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What is the Correct Way to Download Blob in Chrome? [duplicate]

Update
Since asking the question below and arriving at a more fundamental question after finding the error in the code, I found some more information such as in the MDN web docs for the downloads API method downloads.download() it states that a revoke of an object url should be performed only after the file/url has been downloaded. So, I spent some time trying to understand whether or not a web extension makes the downloads API onChanged event 'available' to javascript of a web page and don't think it does. I don't understand why the downloads API is available to extensions only, especailly when there are quite a few questions concerning this same memory-usage/object-url-revocation issue. For example, Wait for user to finish downloading a blob in Javascript.
If you know, would you please explain? Thank you.
Starting with Firefox browser closed, and right clicking on a local html file to open in Firefox, it opens with five firefox.exe processes as viewed in Windows Task Manager. Four of the processes start with between 20,000k and 25,000k of memory and one with about 115,000k.
This html page has an indexedDB database with 50 object stores each containing 50 objects. Each object is extracted from its object store and converted to string using JSON.stringify, and written to a two-dimensional array. Afterward, all elements of the array are concatenated into one large string, converted to a blob and written to the hard disk through a URL object which is revoked immediately afterward. The final file is about 190MB.
If the code is stopped just before the conversion to blob, one of the firefox.exe process's memory usage increases to around 425,000k and then falls back to 25,000k in about 5-10 seconds after the elements of the array have been concatenated into a single string.
If the code is run to completion, the memory usage of that same firefox.exe process grows to about 1,000,000k and then drops to about 225,000k. The firefox.exe process that started at 115,000k also increases at the blob stage of the code to about 325,000k and never decreases.
After the blob is written to disk as a text file, these two firefox.exe processes never release the approximate 2 x 200,000k increase in memory.
I have set every variable used in each function to null and the memory is never freed unless the page is refreshed. Also, this process is initiated by a button click event; and if it is run again without an intermediate refresh, each of these two firefox.exe processes grab an additional 200,000k of memory with each run.
I haven't been able to figure out how to free the memory?
The two functions are quite simple. json[i][j] holds the string version of the jth object from the ith object store in the database. os_data[] is an array of small objects { "name" : objectStoreName, "count" : n }, where n is the number of objects in the store. The build_text fuction appears to release the memory if write_to_disk is not invoked. So, the issue appears to be related to the blob or the url.
I'm probably overlooking something obvious. Thank you for any direction you can provide.
EDIT:
I see from JavaScript: Create and save file that I have a mistake in the revokeObjectURL(blob) statment. It can't revoke blob, the createObjectURL(blob) needed to be saved to a variable like url and then revoke url, not blob.
That worked for the most part and the memory is released from both of the firefox.exe processes mentioned above, in most cases. This leaves me with one small question about the timing of the revoke of the url.
If the revoke is what allows for the release of memory, should the url be revoked only after the file has been successfully downloaded? If the revoke takes place before the user clicks ok to download the file, what happens? Suppose I click the button to prepare the file from the database and after it's ready the browser brings up the window for downloading, but I wait a little while thinking about what to name the file or where to save it, won't the revoke statment be run already but the url is still 'held' by the browser since it is what will be downloaded? I know I can still download the file, but does the revoke still release the memory? From my small amount of experimenting with this one example, it appears that it does not get released in this scenario.
If there was an event that fires when the file has either successfully or unsuccessfully been downloaded to the client, is not that the time when the url should be revoked? Would it be better to set a timeout of a few minutes before revoking the url, since I'm pretty sure there is not an event indicating download to client has ended.
I'm probably not understanding something basic about this. Thanks.
function build_text() {
var i, j, l, txt = "";
for ( i = 1; i <=50; i++ ) {
l = os_data[i-1].count;
for ( j = 1; j <= l; j++ ) {
txt += json[i][j] + '\n';
}; // next j
}; // next i
write_to_disk('indexedDB portfolio', txt);
txt = json = null;
} // close build_text
function write_to_disk( fileName, data ) {
fileName = fileName.replace(".","");
var blob = new Blob( [data], { type: 'text/csv' } ), elem;
if ( window.navigator.msSaveOrOpenBlob ) {
window.navigator.msSaveBlob(blob, fileName);
} else {
elem = window.document.createElement('a');
elem.href = window.URL.createObjectURL(blob);
elem.download = fileName;
document.body.appendChild(elem);
elem.click();
document.body.removeChild(elem);
window.URL.revokeObjectURL(blob);
}; // end if
data = blob = elem = fileName = null;
} // close write_to_disk

I am a bit lost as to what is the question here...
But let's try to answer, at least part of it:
For a starter let's explain what URL.createObjectURL(blob) roughly does:
It creates a blob URI, which is an URI pointing to the Blob blob in memory just like if it was in an reachable place (like a server).
This blob URI will mark blob as being un-collectable by the Garbage Collector (GC) for as long as it has not been revoked, so that you don't have to maintain a live reference to blob in your script, but that you can still use/load it.
URL.revokeObjectURL will then break the link between the blob URI and the Blob in memory. It will not free up the memory occupied by blob directly, it will just remove its own protection regarding the GC, [and won't point to anywhere anymore].
So if you have multiple blob URI pointing to the same Blob object, revoking only one won't break the other blob URIs.
Now, the memory will be freed only when the GC will kick in, and this in only decided by the browser internals, when it thinks it is the best time, or when it sees it has no other options (generally when it misses memroy space).
So it is quite normal that you don't see your memory being freed up instantly, and by experience, I would say that FF doesn't care about using a lot of memory, when it is available, making GC kick not so often, whihc is good for user-experience (GCing often results in lags).
For your download question, indeed, web APIs don't provide a way to know if a download has been successful or failed, nor even if it has just ended.
For the revoking part, it really depends on when you do it.
If you do it directly in the click handler, then the browser won't have done the pre-fetch request yet, so when the default action of the click (the download) will happen, there won't be anything linked by the URI anymore.
Now, if you do revoke the blob URI after the "save" prompt, the browser will have done a pre-fetch request, and thus might be able to mark by itself that the Blob resource should not be cleared. But I don't think this behavior is tied by any specs, and it might be better to wait at least for the window's focus event, at which point the downloading of the resource should already have started.
const blob = new Blob(['bar']);
const uri = URL.createObjectURL(blob);
anchor.href = uri;
anchor.onclick = e => {
window.addEventListener('focus', e=>{
URL.revokeObjectURL(uri);
console.log("Blob URI revoked, you won't be able to download it anymore");
}, {once: true});
};
<a id="anchor" download="foo.txt">download</a>

Can I set the filename of a PDF object displayed in Chrome?

In my Vue app I receive a PDF as a blob, and want to display it using the browser's PDF viewer.
I convert it to a file, and generate an object url:
const blobFile = new File([blob], `my-file-name.pdf`, { type: 'application/pdf' })
this.invoiceUrl = window.URL.createObjectURL(blobFile)
Then I display it by setting that URL as the data attribute of an object element.
<object
:data="invoiceUrl"
type="application/pdf"
width="100%"
style="height: 100vh;">
</object>
The browser then displays the PDF using the PDF viewer. However, in Chrome, the file name that I provide (here, my-file-name.pdf) is not used: I see a hash in the title bar of the PDF viewer, and when I download the file using either 'right click -> Save as...' or the viewer's controls, it saves the file with the blob's hash (cda675a6-10af-42f3-aa68-8795aa8c377d or similar).
The viewer and file name work as I'd hoped in Firefox; it's only Chrome in which the file name is not used.
Is there any way, using native Javascript (including ES6, but no 3rd party dependencies other than Vue), to set the filename for a blob / object element in Chrome?
[edit] If it helps, the response has the following relevant headers:
Content-Type: application/pdf; charset=utf-8
Transfer-Encoding: chunked
Content-Disposition: attachment; filename*=utf-8''Invoice%2016246.pdf;
Content-Description: File Transfer
Content-Encoding: gzip

Chrome's extension seems to rely on the resource name set in the URI, i.e the file.ext in protocol://domain/path/file.ext.
So if your original URI contains that filename, the easiest might be to simply make your <object>'s data to the URI you fetched the pdf from directly, instead of going the Blob's way.
Now, there are cases it can't be done, and for these, there is a convoluted way, which might not work in future versions of Chrome, and probably not in other browsers, requiring to set up a Service Worker.
As we first said, Chrome parses the URI in search of a filename, so what we have to do, is to have an URI, with this filename, pointing to our blob:// URI.
To do so, we can use the Cache API, store our File as Request in there using our URL, and then retrieve that File from the Cache in the ServiceWorker.
Or in code,
From the main page
// register our ServiceWorker
navigator.serviceWorker.register('/sw.js')
.then(...
...
async function displayRenamedPDF(file, filename) {
// we use an hard-coded fake path
// to not interfere with legit requests
const reg_path = "/name-forcer/";
const url = reg_path + filename;
// store our File in the Cache
const store = await caches.open( "name-forcer" );
await store.put( url, new Response( file ) );
const frame = document.createElement( "iframe" );
frame.width = 400
frame.height = 500;
document.body.append( frame );
// makes the request to the File we just cached
frame.src = url;
// not needed anymore
frame.onload = (evt) => store.delete( url );
}
In the ServiceWorker sw.js
self.addEventListener('fetch', (event) => {
event.respondWith( (async () => {
const store = await caches.open("name-forcer");
const req = event.request;
const cached = await store.match( req );
return cached || fetch( req );
})() );
});
Live example (source)
Edit: This actually doesn't work in Chrome...
While it does set correctly the filename in the dialog, they seem to be unable to retrieve the file when saving it to the disk...
They don't seem to perform a Network request (and thus our SW isn't catching anything), and I don't really know where to look now.
Still this may be a good ground for future work on this.
And an other solution, I didn't took the time to check by myself, would be to run your own pdf viewer.
Mozilla has made its js based plugin pdf.js available, so from there we should be able to set the filename (even though once again I didn't dug there yet).
And as final note, Firefox is able to use the name property of a File Object a blobURI points to.
So even though it's not what OP asked for, in FF all it requires is
const file = new File([blob], filename);
const url = URL.createObjectURL(file);
object.data = url;

In Chrome, the filename is derived from the URL, so as long as you are using a blob URL, the short answer is "No, you cannot set the filename of a PDF object displayed in Chrome." You have no control over the UUID assigned to the blob URL and no way to override that as the name of the page using the object element. It is possible that inside the PDF a title is specified, and that will appear in the PDF viewer as the document name, but you still get the hash name when downloading.
This appears to be a security precaution, but I cannot say for sure.
Of course, if you have control over the URL, you can easily set the PDF filename by changing the URL.

I believe Kaiido's answer expresses, briefly, the best solution here:
"if your original URI contains that filename, the easiest might be to simply make your object's data to the URI you fetched the pdf from directly"
Especially for those coming from this similar question, it would have helped me to have more description of a specific implementation (working for pdfs) that allows the best user experience, especially when serving files that are generated on the fly.
The trick here is using a two-step process that perfectly mimics a normal link or button click. The client must (step 1) request the file be generated and stored server-side long enough for the client to (step 2) request the file itself. This requires you have some mechanism supporting unique identification of the file on disk or in a cache.
Without this process, the user will just see a blank tab while file-generation is in-progress and if it fails, then they'll just get the browser's ERR_TIMED_OUT page. Even if it succeeds, they'll have a hash in the title bar of the PDF viewer tab, and the save dialog will have the same hash as the suggested filename.
Here's the play-by-play to do better:
You can use an anchor tag or a button for the "download" or "view in browser" elements
Step 1 of 2 on the client: that element's click event can make a request for the file to be generated only (not transmitted).
Step 1 of 2 on the server: generate the file and hold on to it. Return only the filename to the client.
Step 2 of 2 on the client:
If viewing the file in the browser, use the filename returned from the generate request to then invoke window.open('view_file/<filename>?fileId=1'). That is the only way to indirectly control the name of the file as shown in the tab title and in any subsequent save dialog.
If downloading, just invoke window.open('download_file?fileId=1').
Step 2 of 2 on the server:
view_file(filename, fileId) handler just needs to serve the file using the fileId and ignore the filename parameter. In .NET, you can use a FileContentResult like File(bytes, contentType);
download_file(fileId) must set the filename via the Content-Disposition header as shown here. In .NET, that's return File(bytes, contentType, desiredFilename);
client-side download example:
download_link_clicked() {
// show spinner
ajaxGet(generate_file_url,
{},
(response) => {
// success!
// the server-side is responsible for setting the name
// of the file when it is being downloaded
window.open('download_file?fileId=1', "_blank");
// hide spinner
},
() => { // failure
// hide spinner
// proglem, notify pattern
},
null
);
client-side view example:
view_link_clicked() {
// show spinner
ajaxGet(generate_file_url,
{},
(response) => {
// success!
let filename = response.filename;
// simplest, reliable method I know of for controlling
// the filename of the PDF when viewed in the browser
window.open('view_file/'+filename+'?fileId=1')
// hide spinner
},
() => { // failure
// hide spinner
// proglem, notify pattern
},
null
);

I'm using the library pdf-lib, you can click here to learn more about the library.
I solved part of this problem by using api Document.setTitle("Some title text you want"),
Browser displayed my title correctly, but when click the download button, file name is still previous UUID. Perhaps there is other api in the library that allows you to modify download file name.

How to download file from URL and Alert user that file is downloaded successfully in Javascript? [duplicate]

I have an application that hosts videos, and we recently migrated to Azure.
On our old application we gave the ability for users to either play or download the video. However on Azure it seems like I have to pick between which functionality I want, as the content disposition has to be set on the file and not on the request.
So far I have came up with two very poor solutions.
The first solution is streaming the download through my MVC server.
CloudStorageAccount storageAccount = CloudStorageAccount.Parse(ConfigurationManager.AppSettings["StorageConnectionString"]);
CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
CloudBlobContainer container = blobClient.GetContainerReference("videos");
string userFileName = service.FirstName + service.LastName + "Video.mp4";
Response.AddHeader("Content-Disposition", "attachment; filename=" + userFileName); // force download
container.GetBlobReference(service.Video.ConvertedFilePath).DownloadToStream(Response.OutputStream);
return new EmptyResult();
This option works okay for smaller videos, but it is very taxing on my server. For larger videos the operation times out.
The second option is hosting every video twice.
This option is obviously bad, as I will have to pay double the storage cost.

However on Azure it seems like I have to pick between which
functionality I want, as the content disposition has to be set on the
file and not on the request.
There's a workaround for that. As you may know there's a Content-Disposition property that you can define on a blob. However when you define a value for this property, it will always be applied on that blob. When you want to selectively apply this property on a blob (say on a per request basis), what you do is create a Shared Access Signature (SAS) on that blob and override this request header there. Then you can serve the blob via SAS URL.
Here's the sample code for this:
CloudStorageAccount storageAccount = CloudStorageAccount.Parse(ConfigurationManager.AppSettings["StorageConnectionString"]);
CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
CloudBlobContainer container = blobClient.GetContainerReference("videos");
string userFileName = service.FirstName + service.LastName + "Video.mp4";
CloudBlockBlob blob = container.GetBlockBlobReference(userFileName);
SharedAccessBlobPolicy policy = new SharedAccessBlobPolicy()
{
Permissions = SharedAccessBlobPermissions.Read,
SharedAccessExpiryTime = DateTime.UtcNow.AddHours(1)
};
SharedAccessBlobHeaders blobHeaders = new SharedAccessBlobHeaders()
{
ContentDisposition = "attachment; filename=" + userFileName
};
string sasToken = blob.GetSharedAccessSignature(policy, blobHeaders);
var sasUrl = blob.Uri.AbsoluteUri + sasToken;//This is the URL you will use. It will force the user to download the video.
I wrote a blog post about the same long time ago that you may find useful: http://gauravmantri.com/2013/11/28/new-changes-to-windows-azure-storage-a-perfect-thanksgiving-gift/.

As far as I know, azure blob storage doesn't support add the custom header to the special container.
I suggest you could follow and vote this feedback to push the azure develop team to support this feature.
Here is a workaround, you could compression the video file firstly, then uploaded to the azure blob storage.
It will not be opened by the browser.

Make browser download/save files without user interaction

I need to create a small browser-based application that helps users download/save, and possibly print to the default printer, a large number of files from a webserver we have no control over (but we have all the URIs beforehand).
These files are hidden behind a "single sign-on" (SSO) that can only be performed via a browser and requires a user. Hence, it must be a browser-based solution, where we piggyback on to the session established by the SSO.
The users' platform is Windows 7.
The point is to save the users from going through a lot of clicks per file (download, where to save, etc.) when they need to perform this operation (daily).
At this point all the files are PDF, but that might change in the future.
A browser-agnostic solution is preferred (and I assume more robust when it comes to future browser updates).
But we can base it on a particular browser if needed.
How would you do this from Javascript?

As the comments to my question says, this isn't really allowed by the browsers for security reasons.
My workaround for now (only tested using IE11) is to manually change the security settings of the users browser, and then download the files as a blob into a javascript variable using AJAX, followed by upload of same blob to my own server again using AJAX.
"My own server" is a Django site created for this purpose, that also knows which files to download for the day, and provide the javascript needed. The user goes to this site to initiate the daily download after performing the SSO in a separate browser tab.
On the server I can then perform whatever operations needed for said files.
Many thanks to this post https://stackoverflow.com/a/13887220/833320 for the handling of binary data in AJAX.
1) In IE, add the involved sites to the "Local Intranet Zone", and enable "Access data sources across domains" for this zone to overcome the CORS protection otherwise preventing this.
Of course, consider the security consequences involved in this...
2) In javascript (browser), download the file as a blob and POST the resulting data to my own server:
var x = new XMLHttpRequest();
x.onload = function() {
// Create a form
var fd = new FormData();
fd.append('csrfmiddlewaretoken', '{{ csrf_token }}'); // Needed by Django
fd.append('file', x.response); // x.response is a Blob object
// Upload to your server
var y = new XMLHttpRequest();
y.onload = function() {
alert('File uploaded!');
};
y.open('POST', '/django/upload/');
y.send(fd);
};
x.open('GET', 'https://external.url', true);
x.responseType = 'blob'; // <-- This is necessary!
x.send();
3) Finally (in Django view for '/django/upload/'), receive the uploaded data and save as file - or whatever...
filedata = request.FILES['file'].read()
with open('filename', 'wb') as f:
f.write(filedata)
Thanks all, for your comments.
And yes, the real solution would be to overcome the SSO (that requieres the user), so it all could be done by the server itself.
But at least I learned a little about getting/posting binary data using modern XMLHttpRequests. :)

Actually, I had a problem like it, I wanted to download a binary file(an image) and store it and then use it when I need it, So I decided to download it with Fetch API Get call:
const imageAddress = 'an-address-to-my-image.jpg'; // sample address
fetch(imageAddress)
.then(res => res.blob) // <-- This is necessary!
.then(blobFileToBase64)
.then(base64FinalAnswer => console.log(base64FinalAnswer))
The blobFileToBase64 is a helper function that converts blob binary file to a base64 data string:
const blobToBase64 = blob => {
const reader = new FileReader();
reader.readAsDataURL(blob);
return new Promise(resolve => {
reader.onloadend = () => {
resolve(reader.result);
};
});
};
In the end, I have the base64FinalAnswer and I can do anything with it.

Save File From Local Data in Javascript

Heres the scenario:
User comes to my website and opens a webpage with some javascript functionality.
User edits the data through javascript
User clicks on a save button to save the data, thing is, it seems like they shouldn't need to download this data because its already in javascript on the local machine.
Is it possible to save data from javascript (executing from a foreign webpage) without downloading a file from the server?
Any help would be much appreciated!

For saving data on the client-side, without any server interaction, the best I've seen is Downloadify, is a small JavaScript + Flash library allows you to generate and save files on the fly, directly in the browser...
Check this demo.

I came across this scenario when I wanted to initiate a download without using a server. I wrote this jQuery plugin that wraps up the content of a textarea/div in a Blob, then initiates a download of the Blob. Allows you to specify both file name and type..
jQuery.fn.downld = function (ops) {
this.each(function () {
var _ops = ops || {},
file_name = _ops.name || "downld_file",
file_type = _ops.type || "txt",
file_content = $(this).val() || $(this).html();
var _file = new Blob([file_content],{type:'application/octet-stream'});
window.URL = window.URL || window.webkitURL;
var a = document.createElement('a');
a.href = window.URL.createObjectURL(_file);
a.download = file_name+"."+file_type;
document.body.appendChild(a);
a.click(); $('a').last().remove();
});
}
Default Use : $("#element").downld();
Options : $("#element").downld({ name:"some_file_name", type:"html" });
Codepen example http://codepen.io/anon/pen/cAqzE

JavaScript is run in a sandboxed environment, meaning it only has access to specific browser resources. Specifically, it doesn't have access to the filesystem, or dynamic resources from other domains (web pages, javascript etc). Well, there are other things (I/O, devices), but you get the point.
You will need to post the data to the server which can invoke a file download, or use another technology such as flash, java applets, or silverlight. (i'm not sure about the support for this in the last 2, and I also wouldn't recommend using them, depends what it's for...)

The solution to download local/client-side contents via javascript is not straight forward. I have implemented one solution using smartclient-html-jsp.
Here is the solution:
I am in the project build on SmartClient. We need to download/export data of a grid
(table like structure).
We were using RESTish web services to serve the data from Server side. So I could not hit the url two times; one for grid and second time for export/transform the data to download.
What I did is made two JSPs namely: blank.jsp and export.jsp.
blank.jsp is literally blank, now I need to export the grid data
that I already have on client side.
Now when ever user asks to export the grid data, I do below:
a. Open a new window with url blank.jsp
b. using document.write I create a form in it with one field name text in it and set data to export inside it.
c. Now POST that form to export.jsp of same heirarchy.
d. Contents of export.jsp I am pasting below are self explanatory.
// code start
<%# page import="java.util.*,java.io.*,java.util.Enumeration"%>
<%
response.setContentType ("text/csv");
//set the header and also the Name by which user will be prompted to save
response.setHeader ("Content-Disposition", "attachment;filename=\"data.csv\"");
String contents = request.getParameter ("text");
if (!(contents!= null && contents!=""))
contents = "No data";
else
contents = contents.replaceAll ("NEW_LINE", "\n");
//Open an input stream to the file and post the file contents thru the
//servlet output stream to the client m/c
InputStream in = new ByteArrayInputStream(contents.getBytes ());
ServletOutputStream outs = response.getOutputStream();
int bit = 256;
int i = 0;
try {
while ((bit) >= 0) {
bit = in.read();
outs.write(bit);
}
//System.out.println("" +bit);
} catch (IOException ioe) {
ioe.printStackTrace(System.out);
}
outs.flush();
outs.close();
in.close();
%>
<HTML>
<HEAD>
</HEAD>
<BODY>
<script type="text/javascript">
try {window.close ();} catch (e) {alert (e);}
</script>
</BODY>
</HTML>
// code end
This code is tested and deployed/working in production environment, also this is cross-browser functionality.
Thanks
Shailendra

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