Convert blob to base64

This is a snippet for the code that I want to do Blob to Base64 string:

This commented part works and that when the URL generated by this is set to img src it displays the image:

var blob = items[i].getAsFile();
//var URLObj = window.URL || window.webkitURL;
//var source = URLObj.createObjectURL(blob);
//console.log("image source=" + source);
		                
var reader = new FileReader();
reader.onload = function(event){
console.log(event.target.result)
}; // data url!
var source = reader.readAsBinaryString(blob);

The problem is with the the lower code, the source variable generated is null

Update:

Is there an easier way to do this with JQuery to be able to create Base64 String from Blob file as in the code above?

var reader = new FileReader();
reader.readAsDataURL(blob); 
reader.onloadend = function() {
  var base64data = reader.result;                
  console.log(base64data);
}

Form the docs readAsDataURL encodes to base64

As an awaitable function:

function blobToBase64(blob) {
  return new Promise((resolve, _) => {
    const reader = new FileReader();
    reader.onloadend = () => resolve(reader.result);
    reader.readAsDataURL(blob);
  });
}

> Note: The blob's result cannot be directly decoded as Base64 without first removing the Data-URL declaration preceding the Base64-encoded data. To retrieve only the Base64 encoded string, first remove data:/;base64, from the result.

this worked for me:

var blobToBase64 = function(blob, callback) {
	var reader = new FileReader();
	reader.onload = function() {
		var dataUrl = reader.result;
		var base64 = dataUrl.split(',')[1];
		callback(base64);
	};
	reader.readAsDataURL(blob);
};

There is a pure JavaScript way that is not depended on any stacks:

const blobToBase64 = blob => {
  const reader = new FileReader();
  reader.readAsDataURL(blob);
  return new Promise(resolve => {
    reader.onloadend = () => {
      resolve(reader.result);
    };
  });
};

For using this helper function you should set a callback, example:

blobToBase64(blobData).then(res => {
  // do what you wanna do
  console.log(res); // res is base64 now
});

---

I write this helper function for my problem on React Native project, I wanted to download an image and then store it as a cached image:

fetch(imageAddressAsStringValue)
  .then(res => res.blob())
  .then(blobToBase64)
  .then(finalResult => { 
    storeOnMyLocalDatabase(finalResult);
  });

var audioURL = window.URL.createObjectURL(blob);
audio.src = audioURL;

var reader = new window.FileReader();
reader.readAsDataURL(blob);
reader.onloadend = function () {
     base64data = reader.result;
     console.log(base64data);
}

So the problem is that you want to upload a base 64 image and you have a blob url. Now the answer that will work on all html 5 browsers is: Do:

  var fileInput = document.getElementById('myFileInputTag');
  var preview = document.getElementById('myImgTag');

  fileInput.addEventListener('change', function (e) {
      var url = URL.createObjectURL(e.target.files[0]);
      preview.setAttribute('src', url);
  });
function Upload()
{
     // preview can be image object or image element
     var myCanvas = document.getElementById('MyCanvas');
     var ctx = myCanvas.getContext('2d');
     ctx.drawImage(preview, 0,0);
     var base64Str = myCanvas.toDataURL();
     $.ajax({
         url: '/PathToServer',
         method: 'POST',
         data: {
             imageString: base64Str
         },
     success: function(data) { if(data && data.Success) {}},
     error: function(a,b,c){alert(c);}
     });
 }

function bufferToBinaryString(arrayBuffer){
    return String.fromCharCode(...new Uint8Array(arrayBuffer));
}
(async () => console.log(btoa(bufferToBinaryString(await new Response(blob).arrayBuffer()))))();

or

function bufferToBinaryString(arrayBuffer){
    return String.fromCharCode(...new Uint8Array(arrayBuffer));
}
new Response(blob).arrayBuffer().then(arr_buf => console.log(btoa(bufferToBinaryString(arr_buf)))))

see Response's constructor, you can turn [blob, buffer source form data, readable stream, etc.] into Response, which can then be turned into [json, text, array buffer, blob] with async method/callbacks.

edit: as @Ralph mentioned, turning everything into utf-8 string causes problems (unfortunately Response API doesn't provide a way converting to binary string), so array buffer is use as intermediate instead, which requires two more steps (converting it to byte array THEN to binary string), if you insist on using native btoa method.

async function blobToBase64(blob) {
  return new Promise((resolve, _) => {
    const reader = new FileReader();
    reader.onloadend = () => resolve(reader.result);
    reader.readAsDataURL(blob);
  });
}

let blob = null; // <= your blob object goes here

blobToBase64(blob)
  .then(base64String => console.log(base64String));

See also:

• https://gist.github.com/moosetraveller/723987931308c9ec63725c14cdcbc3e7

you can fix problem by:

var canvas = $('#canvas'); 
var b64Text = canvas.toDataURL();
b64Text = b64Text.replace('data:image/png;base64,','');
var base64Data = b64Text;

I hope this help you

Another way is to use a simple wrapper around FileReader returning Observable (snippet is in TypeScript):

  function toBase64(blob: Blob): Observable<string> {
    const reader = new FileReader();
    reader.readAsDataURL(blob);
    return fromEvent(reader, 'load')
      .pipe(map(() => (reader.result as string).split(',')[1]));
  }

Usage:

toBase64(blob).subscribe(base64 => console.log(base64));

Typescript version :

const blob2Base64 = (blob:Blob):Promise<string> => {
      return new Promise<string> ((resolve,reject)=> {
           const reader = new FileReader();
           reader.readAsDataURL(blob);
           reader.onload = () => resolve(reader.result.toString());
           reader.onerror = error => reject(error);
       })
      }

usage:

blob2Base64(blob).then(res=>console.log(res))

I wanted something where I have access to base64 value to store into a list and for me adding event listener worked. You just need the FileReader which will read the image blob and return the base64 in the result.

createImageFromBlob(image: Blob) {
    const reader = new FileReader();
    const supportedImages = []; // you can also refer to some global variable
    reader.addEventListener(
      'load',
      () => {
        // reader.result will have the required base64 image
        const base64data = reader.result;
        supportedImages.push(base64data); // this can be a reference to global variable and store the value into that global list so as to use it in the other part
      },
      false
    );
    // The readAsDataURL method is used to read the contents of the specified Blob or File.
    if (image) {
      reader.readAsDataURL(image);
    }
 }

Final part is the readAsDataURL which is very important is being used to read the content of the specified Blob

async TypeScript variation:

async function blobToBase64Async(blob: Blob): Promise<string> {
  return new Promise((resolve, reject) => {
    const fileReader = new FileReader();
    fileReader.onerror = (e) => reject(fileReader.error);
    fileReader.onloadend = (e) => {
      const dataUrl = fileReader.result as string;
      // remove "data:mime/type;base64," prefix from data url
      const base64 = dataUrl.substring(dataUrl.indexOf(',') + 1);
      resolve(base64);
    };
    fileReader.readAsDataURL(blob);
  });
}

Sample usage:

async function fetchToBase64Async(url: string, init?: RequestInit): Promise<string> {
  try {
    const response = await fetch(url, init);
    if (!response.ok) {
      const responseText = await response.text();
      throw new Error("server status: " + response.status + "\n" + "server response:" + "\n" + responseText);
    }
    const blob = await response.blob();
    const base64 = await blobToBase64Async(blob);
    return base64;
  } catch (e) {
    throw new Error("failed to fetch: " + url + "\n" + "caused by: " + e);
  }
}

async function demoUsage() {
  const base64 = await fetchToBase64Async("https://httpstat.us/200", {
    method: "POST",
    headers: {
      "Accept": "*/*",
      "Authorization": "Bearer ...",
    }
  });
  console.log(base64);
}

Notes:

• I don't understand why some answers use the load instead of the loadend event

• I don't understand why some answers call readAsDataURL before setting the event handler