random-string.js

let charType = () => {
  let group = [ 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2 ]
  return group[parseInt(Math.random() * group.length)]
}
let charASCII = t => parseInt(Math.random() * (t === 0 ? 9 : 25))

const randomString = l => {
  l = l || 32
  let result = ''
  for (let i = 0; i < l; i++) {
    let t = charType()
    let n = charASCII(t)
    result += t === 0 ? n.toString() : String.fromCharCode(t === 1 ? 97 + n : 65 + n)
  }
  return result
}
console.log('random:', randomString(16))