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);
}
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.
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);}
});
}
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
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
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));
The answer from #Arun Killu is a good snippet if you know what is going on, but nobody has explained what was the error on the original code. For people using async and Promise calls this is error is soo obvious but for people learning or without experience it's not so clear.
Here a simple explanation.
The Bad code
var blob = items[i].getAsFile();
var reader = new FileReader();
reader.onload = function(event){
console.log(event.target.result)
}; // data url!
var source = reader.readAsBinaryString(blob);
Above code is trying to capture a binary string on source variable, however, FileReader.readAsBinaryString() returns undefined. This is because the result will be available whenever the event onload will be triggered. As we can see, he was trying to console.log the event.target.result value, which is a wrong approach.
The Good code
Here is a step by step implementation:
// 1. Create a FileReader instance
const reader = new FileReader()
// 2. Add a handler for the 'onload' event
reader.onload = (e) => {
// 5. Get the result when the 'onload' event is triggered.
const base64data = reader.result
console.log({base64data})
}
// 3. Add a handler for the 'onerror' event
reader.onerror = () => {
console.log('error')
}
// 4. Call 'readAsDataURL' method
reader.readAsDataURL(imageBlob)
As you can see the last step is 5 and it is because it is an asynchronous call.
Here is a working example:
const url = 'https://i.stack.imgur.com/RRuCp.png'
const fetchImage = async url => {
const response = await fetch(url, {mode: 'no-cors',})
const blob = await response.blob()
return blob
}
const loadImage = async () => {
const imageBlob = await fetchImage(url)
const reader = new FileReader()
reader.onload = () => {
const base64data = reader.result
console.log({base64data})
}
reader.onerror = () => {
console.log('error')
}
reader.readAsDataURL(imageBlob)
}
loadImage()
Teo, what means asynchronous?
Well young Padawan, asynchronous means that we don't know when the result will be ready, it can be different in each system and depends on how heavy or complex is the process and also it can find some errors that will not produce any result at all.
So if a process is asynchronous is a good practice to encapsulate it using an async method and returning a Promise like this:
const blobToBase64 = async blob => {
return new Promise((resolve, reject) => {
const reader = new FileReader();
reader.onload = () => resolve(reader.result)
reader.error = (err) => reject(err)
reader.readAsDataURL(blob)
})
}
Ah okay Teo, but what is a Promise?
Good question my young fella. A Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value. In other words, will tell us if the result is ready and will give us its value, otherwise will return an error.
Above code shows a function blobToBase64 that will return a Promise This means that this function will return reader.result when it ready.
How can we integrate it into our code?
Super easy, just replace all the FileReader with the function blobToBase64 defined above and call it like this imageBase64 = await blobToBase64(imageBlob)
Check this snippet:
const url = 'https://i.stack.imgur.com/RRuCp.png'
const fetchImage = async url => {
const response = await fetch(url, {
mode: 'no-cors',
})
const blob = await response.blob()
return blob
}
const blobToBase64 = async blob => {
return new Promise((resolve, reject) => {
const reader = new FileReader();
reader.onload = () => resolve(reader.result)
reader.error = (err) => reject(err)
reader.readAsDataURL(blob)
})
}
const loadImage = async() => {
const imageBlob = await fetchImage(url)
const imageBase64 = await blobToBase64(imageBlob)
console.log({imageBase64})
}
loadImage()
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))
Maybe I'm missing something but
let encoded = btoa(await myblob.text());
... is all you need to do to encode a Blob's data to base64. See Blob.text() and btoa().
Or if you want the whole thing as a promise:
let encode = myblob.text().then(btoa);
PS: To decode back to a Blob: new Blob([atob(encoded)])
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
If your "blob" is an actual Blob Object and not a blob url, the conversion is pretty simple:
const reader = new FileReader()
reader.readAsDataURL(blob)
reader.onload = () => resolve(reader.result)
example of Blob Object:
console.log(blob)
output:
Blob {lastModified: 1658039931443, lastModifiedDate: Sun Jul 17 2022 08:38:51 GMT+0200 (Central European Summer Time), name: '1.jpg', size: 35493, type: 'image/jpeg'}
lastModified: 1658039931443
lastModifiedDate: Sun Jul 17 2022 08:38:51 GMT+0200 (Central European Summer Time) {}
name: "1.jpg"
size: 35493
type: "image/jpeg"
[[Prototype]]: Blob
In my case, the blob was produced by Compressorjs (should you need image compression).
I have this function where I call a function and have a local file as the parameter to convert it to base64.
export const fileToBase64 = (filename, filepath) => {
return new Promise(resolve => {
var file = new File([filename], filepath);
var reader = new FileReader();
// Read file content on file loaded event
reader.onload = function(event) {
resolve(event.target.result);
};
// Convert data to base64
reader.readAsDataURL(file);
});
}
Importing the function
fileToBase64("shield.png", "./form").then(result => {
console.log(result);
console.log("here");
});
gives me an output as
data:application/octet-stream;base64,c2hpZWxkLnBuZw==
here
I want base64 information, but noticing the file the application/octet-stream is wrong? I entered an image so shouldn't it be
data:image/pgn;base64,c2hpZWxkLnBuZw==
https://medium.com/#simmibadhan/converting-file-to-base64-on-javascript-client-side-b2dfdfed75f6
try this I think this should helpfull
let buff = new Buffer(result, 'base64');
let text = buff.toString('ascii');
console.log(text)
So I am trying to convert a file from tag. This is how my javascript code looks like:
var file = document.getElementById("file").files[0];
if (file) {
var filereader = new FileReader();
filereader.readAsDataURL(file);
filereader.onload = function (evt) {
var base64 = evt.target.result;
}
}
That returns undefined.
two little helper and an example.
const blobToDataUrl = blob => new Promise((resolve, reject) => {
const reader = new FileReader();
reader.onload = () => resolve(reader.result);
reader.onerror = reject;
reader.readAsDataURL(blob);
});
const blobToBase64 = blob => blobToDataUrl(blob).then(text => text.slice(text.indexOf(",")));
for(let file of document.getElementById("file").files) {
blobToBase64(file).then(base64 => console.log(file, base64));
}
But why the Promises?
Because your next question will be: How do I get the base64 string out of onload? and the short answer is You don't. A longer answer would be: It's like asking how to get something from the future into the now. You can't.
Promises are placeholder/wrapper for values that will eventually be available; but not yet. And they are the foundation of async functions.
So let's skip messing with callbacks and get right to the point where you write
for(let file of document.getElementById("file").files) {
const base64 = await blobToBase64(file);
console.log(file, base64);
}
but for that you will have to brush up on async and await.
I think you have missed the return statement in the code.
Replace your function with the following lines:
var file = document.getElementById("file").files[0];
if (file) {
var filereader = new FileReader();
filereader.readAsDataURL(file);
filereader.onload = function (evt) {
var base64 = evt.target.result;
return base64
}
}
I'm trying to upload an array of files using FileReader which are base64 encoded and stored in an array for further processing. I'm struggling to understand the pattern I need to create in order to ensure all files have been uploaded because I have to wait for the onload event handler to fire. For example;
If I pass an array of files to the following function it will resolve the promise before the files are actually uploaded.
localUploads( event ) : any {
var response = [];
return new Promise( function(resolve, reject) {
//Retrieve all the files from the FileList object
var files = event.target.files;
var response = [];
if (files) {
for (var i=0, f; f=files[i]; i++) {
var r = new FileReader();
r.onload = (function(f) {
return function(e) {
let contents = e.target['result'];
let file = {
name: f.name,
asset: contents,
private: false
};
console.log('we are pushing into the array');
response.push( file );
};
})(f);
}
resolve( response );
}
r.readAsText(f);
});
}
Can anybody please advise a novice?
Many thanks.
It's all about resolving the promise at the right time. Currently you resolve it when the loop is done, but that doesn't mean that all the items have been processed. I haven't used FileReader, but you should be able to do something like this:
localUploads( event ) : any {
var response = [];
return new Promise(function(resolve, reject) {
//Retrieve all the files from the FileList object
var files = event.target.files;
var response = [];
if (files) {
for (var i=0, f; f=files[i]; i++) {
var r = new FileReader();
r.onload = function(e) { // Possible clean-up?
let contents = e.target['result'];
let file = {
name: f.name,
asset: contents,
private: false
};
console.log('we are pushing into the array');
response.push( file );
if(response.length == files.length)
{
// Everything is done. Resolve the promise.
resolve( response );
}
};
// Moved here to be able to access r and f variables
r.readAsText(f);
}
}
});
}
Or the old way using $q.
var response = [];
var dfd = $q.defer();
//Retrieve all the files from the FileList object
var files = event.target.files;
var response = [];
if (files) {
for (var i=0, f; f=files[i]; i++) {
var r = new FileReader();
r.onload = function(e) { // Possible clean-up?
let contents = e.target['result'];
let file = {
name: f.name,
asset: contents,
private: false
};
console.log('we are pushing into the array');
response.push( file );
if(response.length == files.length)
{
// Everything is done. Resolve the promise.
dfd.resolve(response);
}
};
// Moved here to be able to access r and f variables
r.readAsText(f);
}
}
else {
// Possible resolve promise here to?
}
return dfd.promise;
Note that you might also want to handle the possible errors. If one of the files isn't completed sucessfully the promise will never be resolved.
You might need to resolve the promise in onloadend instead of onload. I really cannot figure it out from the docs.
var r = new FileReader();
r.onload = function(e) {
if(response.length == files.length)
{
// Everything is done. Resolve the promise.
dfd.resolve(response);
}
}
I'm trying to read a text file of over 150,000 lines of text. I want to be able to read the text file and pass it as a parameter for processFileContent.
I tried it this way, but it doesn't work. Also, is there a better way to do this for such big data?
function readFile(file) {
var reader = new FileReader();
reader.onload = function (evt) {
var data = evt.target.result;
};
reader.readAsText(file);
return data;
}
document.getElementById('file').addEventListener('change', readFile, false);
var data = readFile();
function processFileContent(data) {
var list = data.split('\n');
...
FileReader.onload event returns results asynchronously. You can use a callback or Promise to return result of FileReader to processFileContent. Also file at readFile would be event object, not .files property of event.target.
function readFile(event) {
var file = event.target.files[0];
if (file) {
new Promise(function(resolve, reject) {
var reader = new FileReader();
reader.onload = function (evt) {
resolve(evt.target.result);
};
reader.readAsText(file);
reader.onerror = reject;
})
.then(processFileContent)
.catch(function(err) {
console.log(err)
});
}
}
document.getElementById('file')
.addEventListener('change', readFile, false);
function processFileContent(data) {
var list = data.split('\n');
...
One of your problems is with scoping. You declared data as a local variable in the onload event handler, and tried returning it from the outer function, in which it was undefined. To fix this, you need to move the variable declaration out of the event handler.
You also are expecting a file as an argument to your event handler it seems, however events pass Event objects to their event handlers. To get the file, you need to get event.target.files[0]. This should fix it.
function readFile(event) {
var file = event.target.files[0]; // getting the file Blob
var reader = new FileReader();
var data; // moved declaration
reader.onload = function (evt) {
data = evt.target.result;
};
reader.readAsText(file);
return data;
}