code-challenge.js

function flatten(source, k, keyPath = [], result = {}) {
  if(typeof source !== `object` || source === null) {
    result[keyPath.join('.')] = source

    return result
  }
  let iterationResult
  for (k in source) {
      keyPath.push(k)
      iterationResult = flatten(source[k], k, keyPath, result)
      keyPath.pop()
  }
  return iterationResult
}