Published on

Independent Module Url Processing For New Worker()

Authors

Yesterday, I found a problem with file upload in my frontend page.

When I upload a file to server, the browser tells me an error.

DOMException: Failed to construct 'Worker': Script at 'http://localhost:9000/src_env_document_spark-md5_js.UploadFileApp.js' cannot be accessed from origin 'http://localhost:5001'.
    at http://localhost:9000/UploadFileApp.js:234274:27
    at new Promise (<anonymous>)
    at splitBigFile (http://localhost:9000/UploadFileApp.js:234271:12)
    at http://localhost:9000/UploadFileApp.js:234373:17
    at Generator.next (<anonymous>)
    at http://localhost:9000/UploadFileApp.js:234258:69
    at new Promise (<anonymous>)
    at __awaiter (http://localhost:9000/UploadFileApp.js:234240:12)
    at uploadFile (http://localhost:9000/UploadFileApp.js:234339:117)
    at http://localhost:9000/UploadFileApp.js:269092:42
    at new Promise (<anonymous>)
    at isUploadTimeOut (http://localhost:9000/UploadFileApp.js:269088:7)
    at http://localhost:9000/UploadFileApp.js:269101:27
    at Generator.next (<anonymous>)
    at http://localhost:9000/UploadFileApp.js:268900:69
    at new Promise (<anonymous>)
    at __awaiter (http://localhost:9000/UploadFileApp.js:268882:12)
    at uploadFileToNode (http://localhost:9000/UploadFileApp.js:269068:12)
    at http://localhost:9000/UploadFileApp.js:254966:45
    at Generator.next (<anonymous>)
    at fulfilled (http://localhost:9000/UploadFileApp.js:254870:26)

Here is my code.

// `UploadFileApp`: getMD5EventListener.js
import SparkMD5 from 'spark-md5'

addEventListener('message', (e) => {
  const file = e.data.file || []
  const FILE_CHUNK_SIZE = 1 * 1024 * 1024
  const blobSlice = File.prototype.slice || File.prototype.mozSlice || File.prototype.webkitSlice
  const chunks = Math.ceil(file.size / FILE_CHUNK_SIZE)
  const spark = new SparkMD5.ArrayBuffer()
  const fileReader = new FileReader()
  let currentChunk = 0

  fileReader.onload = function (e) {
    spark.append(e.target.result) // Append array buffer
    currentChunk++
    if (currentChunk < chunks) {
      loadNext()
    } else {
      postMessage({ md5: spark.end() })
    }
  }

  fileReader.onerror = function () {
    console.warn('oops, something went wrong.')
  }

  function loadNext() {
    const start = currentChunk * FILE_CHUNK_SIZE,
      end = start + FILE_CHUNK_SIZE >= file.size ? file.size : start + FILE_CHUNK_SIZE
    fileReader.readAsArrayBuffer(blobSlice.call(file, start, end))
  }
  loadNext()
})

// `UploadFileApp`: main.js
const getMD5 = (file: File): Promise<{ md5: string }> => {
  return new Promise((resolve) => {
    //  Here: Failed to construct 'Worker'
    const sparkWorker = new Worker(new URL('./getMD5EventListener.js', import.meta.url))
    sparkWorker.onmessage = (event) => {
      resolve({ md5: event.data.md5 })
    }
  })
}

I think it could work until the file upload module was split as a independent module(We call it UploadFileApp) from a big web application(We call it BigWebApp, it's based on Next and React). So I want to compare what's the difference between them.

First, I try to hit some breakpoints, and use console.log to show me more information

// `UploadFileApp`: main.js
const getMD5 = (file: File): Promise<{ md5: string }> => {
  return new Promise((resolve) => {
   debugger
    // 1
    console.log('import.meta.url', import.meta.url)
    // 2
    console.log(
      "new URL('./spark-md5.js', import.meta.url)",
      new URL('./spark-md5.js', import.meta.url)
    )    const sparkWorker = new Worker(new URL('./getMD5EventListener.js', import.meta.url))
    sparkWorker.onmessage = (event) => {
      resolve({ md5: event.data.md5 })
    }
  })
}

Obviously, import.meta.url return the file absolute path and show it's path in it's file Location, and because of module split, it changed. But, to my surprise, new URL('./spark-md5.js', import.meta.url) return completely different result.

// old
'/_next/static/media/spark-md5.756744a0.js'

// new
'http://localhost:9000/2471a53e42b8c4be0b00.js'

localhost:9000 is my upload module server host and port

It seems that old result is a static file, and it's generated by Next in BigWebApp. And the new result is also a static file, But it's generated by Webpack in UploadFileApp.

The key is that when you use new Worker(), new URL() and import.meta.url, you should consider where will it be used.

So When I use UploadFileApp inside BigWebApp, its incoming parameter is totally wrong.

There is a less elegant solution for me, just replace new Worker() with asynchronously read and process the file. Actually my script in new Worker() is just to calculate MD5. So, I change my code, and calculate MD5 without new Worker().

const asyncGetFileMD5 = ({ file, resolve }) => {
  const blobSlice = File.prototype.slice || File.prototype.mozSlice || File.prototype.webkitSlice
  const chunks = Math.ceil(file.size / FILE_CHUNK_SIZE)
  const spark = new SparkMD5.ArrayBuffer()
  const fileReader = new FileReader()
  let currentChunk = 0

  fileReader.onload = function (e) {
    spark.append(e.target.result) // Append array buffer
    currentChunk++
    if (currentChunk < chunks) {
      loadNext()
    } else {
      resolve({ md5: spark.end() })
    }
  }

  fileReader.onerror = function () {
    console.warn('oops, something went wrong.')
  }

  function loadNext() {
    const start = currentChunk * FILE_CHUNK_SIZE,
      end = start + FILE_CHUNK_SIZE >= file.size ? file.size : start + FILE_CHUNK_SIZE
    fileReader.readAsArrayBuffer(blobSlice.call(file, start, end))
  }
  loadNext()
}

const splitBigFile = (file: File): Promise<{ md5: string }> => {
  return new Promise((resolve) => {
    asyncGetFileMD5({ file, resolve })
  })
}

After some tests, It looks like it's okay, but is there any way we can continue to use new Worker()? I ask chatGPT for help...

chatGPT tell me two ways to solve the problem.

  1. Using a static location to store script, like cdn
  2. Using Blob and URL.createObjectURL()

In my project, I don't a static location to store script, So the only way is using Blob and URL.createObjectURL()

First of all, I can't import SparkMD5 from 'spark-md5' directly in UploadFileApp/getMD5EventListener.js， I need to download spark-md5.js and store it in my local folder(localThirdModule).

// `UploadFileApp`: main.js
function getWorkerUrl() {
  const sparkMD5Url = new URL('localThirdModule/spark-md5.js', import.meta.url)
  const getMD5EventListenerUrl = new URL('./getMD5EventListener.js', import.meta.url)
  const workerScript = `importScripts("${sparkMD5Url.href}", "${getMD5EventListenerUrl.href}");` // please pay attention, the script order is important
  const blob = new Blob([workerScript], { type: 'application/javascript' })
  return window.URL.createObjectURL(blob)
}

const getMD5 = (file: File): Promise<{ md5: string }> => {
  return new Promise((resolve) => {
    const sparkWorker = new Worker(getWorkerUrl())
    sparkWorker.onmessage = (event) => {
      resolve({ md5: event.data.md5 })
    }
  })
}

I don't known why the MDN say the new Worker() first incoming parameter is just a string representing the URL of the script the worker will execute. But you can even pass a function, and use it's result as first incoming parameter

Finally, you can see there is a URL.revokeObjectURL() method to releases an existing object URL which was previously created by calling URL.createObjectURL().But in my application, I can't find a fit place to use it. Maybe, one day, it will give me a hammer blow. o(╥﹏╥)o

View on GitHub