josudoey/promise-solo

const add = function (tail, func, self, args) {
  return new Promise(function (resolve, reject) {
    const next = function () {
      try {
        const r = func.apply(self, args)
        if (!r || !r.then) {
          return resolve(r)
        }
        r.then(resolve).catch(reject)
      } catch (e) {
        reject(e)
      }
    }
    tail.then(next).catch(next)
  })
}

module.exports = function () {
  let tail = Promise.resolve()
  const wrap = function (func, self) {
    if (typeof func !== 'function') {
      throw new TypeError('"func" argument must be a function')
    }
    return function () {
      const args = Array.prototype.slice.call(arguments)
      const song = add(tail, func, self, args)
      tail = song
      return song
    }
  }

  wrap.spy = function (self, method) {
    const func = self[method]
    self[method] = wrap(func, self)
  }
  return wrap
}