmentation.
290
Generated DeclarationsEmitting .d.ts files from TypeScript and generating client types from OpenAPI or GraphQL schemas.

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