https://qiita.com/fukasawah/items/db7f0405564bdc37820e
function getRandomStr(){
const LENGTH = 8 //生成したい文字列の長さ
const SOURCE = "abcdefghijklmnopqrstuvwxyz0123456789" //元になる文字
let result = ''
for(let i=0; i<LENGTH; i++){
result += SOURCE[Math.floor(Math.random() * SOURCE.length)];
}
return result //p9zh1ziw
}