Convert URL to File or Blob for FileReader.readAsDataURL

Question

Reference: FileReader.readAsDataURL

Considering the following example:

function previewFile(file) {

  var reader  = new FileReader();

  reader.onlo         


        

Answer 1

Try this I learned this from @nmaier when I was mucking around with converting to ico: Well i dont really understand what array buffer is but it does what we need:

function previewFile(file) {

  var reader  = new FileReader();

  reader.onloadend = function () {
    console.log(reader.result); //this is an ArrayBuffer
  }
  reader.readAsArrayBuffer(file);
}

notice how i just changed your readAsDataURL to readAsArrayBuffer.

Here is the example @nmaier gave me: https://stackoverflow.com/a/24253997/1828637

it has a fiddle

if you want to take this and make a file out of it i would think you would use file-output-stream in the onloadend

Answer 2

I know this is an expansion off of @tibor-udvari's answer, but for a nicer copy and paste.

async function createFile(url, type){
  if (typeof window === 'undefined') return // make sure we are in the browser
  const response = await fetch(url)
  const data = await response.blob()
  const metadata = {
    type: type || 'video/quicktime'
  }
  return new File([data], url, metadata)
}

Answer 3

This information is outdated as of now, but cannot be deleted.

1. You can create File instances just by specifying a path when your code is chrome-privileged:

   new File("/path/to/file");
   

   File is a sub-class of Blob, so all File instances are also valid Blobs. Please note that this requires a platform path, and not a file URL.

2. Yes, FileReader is available to addons.

File and FileReader are available in all windows. If you want to use them in a non-window scope (like bootstrap.js or a code module), you may use nsIDOMFile/nsIDOMFileReader.

Answer 4

To convert a URL to a Blob for FileReader.readAsDataURL() do this:

var request = new XMLHttpRequest();
request.open('GET', MY_URL, true);
request.responseType = 'blob';
request.onload = function() {
    var reader = new FileReader();
    reader.readAsDataURL(request.response);
    reader.onload =  function(e){
        console.log('DataURL:', e.target.result);
    };
};
request.send();

Answer 5

Here is my code using async awaits and promises

const getBlobFromUrl = (myImageUrl) => {
    return new Promise((resolve, reject) => {
        let request = new XMLHttpRequest();
        request.open('GET', myImageUrl, true);
        request.responseType = 'blob';
        request.onload = () => {
            resolve(request.response);
        };
        request.onerror = reject;
        request.send();
    })
}

const getDataFromBlob = (myBlob) => {
    return new Promise((resolve, reject) => {
        let reader = new FileReader();
        reader.onload = () => {
            resolve(reader.result);
        };
        reader.onerror = reject;
        reader.readAsDataURL(myBlob);
    })
}

const convertUrlToImageData = async (myImageUrl) => {
    try {
        let myBlob = await getBlobFromUrl(myImageUrl);
        console.log(myBlob)
        let myImageData = await getDataFromBlob(myBlob);
        console.log(myImageData)
        return myImageData;
    } catch (err) {
        console.log(err);
        return null;
    }
}

export default convertUrlToImageData;

Answer 6

Add cors mode to prevent files cors blocked

fetch(url,{mode:"cors"})
                    .then(res => res.blob())
                    .then(blob => {
                        const file = new File([blob], 'dot.png', {type:'image/png'});
                        console.log(file);
                    });