abcd.js

const arrStr = 'a.b.c.d'.split('.')

const res = arrStr.reduce((acc, curr, i, j) => {
  acc.push({ [`${curr}`]: {} })

  return acc
}, []).reduce((acc, curr, i, arr) => {
  acc = arr[0]

  if(i < arr.length) {
    arr[i][Object.keys(arr[i])] = arr[i+1]
  }

  if(i === arr.length - 2) {
    const key = Object.keys(arr[i+1])

    arr[i][Object.keys(arr[i])] = { [`${key}`]: {} }
  }

  return acc
}, {})


console.log(JSON.stringify(res, null, 2))