example_1

example_1.ts

// 1) Types that have a common, singleton type property — the discriminant.
// In this example the "kind" property is the discriminant.
interface Square {
    kind: "square";
    size: number;
}

interface Rectangle {
    kind: "rectangle";
    width: number;
    height: number;
}

interface Circle {
    kind: "circle";
    radius: number;
}

// 2) A type alias that takes the union of those types — the union.
type Shape = Square | Rectangle | Circle;

function area(s: Shape) {
    // 3) Type guards on the common property.
    // A switch statement acts as a "type guard" on 
    switch (s.kind) {
        case "square": return s.size * s.size;
        case "rectangle": return s.height * s.width;
        case "circle": return Math.PI * s.radius ** 2;
    }
}