HTML5 / Javascript - DataURL to Blob & Blob to DataURL

Question

I have a DataURL from a canvas that shows my webcam. I turn this dataURL into a blob using Matt's answer from here: How to convert dataURL to file object in javascript?

How do I convert this blob back into the same DataURL? I've spent a day researching this and I'm astouned this isn't documented better, unless I'm blind.

EDIT: There is

var urlCreator = window.URL || window.webkitURL; 
var imageUrl = urlCreator.createObjectURL(blob); 

but it only returns a really short URL that seems to point at a local file, but I need to send the webcam data over a network.

Answer 1

Nevermind, it ended up working after sticking to the instructions in this thread Display image from blob using javascript and websockets and making my server forward raw (yet) unmodified BinaryWebSocketFrames.

Now I'm still fighting bad performance of the canvas (<1fp) but that might become subject of a new thread.

Answer 2

Use my code to convert between dataURL and blob object in javascript. (better than the code in your link.)

//**dataURL to blob**
function dataURLtoBlob(dataurl) {
    var arr = dataurl.split(','), mime = arr[0].match(/:(.*?);/)[1],
        bstr = atob(arr[1]), n = bstr.length, u8arr = new Uint8Array(n);
    while(n--){
        u8arr[n] = bstr.charCodeAt(n);
    }
    return new Blob([u8arr], {type:mime});
}

//**blob to dataURL**
function blobToDataURL(blob, callback) {
    var a = new FileReader();
    a.onload = function(e) {callback(e.target.result);}
    a.readAsDataURL(blob);
}

//test:
var blob = dataURLtoBlob('data:text/plain;base64,YWFhYWFhYQ==');
blobToDataURL(blob, function(dataurl){
    console.log(dataurl);
});

Answer 3

Here is a Promise-based Typescript version with proper error checking.

function blobToDataURL(blob: Blob): Promise<string> {
  return new Promise<string>((resolve, reject) => {
    const reader = new FileReader();
    reader.onload = _e => resolve(reader.result as string);
    reader.onerror = _e => reject(reader.error);
    reader.onabort = _e => reject(new Error("Read aborted"));
    reader.readAsDataURL(blob);
  });
}

Answer 4

DataURL to Blob has a better solution:

const dataURL = "dataURL string...";
const res = await fetch(dataURL);
const blob = await res.blob();

Answer 5

I like one-liners, so here's a Promise-based one to convert a blob to a data URL:

let blob = ...;
let dataUrl = await new Promise(r => {let a=new FileReader(); a.onload=r; a.readAsDataURL(blob)}).then(e => e.target.result);

Or, as a function:

function blobToDataUrl(blob) {
  return new Promise(r => {let a=new FileReader(); a.onload=r; a.readAsDataURL(blob)}).then(e => e.target.result);
}

let dataUrl = await blobToDataUrl(blob);