What type of  it ?

typeof 

console.log(typeof  "中林大魔王")

console.log(typeof  yes)

console.log(typeof  "yes")

console.log(typeof  12)

console.log(typeof  1.9872)

console.log(typeof  true)

console.log(typeof  null)

猜猜看如果是undefined 呢？

Math.random()

Math.random()*10

Math.floor(Math.random()*10)

setInterval(function(){
console.log('hello');
},1000);

setInterval("console.log('hello');",1000);