merge2DArray.js

const merge2DArray = arr => {
  return arr.reduce((acc, secondData) => {
    if (acc.length < secondData.length) {
      const loopTimes = secondData.length - acc.length
      for (let i = 1; i <= loopTimes; i++) {
        acc.push('')
      }
    }

    secondData.map((value, index) => {
      acc[index] += value
    })

    return acc
  }, [])
}

const data = [
  ['A', 'B', 'C'],
  ['D', 'E', 'F', 'G'],
]

const expected = ['AD', 'BE', 'CF', 'G']

console.log(merge2DArray(data))
console.log(expected)