将对象，数组，数字转为字符串JSON.stringify()

let a={a:1,b:2}

let b=[1,2,3,4]

let c = JSON.stringify(a)

let d = JSON.stringify(b)

console.log(c);//"{"a":1,"b":2}"

console.log(d);//"[1,2,3,4]"