assign-deepassign.js

function assign(target, ...sources) {
  if (target == null) throw new TypeError

  let to = Object(target)

  if (!sources.length) return to

  sources.forEach(nextSource => {
    if (nextSource == null) return

    let from = Object(nextSource)

    let keys = Reflect.ownKeys(from)

    keys.forEach(nextKey => {
      let desc = Object.getOwnPropertyDescriptor(from, nextKey)

      if (desc && desc.enumerable)
        to[nextKey] = from[nextKey]
    })
  })

  return to
}

function any(...args)
{
  return {
    [Symbol.hasInstance]: (v) => args.some(arg => v instanceof arg)
  }
}

function deepAssign(target, ...sources) {
  if (target == null) throw new TypeError

  let to = Object(target)

  if (!sources.length) return to

  sources.forEach(nextSource => {
    if (nextSource == null) return

    let from = Object(nextSource)

    let keys = Reflect.ownKeys(from)

    keys.forEach(nextKey => {
      let desc = Object.getOwnPropertyDescriptor(from, nextKey)

      if (desc && desc.enumerable) {

        let val = from[nextKey]

        if (val === Object(val))
          to[nextKey] = val instanceof any(RegExp, Date, Map, Set) 
            ? deepAssign(new val.constructor(val), val) 
            : deepAssign(new val.constructor(), val)
        else
          to[nextKey] = val
      }
    })
  })

  return to
}