Discriminated unions in TypeScript

A shared literal field that makes a union exhaustive.

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

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

How it works

1. Every member carries a distinct kind.
2. switch on kind narrows to that member.
3. Covering all cases satisfies the return type.

Keywords and builtins used here

• Math
• Shape
• case
• function
• number
• return
• switch
• type

The run, in numbers

Lines

15

Characters to type

338

Tokens

93

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 51 seconds.

Type this snippet

Step 1 of 3 in Exhaustiveness & tests, step 9 of 12 in Type-level programming.

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