/**
 * Just some interfaces
 */
interface Foo {
  foo: number
  common: string
}
interface Bar {
  bar: number
  common: string
}
/**
 * User Defined Type Guard!
 */
function isFoo(arg: any): arg is Foo {
  return arg.foo !== undefined
}
/**
 * Sample usage of the User Defined Type Guard
 */
function doStuff(arg: Foo | Bar) {
  if (isFoo(arg)) {
    console.log(arg.foo) // OK
    console.log(arg.bar) // Error!
  } else {
    console.log(arg.foo) // Error!
    console.log(arg.bar) // OK
  }
}
doStuff({ foo: 123, common: "123" })
doStuff({ bar: 123, common: "123" })