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TypeScript

27 cheat sheets

TypeScript#

TypeScript adds a static type layer on top of JavaScript that is erased at compile time. It ships as a compiler (tsc) and a language server that powers editor autocompletion across every major IDE.

What’s in this section#

TopicPages
Setupinstallation · tsconfig
Type systemtypes vs interfaces · generics · utility types
Narrowingnarrowing · type guards · mapped & conditional types
Importstype-only imports
Advancedenums · decorators
Troubleshootingcommon errors
  1. Installation — install TypeScript globally or per-project with npm/pnpm
  2. tsconfig — understand strict, target, module, paths, and lib
  3. Types vs interfaces — when to use each and the key differences
  4. Generics — write reusable, type-safe functions and classes
  5. Utility types — Partial, Required, Pick, Omit, Record, ReturnType, and more
  6. Narrowing and type guards — runtime checks that refine types
  7. Mapped & conditional types — meta-programming at the type level
  8. Type-only imports — import type and its effect on output
  9. Enums — numeric vs string enums, and when to use const enums or plain objects instead
  10. Decorators — stage 3 decorators and experimentalDecorators legacy mode
  11. Common errors — TS2345, TS2322, TS2339, and the rest explained with fixes

[!TIP] Enable strict: true in your tsconfig.json from the start of every project. It catches far more bugs than the default loose settings and is much harder to add later.

TypeScript Installation & Running

Install the TypeScript compiler, run .ts files without a build step using ts-node or tsx, and compile projects with tsc. Covers tsc flags, watch mode, and project references.

Apr 26, 2026
typescript installation tsc tooling

tsconfig.json Reference

Complete reference for tsconfig.json — compiler options for type checking, module resolution, output, paths, and JSX. Includes ready-to-use presets for Node 20, browser libraries, and Vite/React apps.

Apr 26, 2026
typescript tsconfig compiler tooling

Types vs Interfaces

TypeScript has two ways to define object shapes — type aliases and interface declarations. Learn when each is appropriate, how they differ in extension, merging, and composability.

Apr 26, 2026
typescript types interfaces language

Generics

TypeScript generics allow writing reusable, type-safe code that works over many types. Covers generic functions, interfaces, classes, constraints, keyof, conditional types, and the infer keyword.

Apr 26, 2026
typescript generics language

Utility Types

TypeScript's built-in generic utility types that transform existing types into new ones. Covers Partial, Required, Readonly, Record, Pick, Omit, Exclude, Extract, ReturnType, Awaited, and more.

Apr 26, 2026
typescript utility-types language

Type Narrowing

TypeScript narrowing refines broad types to specific ones within code branches. Covers typeof, instanceof, in, equality, assignment narrowing, discriminated unions, control flow analysis, and the never type.

Apr 26, 2026
typescript narrowing type-guards language

typeof & keyof — Type-level operators

TypeScript's typeof promotes a runtime value into the type position; keyof extracts the union of property keys. Together they form the backbone of type-safe lookups, enum-from-object patterns, and inferred shapes.

May 25, 2026
typescript types operators language

satisfies Operator — Type-check without widening

TypeScript's satisfies operator checks a value against a type while preserving its narrow literal inferred type. The middle ground between an annotation that widens and an as cast that lies.

May 25, 2026
typescript types operators language

Type Guards

User-defined type guards narrow types at runtime using the `is` predicate, assertion functions, generic guards, and class-based patterns. Covers API validation, DOM guards, and when to use Zod.

Apr 26, 2026
typescript type-guards narrowing language

Mapped & Conditional Types

Mapped types iterate over type keys to create new types; conditional types pick between types based on a condition. Together they power all built-in utility types and advanced type composition.

Apr 26, 2026
typescript mapped-types conditional-types language

Discriminated Unions — Tagged Variants & Exhaustiveness

Discriminated unions model finite states with a shared literal tag, enabling exhaustive narrowing in switch statements, Result-style error handling, reducer actions, and pattern-matched API responses.

May 25, 2026
typescript types patterns narrowing language

infer Keyword — Extracting types from patterns

TypeScript's infer keyword declares a fresh type variable inside a conditional type, letting you pull a sub-type out of a matched shape. It powers ReturnType, Parameters, Awaited, and most advanced type extractors.

May 25, 2026
typescript types advanced language

Type-Only Imports & Exports

import type and export type erase at compile time, preventing runtime side-effects and enabling better tree-shaking. Covers inline type qualifiers, verbatimModuleSyntax, and common gotchas with enums and namespaces.

Apr 26, 2026
typescript imports modules language

Enums

TypeScript enums create named constant sets as real runtime objects. Covers numeric, string, and const enums; reverse mapping; enum pitfalls; and when to prefer a union of string literals instead.

Apr 26, 2026
typescript enums language

Template Literal Types — String Algebra at the Type Level

Template literal types let TypeScript pattern-match and synthesize string types — covering Uppercase/Lowercase intrinsics, infer-based parsers (Split, Join, CamelCase), route-param extraction, JSON path keys, and typed i18n helpers.

May 25, 2026
typescript types advanced template-literals language

Branded Types — Nominal Typing in a Structural System

Branded (a.k.a. nominal, opaque, tagged) types add a phantom marker to a primitive so the compiler refuses to mix two strings — useful for IDs, units, validated input, and money. Covers hand-rolled brands, type-fest's Opaque/Tagged, Zod's .brand(), class-based brands, and runtime validation pairings.

May 25, 2026
typescript types patterns language branded

Decorators

TypeScript decorators annotate and transform classes and class members. Covers the TC39 stage-3 standard decorators (TS 5.0+), legacy experimentalDecorators, decorator factories, method/class/field decorators, and reflect-metadata for NestJS and Angular patterns.

Apr 26, 2026
typescript decorators language

Common TypeScript Errors

A reference for the most frequent TypeScript compiler errors — their meaning, a minimal reproduction, and the correct fix. Covers TS2304, TS2345, TS2339, TS2322, TS2532, TS2554, TS2307, TS2366, TS7006, TS2571, TS2769, TS2693, plus type assertions and ts-expect-error.

Apr 26, 2026
typescript errors debugging language

TypeScript Modules — module, moduleResolution, .mts / .cts

Deep dive into TypeScript's module system — module/moduleResolution pairs, .mts/.cts file extensions, NodeNext vs Bundler resolution, paths, and tsconfig extends.

May 25, 2026
typescript modules esm nodenext tooling

.d.ts Files — Ambient Declarations, Asset Imports, Module Typing

Authoring .d.ts files — ambient declarations, declare module 'foo', asset typing (*.svg?raw), declare global, triple-slash refs, and the home-hero.svg?raw pattern used in this project.

May 25, 2026
typescript declarations modules ambient dts

Declaration Merging — Interfaces, Namespaces, Modules, Globals

Deep dive into TypeScript declaration merging — interface merging, namespace + class, namespace + function, module augmentation, global augmentation, and the Express Request pattern.

May 25, 2026
typescript declarations modules patterns advanced

TypeScript Project References — Composite Builds & Monorepos

Split a TypeScript codebase into composite sub-projects, build them incrementally in dependency order with tsc --build, and integrate the result with bundlers, Turborepo, and Nx.

May 25, 2026
typescript build monorepo tooling tsconfig

ts-node, tsx & Friends — Running TypeScript Without a Build Step

Compare ts-node, tsx, Node 22.6+ --experimental-strip-types, Bun, and Deno for running .ts files directly; pick the right tool, configure watch mode, and avoid the classic ESM / type-strip pitfalls.

May 25, 2026
typescript runtime tooling node esm

Structural Typing — Duck Typing at the Type Level

Understand TypeScript's structural type system — assignability is based on shape, not name; excess property checks are the one exception; nominal typing requires branded types or class privates.

May 25, 2026
typescript types concepts language

React with TypeScript — Components, hooks, generics, and forms

Type-safe React patterns — function components, prop types, children, event handlers, refs, generic components, polymorphic `as` props, and Zod-powered forms.

May 25, 2026
typescript react ui framework generics

type-fest — Community utility types for TypeScript

Sindre Sorhus's collection of essential TypeScript utility types — PartialDeep, ReadonlyDeep, SetOptional, RequireAtLeastOne, Merge, Tagged, JsonValue, Opaque, and dozens more — so you don't hand-roll them.

May 25, 2026
typescript library utility-types tooling

TypeScript Compiler API — Codegen, codemods, and AST walks

Drive the TypeScript compiler programmatically — parse SourceFiles, query the type checker, transform ASTs with the factory, and write codemods that scale across a monorepo.

May 25, 2026
typescript tooling advanced compiler codegen