typescript type guard

type_guard.ts

interface A {
    a: string
}

interface B extends A {
    b: string
}

const isB = <T>(v: T):
  v is (T & B) => 'b' in v

const a: A = { a: 'a' }

const b: B = { a: 'a', b: 'b' }

console.log(isB(a))

console.log(isB(b))