convert-1d-to-2d-array.js

const array2d = array1d
    .reduce((cur, i) => {
        /* transform 1d array to 2d */
        let lastInd = cur.length-1
        let lastVal = cur[lastInd]

        if (lastVal.length === 3) {
            return cur.concat([[i]])
        }

        return cur
            .slice(0, lastInd)
            .concat([lastVal.concat(i)])
    }, [[]])