SOURCE

function typeOf(obj){
    // 获取 [object Object] [object Array] [object Null] [object Undefined]
   let str = Object.prototype.toString.call(obj).split(" ")[1];
   // 截取后半段并转为小写
   str = str.substring(0, str.length-1).toLowerCase();
   console.log(str);
}

typeOf(new Date())