discriminatedUnions.ts

type DemoEnum = Foo | Bar;

interface Foo {
  kind: "Foo",
  n: number
}

interface Bar {
  kind: "Bar",
  s: string
}

const values: DemoEnum[] = [
  { kind: "Foo", n: 10001 },
  { kind: "Bar", s: "Hello world!" }
];

for (const value of values) {
  switch (value.kind) {
    case "Foo":
      // TypeScript knows value: A in this block
      console.log(`Got a Foo with a number: ${value.n}`);
      break;
    case "Bar":
      // and here, value: B
      console.log(`Got a Bar with a string: ${value.s}`);
      break;
  }
}