da.test.tsx

import { typeSwitch } from './typeSwitch';
interface Circle {
  type: 'circle';
  radius: number;
}
interface Square {
  type: 'square';
  width: number;
}
type Shape = Circle | Square;
it('forces you to handle all possible types, unlike if', () => {
  const circle = { type: 'circle', radius: 3.14 } as Shape;
  const result = typeSwitch(circle, {
    circle: () => 'round',
    square: () => 'pointy',
  });
  expect(result).toEqual('round');
});

typeswitch.ts

export function typeSwitch<
  TResult,
  T extends { type: string },
>(
  object: T,
  handlers: {[k in T['type']]: () => TResult},
): TResult {
  const { type } = object;
  // https://basarat.gitbooks.io/typescript/docs/types/index-signatures.html
  const handler = (handlers as any)[type] as () => TResult;
  if (!handler) {
    throw new Error(`Invariant Error: Unexpected type "${type}" given to typeSwitch`);
  }
  return handler();
}