Rail fence cipher

railFenceCipher.js

function getSteps(currentRail, numberRails) {
    let step1 = (currentRail === numberRails) || (currentRail === 1) ? ((numberRails - 1) * 2) : ((numberRails - currentRail) * 2);
    let step2 = (currentRail === numberRails) || (currentRail === 1) ? (step1) : ((currentRail - 1) * 2);
    return [step1, step2]
}

function encodeRailFenceCipher(string, numberRails) {
    let res = ''
    for (let currentRail = 1; currentRail <= numberRails; currentRail++) {
        const [step1, step2] = getSteps(currentRail, numberRails)
        let step;
        for (let letter = currentRail - 1; letter < string.length; letter += step) {
            res += string[letter]
            step = step === step1 ? step2 : step1
        }
    }
    return res
}

function decodeRailFenceCipher(string, numberRails) {
    let res = []
    const iterator = string[Symbol.iterator]()
    for (let currentRail = 1; currentRail <= numberRails; currentRail++) {
        const [step1, step2] = getSteps(currentRail, numberRails)
        let step;
        for (let letterIndex = currentRail - 1; letterIndex < string.length; letterIndex += step) {
            res[letterIndex] = iterator.next().value
            step = step === step1 ? step2 : step1
        }
    }
    return res.join('')
}