Flatten object function in plain ES5 javascript and ES2015

flattenObject.es5.js

'use strict'

function flatten (source, dest, separator, path) {
  dest = typeof dest !== 'undefined' ? dest : {}
  separator = typeof separator !== 'undefined' ? separator : '-'
  path = typeof path !== 'undefined' ? path : []

  for (var key in source) {
    if (!source.hasOwnProperty(key)) continue

    if (source[key] && typeof source[key] === 'object') {
      flatten(source[key], dest, separator, path.concat(key))
    } else {
      dest[path.concat(key).join(separator)] = source[key]
    }
  }

  return dest
}

flattenObject.es6.js

const flattenObject = (source, separator = '-', path = []) =>
  Object.keys(source).reduce(
    (acc, key) =>
      Object.assign(
        {},
        acc,
        source[key] === Object(source[key])
          ? flattenObject(source[key], separator, path.concat(key))
          : { [path.concat(key).join(separator)]: source[key] }
      ),
    {}
  )

export default flattenObject