Solution.js

/**
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */
var convert = function (s, numRows) {
    if (numRows == 1) return s
    let arr = new Array(numRows)
    for (let i = 0; i < numRows; i++) arr[i] = []
    let index = 0, len = s.length, mi = 0, isDown = true
    while (index < len) {
        arr[mi].push(s[index])
        index++

        if (mi >= numRows - 1)
            isDown = false
        else if (mi <= 0)
            isDown = true

        if (isDown) mi++
        else mi--
    }
    let ans = []
    for (let item of arr) {
        ans = ans.concat(item)
    }
    return ans.join("")
};

const s = "AB", numRows = 1

console.log(convert(s, numRows))