TypeScript

example.ts

function divide(num1: number | undefined, num2: number | undefined): number | 'no!' | undefined {
  if (num1 === undefined || num2 === undefined) { return }
  // num1 and num2 now have type number 
  if (num2 === 0) { return 'no!'}
  return num1 / num2
}

example2.ts

type Foo = {
  foo: number,
  bar: 1 | true
}

type Bar = {
  foo: 'hello' | 'world',
  bar: 2 | false
}

function example(arg: Foo | Bar): void {
  // do something
}

example({foo: 10, bar: true}) // typechecks
example({foo: 'hello', bar: false}) // typechecks
example({foo: 'world', bar: 2}) // typechecks
example({foo: 'nope', bar: false}) // doesn't typecheck