Discriminated unions

A single object with an optional field for every possibility, like { kind: string; radius?: number; width?: number }, looks flexible. But it accepts a circle with no radius and makes you write ! every time you read a field, and ! is just a promise you make to the compiler that nobody checks.

A discriminated union describes each case on its own and gives every case a shared field with a fixed value, like this: { kind: "circle"; radius: number } | { kind: "rect"; width: number; height: number }. Checking kind is all it takes for the compiler to know which branch you're in.

Inside case "circle":, shape.radius is a guaranteed number, and shape.width is a type error, because a circle really has no width.