判断undefined

function judgeUndefined (arg) {
   if ( typeof arg === 'undefined') {
        console.log('undefined')
      }
}
judgeUndefined(undefined)

判断null

法一

function judgeNull (arg) {
   if ( !arg && typeof arg !== 'undefined' && arg != 0) {
        console.log('null')
      }
}
judgeNull(null)

法二

function getType (obj) {
 let type = Object.prototype.toString.call(obj)
     return type.split(' ')[1].replace(']', '')
}
console.log(getType([])) // Array
console.log(getType(null)) // Null