TypeScript · Const

never

never represents impossible values and unreachable execution paths. It is especially useful for exhaustive checking of narrowed unions and for functions that throw or never terminate.

Since: 2.0

How it works

never describes an impossible value or an execution path that cannot complete normally. Use it for functions that always throw or terminate, and as an exhaustiveness check after narrowing a union.

Syntax

type never
function assertNever(value: never): never

Key points

Control-flow narrowing can reduce a discriminated union to an impossible branch represented by never.
A never check in a switch default branch catches newly added union members at compile time.
Its one-way assignability makes never safe as the required type for an impossible state.

Common mistakes

A value cannot normally be assigned to never; the intended direction is that never can be assigned to other types.
A union containing never usually simplifies to its other members, so never may disappear from displayed or inferred union types.

Example: Exhaustive discriminated-union handling

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number };

function assertNever(value: never): never {
  throw new Error("Unexpected shape: " + JSON.stringify(value));
}

function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle": return Math.PI * shape.radius ** 2;
    case "square": return shape.side ** 2;
    default: return assertNever(shape);
  }
}

The default branch is accepted only when narrowing has removed every valid Shape variant.

When to use it

Non-returning functions

A function always throws or terminates without returning.

Exhaustiveness checking

A discriminated union is handled in a switch statement.

Type-level filtering

A conditional type needs a fallback for unsupported inputs.

Interview questions

QWhat does the never type represent?

never represents a value that cannot occur. In a return position, it indicates that a function throws or terminates execution rather than producing a value.

Follow-up: How does it differ from void? void describes a function whose result is not useful or has no value, whereas never means normal completion cannot happen.

QHow can never enforce exhaustive switch handling?

After narrowing removes every member of a union, TypeScript uses never for the remaining impossible state. Assigning that state to a never parameter lets a default branch verify that all cases were handled.

Follow-up: What happens when a union gains a new variant? The default branch’s argument is no longer never, so the compiler reports the missing case.

QWhat are the assignability rules for never?

never is assignable to every type, but no other type is assignable to it except never itself. This makes it suitable as the parameter type of an assertion function used at an unreachable branch.

Follow-up: Why can an assertion function return never too? Because it throws or otherwise never reaches its caller, so the call expression itself has an impossible continuation.

void
unknown
Narrowing
Exhaustiveness checking
Conditional types
Discriminated unions

Source: Documentation - Narrowing. Checked 2026-10-15.