multiplyOut.js

function multiplyOut (matrix) {
  const partLengths = matrix.map(part => part.length) // how large each part is [3, 3, 2]
  const subPartIndices = '0'.repeat(matrix.length).split('').map(Number) // how far we've gone in each part [0, 0, 0]
  let isEnded = false
  const sumsToPerform = []
  while (isEnded === false) {
    sumsToPerform.push(subPartIndices.reduce((sum, subPartIndex, partIndex) => {
      sum.push(matrix[partIndex][subPartIndex])
      return sum
    }, []))
    isEnded = !subPartIndices.some((subPartIndex, partIndex) => {
      subPartIndex++
      if (subPartIndex < partLengths[partIndex]) {
        subPartIndices[partIndex] = subPartIndex
        return true
      }
      subPartIndices[partIndex] = 0
      return false
    })
  }
  return sumsToPerform
}
console.log(multiplyOut([[1, 2, 3], [10, 20, 30], [100, 200]]))