TypeScript Pattern Matching Approaches

JavaScript lacks native pattern matching, but TypeScript’s type system lets us approximate it in several ways.

Discriminated Unions

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "rect"; width: number; height: number }
  | { kind: "triangle"; base: number; height: number };

function area(s: Shape): number {
  switch (s.kind) {
    case "circle": return Math.PI * s.radius ** 2;
    case "rect": return s.width * s.height;
    case "triangle": return (s.base * s.height) / 2;
  }
}

Exhaustive Checking

Use never to ensure all cases are handled:

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

function area(s: Shape): number {
  switch (s.kind) {
    case "circle": return Math.PI * s.radius ** 2;
    case "rect": return s.width * s.height;
    case "triangle": return (s.base * s.height) / 2;
    default: return assertNever(s);
  }
}

Adding a new variant to Shape will now produce a compile error until every switch is updated — a small discipline that prevents large runtime surprises.