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JS: how can I base64 encode a local file without XMLHttpRequest?

I'm trying to base64 encode a local file. It's next to my .js file so there's no uploading going on. Solutions like this (using XMLHttpRequest) get a cross-site scripting error.
I'm trying something like this (which doesn't work but it might help explain my problem):
var file = 'file.jpg'
var reader = new FileReader();
reader.onload = function(e) {
var res = e.target.result;
console.log(res);
};
var f = reader.readAsDataURL(file);
Anyone have any experience doing this locally?

Solutions like this (using XMLHttpRequest) get a cross-site
scripting error.
If using chrome or chromium browser, you could launch with --allow-file-access-from-files flag set to allow request of resource from local filesystem using XMLHttpRequest() or canvas.toDataURL().
You can use <img> element, <canvas> element .toDataURL() to create data URL of local image file without using XMLHttpRequest()
var file = "file.jpg";
var img = new Image;
var canvas = document.createElement("canvas");
var ctx = canvas.getContext("2d");
img.onload = function() {
canvas.width = this.naturalWidth;
canvas.height = this.naturalHeight;
ctx.drawImage(this, 0, 0);
var res = canvas.toDataURL("image/jpeg", 1); // set image `type` to `image/jpeg`
console.log(res);
}
img.src = file;
You could alternatively use XMLHttpRequest() as described at Convert local image to base64 string in Javascript.
See also How to print all the txt files inside a folder using java script .
For a details of difference of returned data URI from either approach see canvas2d toDataURL() different output on different browser
As described by #Kaiido at comment below
it will first decode it, at this stage it's still your file, then it
will paint it to the canvas (now it's just raw pixels) and finally it
will reencode it (it has nothing to do with your original file
anymore) check the dataURI strings... They're compeltely different and
even if you do the canvas operation from two different browsers,
you'll have different outputs, while FileReader will always give you
the same output, since it encode the file directly, it doesn't decode
it.

How do I save and restore a File object in local storage

I have an HTML5/javscript app which uses
<input type="file" accept="image/*;capture=camera" onchange="gotPhoto(this)">
to capture a camera image. Because my app wants to be runnable offline, how do I save the File (https://developer.mozilla.org/en-US/docs/Web/API/File) object in local storage, such that it can be retrieved later for an ajax upload?
I'm grabbing the file object from the using ...
function gotPhoto(element) {
var file = element.files[0];
//I want to save 'file' to local storage here :-(
}
I can Stringify the object and save it, but when I restore it, it is no longer recognised as a File object, and thus can't be used to grab the file content.
I have a feeling it can't be done, but am open to suggestions.
FWIW My workaround is to read the file contents at store time and save the full contents to local storage. This works, but quickly consumes local storage since each file is a 1MB plus photograph.

You cannot serialize file API object.
Not that it helps with the specific problem, but ...
Although I haven't used this, if you look at the article it seems that there are ways (although not supported yet by most browsers) to store the offline image data to some files so as to restore them afterward when the user is online (and not to use localStorage)

Convert it to base64 and then save it.
function gotPhoto(element) {
var file = element.files[0];
var reader = new FileReader()
reader.onload = function(base64) {
localStorage["file"] = base64;
}
reader.readAsDataURL(file);
}
// Saved to localstorage
function getPhoto() {
var base64 = localStorage["file"];
var base64Parts = base64.split(",");
var fileFormat = base64Parts[0].split(";")[1];
var fileContent = base64Parts[1];
var file = new File([fileContent], "file name here", {type: fileFormat});
return file;
}
// Retreived file object

Here is a workaround that I got working with the code below. I'm aware with your edit you talked about localStorage but I wanted to share how I actually implemented that workaround. I like to put the functions on body so that even if the class is added afterwards via AJAX the "change" command will still trigger the event.
See my example here: http://jsfiddle.net/x11joex11/9g8NN/
If you run the JSFiddle example twice you will see it remembers the image.
My approach does use jQuery. This approach also demonstrates the image is actually there to prove it worked.
HTML:
<input class="classhere" type="file" name="logo" id="logo" />
<div class="imagearea"></div>
JS:
$(document).ready(function(){
//You might want to do if check to see if localstorage set for theImage here
var img = new Image();
img.src = localStorage.theImage;
$('.imagearea').html(img);
$("body").on("change",".classhere",function(){
//Equivalent of getElementById
var fileInput = $(this)[0];//returns a HTML DOM object by putting the [0] since it's really an associative array.
var file = fileInput.files[0]; //there is only '1' file since they are not multiple type.
var reader = new FileReader();
reader.onload = function(e) {
// Create a new image.
var img = new Image();
img.src = reader.result;
localStorage.theImage = reader.result; //stores the image to localStorage
$(".imagearea").html(img);
}
reader.readAsDataURL(file);//attempts to read the file in question.
});
});
This approach uses the HTML5 File System API's to read the image and put it into a new javascript img object. The key here is readAsDataURL. If you use chrome inspector you will notice the images are stored in base64 encoding.
The reader is Asynchronous, this is why it uses the callback function onload. So make sure any important code that requires the image is inside the onLoad or else you may get unexpected results.

You could use this lib:
https://github.com/carlo/jquery-base64
then do something similar to this:
//Set file
var baseFile = $.base64.encode(fileObject);
window.localStorage.setItem("file",basefile);
//get file
var outFile = window.localStorage.getItem("file");
an other solution would be using json (I prefer this method)
using: http://code.google.com/p/jquery-json/
//Set file
window.localStorage.setItem("file",$.toJSON(fileobject));
//get file
var outFile = $.evalJSON(window.localStorage.getItem("file"));

I don't think that there is a direct way to Stringify and then deserialize the string object into the object of your interest. But as a work around you can store the image paths in your local storage and load the images by retrieving the URL for the images. Advantages would be, you will never run out of storage space and you can store 1000 times more files there.. Saving an image or any other file as a string in local storage is never a wise decision..

create an object on the global scope
exp: var attmap = new Object();
after you are done with file selection, put your files in attmap variable as below,
attmap[file.name] = attachmentBody;
JSON.stringify(attmap)
Then you can send it to controller via input hidden or etc. and use it after deserializing.
(Map<String, String>)JSON.deserialize(attachments, Map<String,String>.class);
You can create your files with those values in a for loop or etc.
EncodingUtil.base64Decode(CurrentMapValue);
FYI:This solution will also cover multiple file selection

You could do something like this:
// fileObj = new File(); from file input
const buffer = Buffer.from(await new Response(fileObj).arrayBuffer());
const dataUrl = `data:${fileObj.type};base64,${buffer.toString("base64")}`;
localStorage.setItem('dataUrl', dataUrl);
then you can do:
document.getElementById('image').src = localStorage.getItem('dataUrl');

Image not showing using chrome filesystem toURL

I have the following code to write an image into the filesystem, and read it back for display. Prior to trying out the filesystem API, I loaded the whole base64 image into the src attribute and the image displayed fine. Problem is the images can be large so if you add a few 5MB images, you run out of memory. So I thought I'd just write them to the tmp storage and only pass the URL into the src attribute.
Trouble is, nothing gets displayed.
Initially I thought it might be something wrong with the URL, but then I went into the filesystem directory, found the image it was referring to and physically replaced it with the real binary image and renamed it to the same as the replaced image. This worked fine and the image is displayed correctly, so the URL looks good.
The only conclusion I can come to is that the writing of the image is somehow wrong - particularly the point where the blob is created. I've looked through the blob API and can't see anything that I may have missed, however I'm obviously doing something wrong because it seems to be working for everyone else.
As an aside, I also tried to store the image in IndexedDB and use the createObjectURL to display the image - again, although the URL looks correct, nothing is displayed on the screen. Hence the attempt at the filesystem API. The blob creation is identical in both cases, with the same data.
The source data is a base64 encoded string as I mentioned. Yes, I did also try to store the raw base64 data in the blob (with and without the prefix) and that didn't work either.
Other info - chrome version 28, on linux Ubuntu
//strip the base64 `enter code here`stuff ...
var regex = /^data.+;base64,/;
if (regex.test(imgobj)) { //its base64
imgobj = imgobj.replace(regex,"");
//imgobj = B64.decode(imgobj);
imgobj = window.atob(imgobj);
} else {
console.log("it's already :", typeof imgobj);
}
// store the object into the tmp space
window.requestFileSystem(window.TEMPORARY, 10*1024*1024, function(fs) {
// check if the file already exists
fs.root.getFile(imagename, {create: false}, function(fileEntry) {
console.log("File exists: ", fileEntry);
callback(fileEntry.toURL(), fileEntry.name);
//
}, function (e) { //file doesn't exist
fs.root.getFile(imagename, {create: true}, function (fe) {
console.log("file is: ", fe);
fe.createWriter(function(fw){
fw.onwriteend = function(e) {
console.log("write complete: ", e);
console.log("size of file: ", e.total)
callback(fe.toURL(), fe.name);
};
fw.onerror = function(e) {
console.log("Write failed: ", e.toString());
};
var data = new Blob([imgobj], {type: "image/png"});
fw.write(data);
}, fsErrorHandler);
}, fsErrorHandler);
});
// now create a file
}, fsErrorHandler);
Output from the callback is:
<img class="imgx" src="filesystem:file:///temporary/closed-padlock.png" width="270px" height="270px" id="img1" data-imgname="closed-padlock.png">
I'm at a bit of a standstill unless someone can provide some guidance...
UPDATE
I ran a test to encode and decode the base64 image with both the B64encoder/decoder and atob/btoa -
console.log(imgobj); // this is the original base64 file from the canvas.toDataURL function
/* B64 is broken*/
B64imgobjdecode = B64.decode(imgobj);
B64imgobjencode = B64.encode(B64imgobjdecode);
console.log(B64imgobjencode);
/* atob and btoa decodes and encodes correctly*/
atobimgobj = window.atob(imgobj);
btoaimgobj = window.btoa(atobimgobj);
console.log(btoaimgobj);
The results show that the btoa/atob functions work correctly but the B64 does not - probably because the original encoding didn't use the B64.encode function...
The resulting file in filesystem TEMPORARY, I ran through an online base64 encoder for comparison and the results are totally different. So the question is - while in the filesystem temp storage, is the image supposed to be an exact image, or is it padded with 'something' which only the filesystem API understands? Remember I put the original PNG in the file system directory and the image displayed correctly, which tends to indicate that the meta-data about the image (eg. the filename) is held elsewhere...
Can someone who has a working implementation of this confirm if the images are stored as images in the filesystem, or are padded with additional meta-data?

So to answer my own question - the core problem was in the base64 encoding/decoding - I've since then changed this to use things like ajax and responseTypes like arraybuffer and blob and things have started working.
To answer the last part of the question, this is what I've found - in the filesystem tmp storage, yes the file is supposed to be an exact binary copy - verified this in chrome and phonegap.

Save PNG Canvas Image to HTML5 Storage (JAVASCRIPT)?

I am developing a chrome extension.
I open an image file in canvas, I apply some changes to it, then I am trying to save it to the HTML5 filesystem api.
First I get the dataURL from the canvas:
var dataURL = canvas.toDataURL('image/png;base64');
Then just the data:
var image64 = dataURL.replace(/data:image\/png;base64,/, '');
Then I make a Blob.
var bb = new BlobBuilder();
bb.append(image64);
var blob = bb.getBlob('image/png');
Then I request the file system with the following function onInitFs();
function onInitFs(fs) {
fs.root.getFile('image.png', {create: true}, function(fileEntry) {
// Create a FileWriter object for our FileEntry (log.txt).
fileEntry.createWriter(function(fileWriter) {
//WRITING THE BLOB TO FILE
fileWriter.write(blob);
}, errorHandler);
}, errorHandler);
}
window.requestFileSystem(window.PERSISTENT, 5*1024*1024, onInitFs, errorHandler);
This results in a corrupted file being written to the file system.
I don't know what else I can do to make this work.
Could someone please guide me in the right direction.
The following are some of the sources to the functions I am using to accomplish this task.
http://dev.w3.org/html5/canvas-api/canvas-2d-api.html#todataurl-method
http://www.html5rocks.com/en/tutorials/file/filesystem/#toc-file-creatingempty
Thank You!

I've found a function that converts a data URL to a blob.
Great for when you need to save a canvas image to the sandboxed FileSystem. Works in Chrome 13.
function dataURItoBlob(dataURI, callback) {
// convert base64 to raw binary data held in a string
// doesn't handle URLEncoded DataURIs
var byteString = atob(dataURI.split(',')[1]);
// separate out the mime component
var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0]
// write the bytes of the string to an ArrayBuffer
var ab = new ArrayBuffer(byteString.length);
var ia = new Uint8Array(ab);
for (var i = 0; i < byteString.length; i++) {
ia[i] = byteString.charCodeAt(i);
}
// write the ArrayBuffer to a blob, and you're done
var bb = new window.WebKitBlobBuilder(); // or just BlobBuilder() if not using Chrome
bb.append(ab);
return bb.getBlob(mimeString);
};
Usage:
window.requestFileSystem(window.PERSISTENT, 1024*1024, function(fs){
fs.root.getFile("image.png", {create:true}, function(fileEntry) {
fileEntry.createWriter(function(fileWriter) {
fileWriter.write(dataURItoBlob(myCanvas.toDataURL("image/png")));
}, errorHandler);
}, errorHandler);
}, errorHandler);
Source trail:
http://mustachified.com/master.js
via http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2011-April/031243.html
via https://bugs.webkit.org/show_bug.cgi?id=51652
via http://code.google.com/p/chromium/issues/detail?id=67587

There is now a canvas.toBlob() polyfill: https://github.com/eligrey/canvas-toBlob.js
Other interesting link: https://github.com/eligrey/BlobBuilder.js
So with these it become easy to save image from canvas:
<script type="text/javascript" src="https://raw.github.com/eligrey/BlobBuilder.js/master/BlobBuilder.min.js"></script>
<script type="text/javascript" src="https://raw.github.com/eligrey/canvas-toBlob.js/master/canvas-toBlob.min.js"></script>
...
canvas.toBlob(function(blob) {
fileWriter.write(blob);
}, "image/png");
Here is a little example using the FileSaver: http://jsfiddle.net/JR5XE/

You seem to be directly writing the base64 representation to disk. You need to decode it first.

I had been wondering the same thing and had a look at finding an answer.
From what I can tell you have to convert the raw string you get from an atob to an unit8array and then you can append it to a blob.
Here's an example that converts an image to a canvas and then the data from the canvas to a blob and then the third images src is set to the uri of the blob....you should be able to add the fs stuff from there....
http://jsfiddle.net/PAEz/XfDUS/
..sorry to not put the code here, but to be honest I couldnt figure out how to ;) and anyways, JSFiddle is a nice way to play with it.
References
https://gist.github.com/1032746
http://code.google.com/p/html5wow/source/browse/src/demos/photo-gallery/js/utils.js
Get image data in JavaScript?
http://jsdo.it/tsmallfield/typed_array

You want HTMLCanvasElement.toBlob and then write the blob to a filesystem with the W3C filesystem API, but most browsers haven't implemented it yet.

Using HTML5/JavaScript to generate and save a file

I've been fiddling with WebGL lately, and have gotten a Collada reader working. Problem is it's pretty slow (Collada is a very verbose format), so I'm going to start converting files to a easier to use format (probably JSON). I already have the code to parse the file in JavaScript, so I may as well use it as my exporter too! The problem is saving.
Now, I know that I can parse the file, send the result to the server, and have the browser request the file back from the server as a download. But in reality the server has nothing to do with this particular process, so why get it involved? I already have the contents of the desired file in memory. Is there any way that I could present the user with a download using pure JavaScript? (I doubt it, but might as well ask...)
And to be clear: I am not trying to access the filesystem without the users knowledge! The user will provide a file (probably via drag and drop), the script will transform the file in memory, and the user will be prompted to download the result. All of which should be "safe" activities as far as the browser is concerned.
[EDIT]: I didn't mention it upfront, so the posters who answered "Flash" are valid enough, but part of what I'm doing is an attempt to highlight what can be done with pure HTML5... so Flash is right out in my case. (Though it's a perfectly valid answer for anyone doing a "real" web app.) That being the case it looks like I'm out of luck unless I want to involve the server. Thanks anyway!

Simple solution for HTML5 ready browsers...
function download(filename, text) {
var pom = document.createElement('a');
pom.setAttribute('href', 'data:text/plain;charset=utf-8,' + encodeURIComponent(text));
pom.setAttribute('download', filename);
if (document.createEvent) {
var event = document.createEvent('MouseEvents');
event.initEvent('click', true, true);
pom.dispatchEvent(event);
}
else {
pom.click();
}
}
Usage
download('test.txt', 'Hello world!');

OK, creating a data:URI definitely does the trick for me, thanks to Matthew and Dennkster pointing that option out! Here is basically how I do it:
1) get all the content into a string called "content" (e.g. by creating it there initially or by reading innerHTML of the tag of an already built page).
2) Build the data URI:
uriContent = "data:application/octet-stream," + encodeURIComponent(content);
There will be length limitations depending on browser type etc., but e.g. Firefox 3.6.12 works until at least 256k. Encoding in Base64 instead using encodeURIComponent might make things more efficient, but for me that was ok.
3) open a new window and "redirect" it to this URI prompts for a download location of my JavaScript generated page:
newWindow = window.open(uriContent, 'neuesDokument');
That's it.

HTML5 defined a window.saveAs(blob, filename) method. It isn't supported by any browser right now. But there is a compatibility library called FileSaver.js that adds this function to most modern browsers (including Internet Explorer 10+). Internet Explorer 10 supports a navigator.msSaveBlob(blob, filename) method (MSDN), which is used in FileSaver.js for Internet Explorer support.
I wrote a blog posting with more details about this problem.

Saving large files
Long data URIs can give performance problems in browsers. Another option to save client-side generated files, is to put their contents in a Blob (or File) object and create a download link using URL.createObjectURL(blob). This returns an URL that can be used to retrieve the contents of the blob. The blob is stored inside the browser until either URL.revokeObjectURL() is called on the URL or the document that created it is closed. Most web browsers have support for object URLs, Opera Mini is the only one that does not support them.
Forcing a download
If the data is text or an image, the browser can open the file, instead of saving it to disk. To cause the file to be downloaded upon clicking the link, you can use the the download attribute. However, not all web browsers have support for the download attribute. Another option is to use application/octet-stream as the file's mime-type, but this causes the file to be presented as a binary blob which is especially user-unfriendly if you don't or can't specify a filename. See also 'Force to open "Save As..." popup open at text link click for pdf in HTML'.
Specifying a filename
If the blob is created with the File constructor, you can also set a filename, but only a few web browsers (including Chrome & Firefox) have support for the File constructor. The filename can also be specified as the argument to the download attribute, but this is subject to a ton of security considerations. Internet Explorer 10 and 11 provides its own method, msSaveBlob, to specify a filename.
Example code
var file;
var data = [];
data.push("This is a test\n");
data.push("Of creating a file\n");
data.push("In a browser\n");
var properties = {type: 'text/plain'}; // Specify the file's mime-type.
try {
// Specify the filename using the File constructor, but ...
file = new File(data, "file.txt", properties);
} catch (e) {
// ... fall back to the Blob constructor if that isn't supported.
file = new Blob(data, properties);
}
var url = URL.createObjectURL(file);
document.getElementById('link').href = url;
<a id="link" target="_blank" download="file.txt">Download</a>

function download(content, filename, contentType)
{
if(!contentType) contentType = 'application/octet-stream';
var a = document.createElement('a');
var blob = new Blob([content], {'type':contentType});
a.href = window.URL.createObjectURL(blob);
a.download = filename;
a.click();
}

Take a look at Doug Neiner's Downloadify which is a Flash based JavaScript interface to do this.
Downloadify is a tiny JavaScript + Flash library that enables the generation and saving of files on the fly, in the browser, without server interaction.

Simple Solution!
<a download="My-FileName.txt" href="data:application/octet-stream,HELLO-WORLDDDDDDDD">Click here</a>
Works in all Modern browsers.

I've used FileSaver (https://github.com/eligrey/FileSaver.js) and it works just fine.
For example, I did this function to export logs displayed on a page.
You have to pass an array for the instanciation of the Blob, so I just maybe didn't write this the right way, but it works for me.
Just in case, be careful with the replace: this is the syntax to make this global, otherwise it will only replace the first one he meets.
exportLogs : function(){
var array = new Array();
var str = $('#logs').html();
array[0] = str.replace(/<br>/g, '\n\t');
var blob = new Blob(array, {type: "text/plain;charset=utf-8"});
saveAs(blob, "example.log");
}

You can generate a data URI. However, there are browser-specific limitations.

I found two simple approaches that work for me. First, using an already clicked a element and injecting the download data. And second, generating an a element with the download data, executing a.click() and removing it again. But the second approach works only if invoked by a user click action as well. (Some) Browser block click() from other contexts like on loading or triggered after a timeout (setTimeout).
<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<script type="text/javascript">
function linkDownload(a, filename, content) {
contentType = 'data:application/octet-stream,';
uriContent = contentType + encodeURIComponent(content);
a.setAttribute('href', uriContent);
a.setAttribute('download', filename);
}
function download(filename, content) {
var a = document.createElement('a');
linkDownload(a, filename, content);
document.body.appendChild(a);
a.click();
document.body.removeChild(a);
}
</script>
</head>
<body>
download
<button onclick="download('test.txt', 'Hello World!');">download</button>
</body>
</html>

try
let a = document.createElement('a');
a.href = "data:application/octet-stream,"+encodeURIComponent('"My DATA"');
a.download = 'myFile.json';
a.click(); // we not add 'a' to DOM so no need to remove
If you want to download binary data look here
Update
2020.06.14 I upgrade Chrome to 83.0 and above SO snippet stop works (due to sandbox security restrictions) - but JSFiddle version works - here

Here is a link to the data URI method Mathew suggested, it worked on safari, but not well because I couldn't set the filetype, it gets saved as "unknown" and then i have to go there again later and change it in order to view the file...
http://www.nihilogic.dk/labs/canvas2image/

You can use localStorage. This is the Html5 equivalent of cookies. It appears to work on Chrome and Firefox BUT on Firefox, I needed to upload it to a server. That is, testing directly on my home computer didn't work.
I'm working up HTML5 examples. Go to http://faculty.purchase.edu/jeanine.meyer/html5/html5explain.html
and scroll to the maze one. The information to re-build the maze is stored using localStorage.
I came to this article looking for HTML5 JavaScript for loading and working with xml files. Is it the same as older html and JavaScript????

As previously mentioned the File API, along with the FileWriter and FileSystem APIs can be used to store files on a client's machine from the context of a browser tab/window.
However, there are several things pertaining to latter two APIs which you should be aware of:
Implementations of the APIs currently exist only in Chromium-based browsers (Chrome & Opera)
Both of the APIs were taken off of the W3C standards track on April 24, 2014, and as of now are proprietary
Removal of the (now proprietary) APIs from implementing browsers in the future is a possibility
A sandbox (a location on disk outside of which files can produce no effect) is used to store the files created with the APIs
A virtual file system (a directory structure which does not necessarily exist on disk in the same form that it does when accessed from within the browser) is used represent the files created with the APIs
Here are simple examples of how the APIs are used, directly and indirectly, in tandem to do this:
BakedGoods*
bakedGoods.get({
data: ["testFile"],
storageTypes: ["fileSystem"],
options: {fileSystem:{storageType: Window.PERSISTENT}},
complete: function(resultDataObj, byStorageTypeErrorObj){}
});
Using the raw File, FileWriter, and FileSystem APIs
function onQuotaRequestSuccess(grantedQuota)
{
function saveFile(directoryEntry)
{
function createFileWriter(fileEntry)
{
function write(fileWriter)
{
var dataBlob = new Blob(["Hello world!"], {type: "text/plain"});
fileWriter.write(dataBlob);
}
fileEntry.createWriter(write);
}
directoryEntry.getFile(
"testFile",
{create: true, exclusive: true},
createFileWriter
);
}
requestFileSystem(Window.PERSISTENT, grantedQuota, saveFile);
}
var desiredQuota = 1024 * 1024 * 1024;
var quotaManagementObj = navigator.webkitPersistentStorage;
quotaManagementObj.requestQuota(desiredQuota, onQuotaRequestSuccess);
Though the FileSystem and FileWriter APIs are no longer on the standards track, their use can be justified in some cases, in my opinion, because:
Renewed interest from the un-implementing browser vendors may place them right back on it
Market penetration of implementing (Chromium-based) browsers is high
Google (the main contributer to Chromium) has not given and end-of-life date to the APIs
Whether "some cases" encompasses your own, however, is for you to decide.
*BakedGoods is maintained by none other than this guy right here :)

This thread was invaluable to figure out how to generate a binary file and prompt to download the named file, all in client code without a server.
First step for me was generating the binary blob from data that I was saving. There's plenty of samples for doing this for a single binary type, in my case I have a binary format with multiple types which you can pass as an array to create the blob.
saveAnimation: function() {
var device = this.Device;
var maxRow = ChromaAnimation.getMaxRow(device);
var maxColumn = ChromaAnimation.getMaxColumn(device);
var frames = this.Frames;
var frameCount = frames.length;
var writeArrays = [];
var writeArray = new Uint32Array(1);
var version = 1;
writeArray[0] = version;
writeArrays.push(writeArray.buffer);
//console.log('version:', version);
var writeArray = new Uint8Array(1);
var deviceType = this.DeviceType;
writeArray[0] = deviceType;
writeArrays.push(writeArray.buffer);
//console.log('deviceType:', deviceType);
var writeArray = new Uint8Array(1);
writeArray[0] = device;
writeArrays.push(writeArray.buffer);
//console.log('device:', device);
var writeArray = new Uint32Array(1);
writeArray[0] = frameCount;
writeArrays.push(writeArray.buffer);
//console.log('frameCount:', frameCount);
for (var index = 0; index < frameCount; ++index) {
var frame = frames[index];
var writeArray = new Float32Array(1);
var duration = frame.Duration;
if (duration < 0.033) {
duration = 0.033;
}
writeArray[0] = duration;
writeArrays.push(writeArray.buffer);
//console.log('Frame', index, 'duration', duration);
var writeArray = new Uint32Array(maxRow * maxColumn);
for (var i = 0; i < maxRow; ++i) {
for (var j = 0; j < maxColumn; ++j) {
var color = frame.Colors[i][j];
writeArray[i * maxColumn + j] = color;
}
}
writeArrays.push(writeArray.buffer);
}
var blob = new Blob(writeArrays, {type: 'application/octet-stream'});
return blob;
}
The next step is to get the browser to prompt the user to download this blob with a predefined name.
All I needed was a named link I added in the HTML5 that I could reuse to rename the initial filename. I kept it hidden since the link doesn't need display.
<a id="lnkDownload" style="display: none" download="client.chroma" href="" target="_blank"></a>
The last step is to prompt the user to download the file.
var data = animation.saveAnimation();
var uriContent = URL.createObjectURL(data);
var lnkDownload = document.getElementById('lnkDownload');
lnkDownload.download = 'theDefaultFileName.extension';
lnkDownload.href = uriContent;
lnkDownload.click();

When testing the "ahref" method, I found that the web developer tools of Firefox and Chrome gets confused. I needed to restart the debugging after the a.click() was issued. Same happened with the FileSaver (it uses the same ahref method to actually make the saving). To work around it, I created new temporary window, added the element a into that and clicked it there.
function download_json(dt) {
var csv = ' var data = ';
csv += JSON.stringify(dt, null, 3);
var uricontent = 'data:application/octet-stream,' + encodeURI(csv);
var newwin = window.open( "", "_blank" );
var elem = newwin.document.createElement('a');
elem.download = "database.js";
elem.href = uricontent;
elem.click();
setTimeout(function(){ newwin.close(); }, 3000);
}

You can use this to save text and other data:
function downloadFile(name, data) {
let a = document.createElement("a");
if (typeof a.download !== "undefined") a.download = name;
a.href = URL.createObjectURL(new Blob([data], {
type: "application/octet-stream"
}));
a.dispatchEvent(new MouseEvent("click"));
}
This function will create an Anchor element, set the name via .download (if supported), assign a url (.href) created from an object (URL.createObjectURL), in this case a Blob object, and dispatch a click event. In short: it's as if you're clicking a download link.
Example code
downloadFile("textfile.txt", "A simple text file");
downloadFile(
"circle.svg",
`<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
<circle cx="50" cy="50" r="42" />
</svg>`
);
downloadFile(
"utf8string.txt",
new Uint8Array([85, 84, 70, 45, 56, 32, 115, 116, 114, 105, 110, 103]) // "UTF-8 string"
);
This function also accepts File, Blob and MediaSource:
function downloadFile(name, data) {
if (!(data instanceof File || data instanceof Blob || data instanceof MediaSource)) {
return downloadFile(name, new Blob([data], {
type: "application/octet-stream"
}));
}
let a = document.createElement("a");
if (typeof a.download !== "undefined") a.download = name;
a.href = URL.createObjectURL(data);
a.dispatchEvent(new MouseEvent("click"));
}
Or you could use two functions:
function downloadFile(name, data) {
return downloadObject(new Blob([data], {
type: "application/octet-stream"
}));
}
function downloadObject(name, object) {
let a = document.createElement("a");
if (typeof a.download !== "undefined") a.download = name;
a.href = URL.createObjectURL(object);
a.dispatchEvent(new MouseEvent("click"));
}

Here is a tutorial to export files as ZIP:
Before getting started, there is a library to save files, the name of library is fileSaver.js, You can find this library here. Let's get started, Now, include the required libraries:
<script src="https://cdnjs.cloudflare.com/ajax/libs/jszip/3.1.4/jszip.min.js" type="text/javascript"></script>
<script type="text/javascript" src="https://fastcdn.org/FileSaver.js/1.1.20151003/FileSaver.js" ></script>
Now copy this code and this code will download a zip file with a file hello.txt having content Hello World. If everything thing works fine, this will download a file.
<script type="text/javascript">
var zip = new JSZip();
zip.file("Hello.txt", "Hello World\n");
zip.generateAsync({type:"blob"})
.then(function(content) {
// see FileSaver.js
saveAs(content, "file.zip");
});
</script>
This will download a file called file.zip. You can read more here: http://www.wapgee.com/story/248/guide-to-create-zip-files-using-javascript-by-using-jszip-library

For simple files like 'txt' or'js' you can use the package fs-browsers.
It has nice and easy download and export methods for client-side which do not invole any server.
import { exportFile } from 'fs-browsers';
const onExportClick = (textToExport) => {
// Export to txt file
exportFile(textToExport);
}
If you want to change the name of the file, or even it's type you can do it easily with this:
import { exportFile } from 'fs-browsers';
const onExportClick = (textToExport) => {
// Export to js file called 'file.js'
exportFile(textToExport, { fileName: 'file.js' });
}
For more complex files you will need to involve a server as you said.
The package can also does that with excel files ('xls') if that is what you need.
import { exportFile, EXCEL_FILE } from 'fs-browsers';
const data = [{ "id": 5, "name": "John", "grade": 90, "age": 15 }, { "id": 7, "name": "Nick", "grade": 70, "age": 17 }];
const headings = ["Student ID", "Student Name", "Test Grade", "Student Age"];
exportFile(data, { type: EXCEL_FILE, headings: headings, fileName: 'grades.xls' });
Maybe in the future there eill be other kind of files too.