MultiplyingNumbersAsStrings.js

/**
 * Multiply two numbers! Simple!
 *
 * The arguments are passed as strings.
 * The numbers may be way very large
 * Answer should be returned as a string
 * The returned "number" should not start with zeros e.g. 0123 is invalid
 */
function multiply(a, b) {
    const aRev = Array.from(a.toString()).reverse()
        , bRev = Array.from(b.toString()).reverse()
        , res = []
        , reversedMultiply = (k, currM, currA) => {
        res[k] = currM + currA
        if (res[k] > 9) {
            const prevD = Math.floor(res[k] / 10)
                , prevA = k + 1 >= res.length ? 0 : res[k + 1]
            res[k + 1] = prevD + prevA
            res[k] -= prevD * 10
        }
    }
        , prepareLoop = (currA, aI) => {
        bRev.map((currB, bI) => {
            const keyI = aI + bI
                , multiplyCurrent = currA * currB
                , additionCurrent = keyI >= res.length ? 0 : res[keyI]
            reversedMultiply(keyI, multiplyCurrent, additionCurrent)
        })
    }
    aRev.map(prepareLoop)
    return res.reverse().join('').replace(/^0+/, '') || '0'
}