typeof

  1. let num = 12;
  2. let str = '中华人民共和国';
  3. let boo = true;
  4. let und = undefined;
  5. let nul = null;
  7. console.log(num); // number
  8. console.log(str); // string
  9. console.log(boo); // boolean
  10. console.log(und); // undefined
  11. console.log(nul); // object