How to convert Blob to File in JavaScript

Question

I need to upload an image to NodeJS server to some directory. I am using connect-busboy node module for that.

I had the dataURL of the image that I converted to blob using the following code:

dataURLToBlob: function(dataURL) {
    var BASE64_MARKER = ';base64,';
    if (dataURL.indexOf(BASE64_MARKER) == -1) {
        var parts = dataURL.split(',');
        var contentType = parts[0].split(':')[1];
        var raw = decodeURIComponent(parts[1]);
        return new Blob([raw], {type: contentType});
    }
    var parts = dataURL.split(BASE64_MARKER);
    var contentType = parts[0].split(':')[1];
    var raw = window.atob(parts[1]);
    var rawLength = raw.length;
    var uInt8Array = new Uint8Array(rawLength);
    for (var i = 0; i < rawLength; ++i) {
        uInt8Array[i] = raw.charCodeAt(i);
    }
    return new Blob([uInt8Array], {type: contentType});
}

I need a way to convert the blob to a file to upload the image.

Could somebody help me with it?

Edouard

Files are Blobs, just tack on the meta properties and you're good to go. – Leodora

The default for a blob when uploading it is blob. So, I first extracted the name of the file I was cropping and then gave the same filename so the cropped file while uploading it to server by doing form.append("blob",blob, filename);. – Edouard

@Edouard did my answer below help out? Is so, please mark it as the correct answer. – Walter

Answer 1

This function converts a Blob into a File and it works great for me.

Vanilla JavaScript

function blobToFile(theBlob, fileName){
  //A Blob() is almost a File() - it's just missing the two properties below which we will add
  theBlob.lastModifiedDate = new Date();
  theBlob.name = fileName;
  return theBlob;
}

TypeScript (with proper typings)

public blobToFile = (theBlob: Blob, fileName:string): File => {
  const b: any = theBlob;
  //A Blob() is almost a File() - it's just missing the two properties below which we will add
  b.lastModifiedDate = new Date();
  b.name = fileName;
    
  //Cast to a File() type
  return theBlob as File;
}

Usage

const myBlob = new Blob();

//do stuff here to give the blob some data...

const myFile = blobToFile(myBlob, "my-image.png");

Walter

I think the cast should happen first so you don't need to use any in TypeScript. See this example. – Boni

This does not work for all purposes. Adding this "File" to an ajax call will not set the filename correctly. – Anastassia

This is not a good solution, the produced object still a blob. – Josey

@Josey It's been a while now, but if I recall correctly a file is a type of blob, so this should work – Walter

Hey Chris, I did not want to be offended you. Just wanted point out the the object type will be Blob and not File :(. Maybe I just needed something else. – Josey

Just add b.__proto__ = File.prototype, and your solution becomes a dream that even tricks b instanceOf File to true – Mufinella

Please note that lastModifiedDate is deprecated ( developer.mozilla.org/en-US/docs/Web/API/File/lastModifiedDate ), you should now use lastModified, setting lastModified throws an error in Firefox if the blob accidentally happens to be an instance of File already. – Dragon

this throws error Cannot assign to read only property 'name' of object '[object File]' – Cuman

This should not be the accepted answer, https://mcmap.net/q/108776/-how-to-convert-blob-to-file-in-javascript or https://mcmap.net/q/108776/-how-to-convert-blob-to-file-in-javascript should be. – Czardom

Thanks a lot for this. new File() does not work straightforward on Cordova. Didn't realize that a file was just a type of blob if it weren't for this answer. – Evasion

@JacobPoulRichardt You're correct, but this can can be fixed by adding the file name to the formdata like this: formData.append("file", yourFileObj, yourFileObj.name); – Freckly

Answer 2

You can use the File constructor:

var file = new File([myBlob], "name");

As per the w3 specification this will append the bytes that the blob contains to the bytes for the new File object, and create the file with the specified name http://www.w3.org/TR/FileAPI/#dfn-file

Brobdingnagian

This is what I was looking for, the one difference is the first argument is actually an array so I used it as: var file = new File([myBlob], "name"); – Hawkshaw

Yes. I am looking for this solution too. Stamplay server can not get the file name from blob object. So I need to convert it back to file. But Safari do not support file API... back to 0 now :( – Trull

Exactly what I was looking for. Thanks for this. Other solutions keep the file name to be 'bolb' whatever I changes it to, but your solution is keeping my file name. – Lithotomy

File constructors do not work e.g. in PhantomJS: TypeError: FileConstructor is not a constructor (evaluating 'new File([''], 'file.pdf', {'size': 1000, 'type': 'application/pdf'})') – Carrollcarronade

Edge has currently (2017-10-04) a bug that prevents the usage of this constructor. developer.microsoft.com/en-us/microsoft-edge/platform/issues/… – Weil

Be sure to add file type, otherwise it will not work properly. new File([myBlob], "name", { type: "image/jpeg", }); – Affecting

this is throwing ` ReferenceError: File is not defined` for me – Swats

dumb way fd = new FormData; fd.append('f', new Blob, "name"), fd.get('f') :) – Pressey

IE11 is not supporting new File() constructor, is there any alternative to convert a blob to File type ? assigning a filename and lastdateModified to blob is not working. – Merwyn

You can add image name using timestamp like this. var file = new File([blobImage], ${Date.now()}.png); – Directions

The File ctor method does not work for me on Ionic Capacitor on Native Devices - iOS, Android. Using the method that morphs the Blob object does work, however. – Disaffiliate

Answer 3

Joshua P Nixon's answer is correct but I had to set last modified date also. so here is the code.

var file = new File([blob], "file_name", {lastModified: 1534584790000});

1534584790000 is an unix timestamp for "GMT: Saturday, August 18, 2018 9:33:10 AM"

Profanatory

Points for not breaking instanceof like the accepted answer – Fukuoka

side note, if anyone is doing this and doesn't want to hard code that it was last modified in august 2018, you can always put lastModified: Date.now() and that'll return the current time in milliseconds! (just like how the example given is august 18 2018 at 9:33:10 in milliseconds) – Cantina

Answer 4

Typescript

public blobToFile = (theBlob: Blob, fileName:string): File => {       
    return new File(
        [theBlob as any], // cast as any
        fileName, 
        {
            lastModified: new Date().getTime(),
            type: theBlob.type 
        }
    )
}

Javascript

function blobToFile(theBlob, fileName){       
    return new File([theBlob], fileName, { lastModified: new Date().getTime(), type: theBlob.type })
}

Output

File {name: "fileName", lastModified: 1597081051454, lastModifiedDate: Mon Aug 10 2020 19:37:31 GMT+0200 (Eastern European Standard Time), webkitRelativePath: "", size: 601887, …}
lastModified: 1597081051454
lastModifiedDate: Mon Aug 10 2020 19:37:31 GMT+0200 (Eastern European Standard Time) {}
name: "fileName"
size: 601887
type: "image/png"
webkitRelativePath: ""
__proto__: File

Cowslip

Answer 5

For me to work I had to explicitly provide the type although it is contained in the blob by doing so:

const file = new File([blob], 'untitled', { type: blob.type })

Ehling

Answer 6

My modern variant:

function blob2file(blobData) {
  const fd = new FormData();
  fd.set('a', blobData, 'filename');
  return fd.get('a');
}

Columbine

To add a filename to the resulting file use: fd.set('a', blobData, 'filename') – Tolbooth

It fails in some IOS device when fd.set – Anhanhalt

I had to add the file name with the extension to make it work right. Like: fd.set('a', blobData, 'filename.jpg') – Tristan

Answer 7

var blob = new Blob(["Hello, world!"], { type: "text/plain;charset=utf-8" });

var file = new File([blob], "name.txt");

Now you can upload as .txt file

Hueyhuff

Answer 8

This problem was bugging me for hours. I was using Nextjs and trying to convert canvas to an image file.

I used the other guy's solution but the created file was empty.

So if this is your problem you should mention the size property in the file object.

new File([Blob], `my_image${new Date()}.jpeg`, {
  type: "image/jpeg",
  lastModified: new Date(),
  size: 2,
});

Just add the key the value it's not important.

Abrahamsen

size isn't a valid option – Pressey

Not working bro !! , Blob is actually the object right ? – Lath

Answer 9

I have used FileSaver.js to save the blob as file.

This is the repo : https://github.com/eligrey/FileSaver.js/

Usage:

import { saveAs } from 'file-saver';

var blob = new Blob(["Hello, world!"], {type: "text/plain;charset=utf-8"});
saveAs(blob, "hello world.txt");

saveAs("https://httpbin.org/image", "image.jpg");

Cryptanalysis

this dose not answer the question – Pressey

he asked to convert a file not save a file – Cowslip