random-string.js

const dic = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'
const randomInt = (max = 1000, min = 0) => Math.floor(Math.random() * (max - min)) + min
const randomString = () => dic.charAt(randomInt(dic.length -1, 0))
const makeArray = (length, cb) => Array.from({ length }, cb)
const makeArrayOfStrings = length => makeArray(length, randomString)

const joiArr = lists => lists
    .reduce((acc, arr) => {
        return `${acc}${arr.join('')}`
    }, '')

const makeLists = length => makeArray(length, () => makeArrayOfStrings(randomInt(10, 3)))

const st = joiArr(makeLists(2))