如何判断一个对象实现了某个接口

interface A { 
  name: string; 
} 
 
interface B extends A { 
  age: number; 
} 
 
interface C extends A { 
  sex: string; 
} 
 
function test(obj: A) { 
  let some = obj as B; 
  if (some.age !== undefined) { 
    console.log(some.age + ‘’) 
  } 
} 
 
let obj: B = { 
  name: ‘1’, 
  age: 2 
} 
 
test(obj);