any, unknown, never

TypeScript · Reference cheat sheet

📋 Overview

any disables checking; unknown is the safe top type (must narrow before use); never is the bottom type (no values). Prefer unknown over any at boundaries; use never for exhaustiveness and impossible states.

🔧 Core concepts

Type	Assignability	Use
`any`	Assignable to/from almost everything	Escape hatch, gradual typing
`unknown`	Accepts any value; not assignable out without narrowing	Untrusted input, JSON, catch
`never`	Assignable to every type; nothing assignable to it	Exhaustive checks, empty unions

strict / noImplicitAny — missing annotations become errors, not implicit any.
Empty intersection — string & number → never.
Functions that throw / infinite loop — return type never.

💡 Examples

function parseJson(raw: string): unknown {
  return JSON.parse(raw);
}

const data = parseJson('{"n":1}');
if (
  typeof data === "object" &&
  data !== null &&
  "n" in data &&
  typeof (data as { n: unknown }).n === "number"
) {
  console.log((data as { n: number }).n);
}

function assertNever(x: never): never {
  throw new Error(`Unexpected: ${x}`);
}

type Shape = { kind: "circle"; r: number } | { kind: "square"; s: number };

function area(s: Shape): number {
  switch (s.kind) {
    case "circle":
      return Math.PI * s.r ** 2;
    case "square":
      return s.s ** 2;
    default:
      return assertNever(s); // error if a variant is missing
  }
}

function fail(msg: string): never {
  throw new Error(msg);
}

// any: opt-out (avoid in new code)
function legacy(x: any) {
  return x.foo.bar; // no check
}

⚠️ Pitfalls

any infects call sites — one any can silence whole chains.
catch (e) is unknown under useUnknownInCatchVariables (recommended).
never[] is assignable to any array type — don’t confuse with “empty array of T”.
Returning never from a branch that can complete is a type error.
Prefer typed Zod / validators over casting unknown to concrete types blindly.

any, unknown, never

any, unknown, never

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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