TS Discriminant Union

tsdu.ts

type Square = { type: "square", edgeLength: number }
type Circle = { type: "circle", radius: number }

// Here's our Discriminant Union
type Shape = Square | Circle

function exhaustiveCheck(x: never): never {
    throw new Error("Didn't expect to get here");
}

const imASquare: Square = { type: "square", edgeLength: 20 }

const getShapeArea = (x: Shape) => {
    switch(x.type) {
        case "square":
            // The compiler will uniquely identify that x is a Square
            // because it could infer it from the type property, amazing!
            return x.edgeLength * x.edgeLength
        case "circle":
            return x.radius * x.radius * Math.PI
        default:
            // At this point, if you've exhausted all possible values of
            // `type`, then x in this branch is `never`.  If not then the compiler
            // will throw an error for matching a value with `never`.
            return exhaustiveCheck(x)
    }
}

getShapeArea(imASquare)