register.js

'use strict'

function isPromise (obj) {
  // via https://unpkg.com/is-promise@2.1.0/index.js
  return !!obj && (typeof obj === 'object' || typeof obj === 'function') && typeof obj.then === 'function'
}

function registerPromiseWorker (callback) {
  function postOutgoingMessage (e, messageId, error, result) {
    function postMessage (msg) {
      /* istanbul ignore if */
      if (typeof self.postMessage !== 'function') { // service worker
        e.ports[0].postMessage(msg)
      } else { // web worker
        self.postMessage(msg)
      }
    }
    if (error) {
      /* istanbul ignore else */
      if (typeof console !== 'undefined' && 'error' in console) {
        // This is to make errors easier to debug. I think it's important
        // enough to just leave here without giving the user an option
        // to silence it.
        console.error('Worker caught an error:', error)
      }
      postMessage([messageId, {
        message: error.message
      }])
    } else {
      postMessage([messageId, null, result])
    }
  }

  function tryCatchFunc (callback, message) {
    try {
      return { res: callback(message) }
    } catch (e) {
      return { err: e }
    }
  }

  function handleIncomingMessage (e, callback, messageId, message) {
    var result = tryCatchFunc(callback, message)

    if (result.err) {
      postOutgoingMessage(e, messageId, result.err)
    } else if (!isPromise(result.res)) {
      postOutgoingMessage(e, messageId, null, result.res)
    } else {
      result.res.then(function (finalResult) {
        postOutgoingMessage(e, messageId, null, finalResult)
      }, function (finalError) {
        postOutgoingMessage(e, messageId, finalError)
      })
    }
  }

  function onIncomingMessage (e) {
    var payload = e.data
    if (!Array.isArray(payload) || payload.length !== 2) {
      // message doens't match communication format; ignore
      return
    }
    var messageId = payload[0]
    var message = payload[1]

    if (typeof callback !== 'function') {
      postOutgoingMessage(e, messageId, new Error(
        'Please pass a function into register().'))
    } else {
      handleIncomingMessage(e, callback, messageId, message)
    }
  }

  self.addEventListener('message', onIncomingMessage)
}

module.exports = registerPromiseWorker