Exhaustive type check in Typescript

Exhaustive type checking is when your static compiler checks that you’re not leaving a possibility unchecked.

Example with discriminated union:

interface Square {
    kind: "square";
    size: number;
}
 
interface Rectangle {
    kind: "rectangle";
    width: number;
    height: number;
}
 
// Someone just added this new `Circle` Type
// We would like to let TypeScript give an error at any place that *needs* to cater for this
interface Circle {
    kind: "circle";
    radius: number;
}
 
type Shape = Square | Rectangle | Circle;
 
function area(s: Shape) {
    if (s.kind === "square") {
        return s.size * s.size;
    }
    else if (s.kind === "rectangle") {
        return s.width * s.height;
    }
    else {
        // ERROR : `Circle` is not assignable to `never`
        const _exhaustiveCheck: never = s;
    }
}