Glossary

Typescript · Reference cheat sheet

📋 Overview

Alphabetical glossary of core TypeScript terms for types, narrowing, generics, modules, and compiler options.

🔧 Core concepts

Term	Definition
Any	An escape-hatch type that disables most type checking for a value.
Assertion	A `as Type` or angle-bracket cast that tells the checker to trust a type.
Branded type	A nominal-style type created by intersecting a primitive with a unique tag.
Conditional type	A type that chooses between alternatives with `T extends U ? X : Y`.
Const assertion	`as const` that infers the narrowest literal and readonly types.
Declaration file	A `.d.ts` file that describes types for JavaScript or ambient APIs.
Discriminated union	A union of object types sharing a common literal “tag” property.
Enum	A named set of related constants, numeric or string-based.
Generic	A type or function parameterized by one or more type variables.
Infer	A keyword inside conditional types that extracts a nested type into a variable.
Interface	A named object shape that can be extended and merged by declaration.
Intersection	A type combining members of several types with `&`.
Keyof	An operator producing a union of an object type’s property names.
Literal type	A type that is exactly one specific string, number, or boolean value.
Mapped type	A type that transforms each property of another type via `[K in Keys]`.
Module	A file with import/export that creates its own scope and type namespace.
Narrowing	Refining a broad type to a more specific one via checks.
Never	The empty type for values that never occur, such as exhausted switches.
Readonly	A modifier or utility that makes properties immutable at the type level.
Satisfies	An operator that checks a value against a type without widening inference.
Strict mode	Compiler flags like `strictNullChecks` that tighten soundness.
Tuple	A fixed-length array type where each index may have its own type.
Type alias	A `type` name that refers to any type expression.
Type guard	A check (including user-defined predicates) that narrows types.
Typeof	An operator that produces the type of a value in type position.
Union	A type that may be one of several alternatives joined with `
Unknown	A type-safe top type that must be narrowed before use.
Utility type	Built-in helpers such as `Partial`, `Pick`, `Omit`, and `Record`.
Zod	A popular runtime schema library often paired with TypeScript inference.

💡 Examples

Discriminated union and narrowing:

type Result =
  | { ok: true; data: string }
  | { ok: false; error: string };

function message(r: Result) {
  return r.ok ? r.data : r.error;
}

Generics and utility types:

function pickId<T extends { id: string }>(item: T): Pick<T, "id"> {
  return { id: item.id };
}

Satisfies vs assertion:

const routes = {
  home: "/",
  about: "/about",
} satisfies Record<string, `/${string}`>;

⚠️ Pitfalls

Confusing any (unchecked) with unknown (must narrow before use).
Mixing interface and type alias — interfaces merge; aliases are more flexible for unions.
Using a type assertion when a type guard or refactor would be safer.
Treating enum and union of literals as interchangeable — emit and ergonomics differ.
Assuming satisfies changes the value’s inferred type the same way as does — it validates without forcing the annotation type.

Glossary

Glossary

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

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