Async/Await in `onBeforeFileAdded() throw 'Cannot read property 'size' of undefined'


I am struggling with asynchronous calls in onBeforeFileAdded(). I want to only upload images with a certain image resolution of 12MPX. My implementation rejects files lower than 12mpx successfully. However all other files throw this error:

[Uppy] [12:55:55] Error: Multiple errors occurred while adding files:

 * Cannot read property 'size' of undefined
    at Uppy.addFiles (index.js?b880:866)
    at Dashboard._this.addFiles (index.js?ef92:391)
    at Object.Dashboard._this.handleInputChange (index.js?ef92:513)
    at Object.AddFiles._this.onFileInputChange [as change] (AddFiles.js?abc5:28)
    at HTMLInputElement.eventProxy (preact.esm.js?30bf:308)

Here is a minimal code which produces this behavior.

return new Uppy({
        onBeforeFileAdded: async (currentFile, files) => {
          // Check mpx
          const mpx = await this.getMPX(URL.createObjectURL(
          if (mpx < 12) {
            return false
          return true

The function calculating the megapixels for a given imageUrl:

getMPX(imgUrl) {
      return new Promise((resolve, reject) => {
        const img = new Image()
        img.onload = () => {
          resolve((img.width * img.height) / 1000000)
        img.onerror = reject
        img.src = imgUrl

If you remove the await and async and replace the function with a number (e.g. Math.random()*24) it works (i.e. about 50% of the images are getting uploaded).

I am struggling to find what I am doing wrong. Is an async onBeforeFileAdded() not working in general? I do have various things to check server side before the file should be uploaded (e.g. duplicate) and wanted to call the api’s in onBeforeFileAdded() as well.

Any help would be great!

I figured it out to make Uppy work with an async call. You need to create a plugin and hook onto the this.uppy.addPreProcessor function.

However now I am struggling to display an UI with Vue. Using

    const target =
if (target) {
  this.mount(target, this)

results in

Error: Invalid target option given to UploadRestrictions.If you meant to target an HTML element, please make sure that the element exists. Check that the <script> tag initializing Uppy is right before the closing </body> tag at the end of the page.

I also wrote a cue component but this gets me the error in the render hook:

TypeError: h is not a function
    at RestrictionsPlugin.render

There is basically zero documentation on that. I tried to copy as much code as possible from the @uppy/vue components and the @uppy/plugins but have no idea how to adjust this to vue.