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How to access data sent by node sever using client side javascript

I am using a node sever to send a table from a sqlite db to the browser. This table contains filename and path of a pdf file that I want to render on the browser. Until now I was using hard coded paths for the the pdf file and rendering. But now i have setup a get route and a controller in node such that whenever '/content' is hit in browser , the server queries the database and and sends the data to the client. To the send the data I am using
res.render('content/index',{data:queryData});
Now, how do I access this data using client side javascript so that I can pass the path of the pdf file to the function that renders the pdf? I have done research and the nearest answer I got was using XMLHttpRequest. I tried this method
var xhr = new XMLHttpRequest();
const path = "http://localhost:3000/content";
xhr.onreadystatechange = function () {
if (xhr.readyState == 4 && xhr.status == 200)
{
var myResponseText = xhr.responseText;
console.log(myResponseText);
}
};
xhr.open('get', path, true);
xhr.send();
When I do this I get the entire html code for the view. Not the data I expected. How do I solve this issue. I have done some more reading while writing this post and I suppose. I have set a header somewhere? But the documentation says
app.render(view, [locals], callback)
which means res.render can take local variables, shouldn't be setting the headers?

You should return json instead of render template:
app.get('content/index', (req, res) => {
res.json({data: queryData});
});

I am using pdf.js
PDF.js needs the PDF file, e.g.:
pdfjsLib.getDocument('helloworld.pdf')
I'm assuming your queryData goes something like this:
{ filename: 'file.pdf', path: './path/to/file.pdf' }
I'm not sure what's in your content/index or what path this is on, but you obviously need to find a way to make your PDF file ('./path/to/file.pdf') available (as a download). See Express's built-in static server or res.download() to do that.
Once you have the PDF file available as a download, plug that path into PDF.js's .getDocument('/content/file.pdf') and do the rest to render the PDF onto the canvas or whatever.
Hope that helps.

cannot upload files and vars with xhr2 and web workers

I try to create code to upload files using XHR2 and web workers.
I thought I should use web workers , so if a file is big, web page will not freeze.
This is not working for two reasons, I never used web workers before, and I want to post to the server the file and vars at the same time, with the same xhr. When I say vars I mean the name of the file, and an int.
Heres is what I got
Client side
//create worker
var worker = new Worker('fileupload.js');
worker.onmessage = function(e) {
alert('worker says '+e.data);
}
//handle workers error
worker.onerror =werror;
function werror(e) {
console.log('ERROR: Line ', e.lineno, ' in ', e.filename, ': ', e.message);
}
//send stuff to the worker
worker.postMessage({
'files' : files, //img or video
'name' : nameofthepic, //text
'id':imageinsertid //number
});
Inside the worker (fileupload.js file)
onmessage = function (e) {var name=e.data.name; var id=e.data.id ; var file=e.data.files;
//create a var to catch the anser of the server
var datax;
var xhr = new XMLHttpRequest();
xhr.onload = function() {
if (xhr.status == 200) {datax=xhr.response;}
else { datax=525;}//actually, whatever, just give a value
};
xhr.open('POST', 'upload.php');
xhr.send(file,name,id);
//i also tried xhr.send('file=file&name=name&id=id'); and still nothing
//i also tried just the text/int xhr.send('name=name&id=id'); and still nothing
I am confused. I cannot send anything to the server. I get no feedback from the worker. I dont even know if the data are send to the fileupload.js. Server side does not INSERT.
Is that possible, sending files and text at the same time? What am I missing?
I need to pass text and int along with the file, so server side not only will upload the file, but also will INSERT to the database the int and the text, if the file is uploaded succesfully. This was easy just with formData and xhr, but, putting web workers in the middle, I cant get it right.
Also, can I use Transferable Objects to speed things up? Are Transferable Objects supported in all major browsers?
Thanks in advance

File upload using base64 encoded text instead of multipart/form-data - good idea?

I currently have a django formset with dynamic number of forms. Each form has a file field.
While this works, I need a way to allow multiple files to be selected at once. I can do that with input type='file' multiple='multiple' and I can read file data using FileReader. But how can I pass these files to django formset instead of form-0-file, form-1-file etc?
I can think of one way - replace FileField with TextField, and pass around the base64 encoded file as text, but I'm not sure if it's a good solution.

Just use multiple attribute and use FILES to get all of uploaded files.
base64 encoding maybe not help

Using multiple='multiple' is not related to formset or single form. It will work natively with django. So that if you plan to have single form instead of formset, just put multiple attribute and then access request.FILES to get all of uploaded files.

You should store the Images as Files,(Here are some good answers to do that),
I already tried to store images of that way but it have a lot of problems:
Every time you go to that page, it will start loading the image again, and the people don't want to load the same image every time.
It will use a lot of bandwidth.
If you are using a free web hosting service, will spend all you your bandwidth in a couple of hours, or when you store 50 images, so that mean that your site will be out the whole month, even the services that provide unlimited hosting and bandwidth, inforce a monthly bandwidth.

I recently had a problem where I had to implement a solution where a user can upload a document, stream that to the server, and without storing in on the server, post the stream to a SOAP server.
The way I implemented it, is as follows:
I wanted to upload the file via AJAX in order for me to show the progress of the upload.
This is my solution (Please note this only catered for one file at a time - but it might be a starting point.)
JavaScript:
First declare an object for the FormData - This will be used to send any additional info along with the files:
var formData = new FormData();
Secondly append all the data you would like to send to the server:
formData.append("documentDescription", $("#documentDescription textarea").val());
formData.append("afile", file.files[0]);
Now create a new instance of XMLHttpRequest:
var xhr = new XMLHttpRequest();
This is the rest of the code that got everything working:
xhr.open("POST", "UploadDocumentURL", true);
xhr.upload.onprogress = function(e) {
if (e.lengthComputable) {
var percentComplete = (e.loaded / e.total) * 100;
$('.progress .bar').css('width', percentComplete + '%').attr('aria-valuenow', percentComplete);
}
}
xhr.onload = function() {
if (this.status == 200) {
var resp = JSON.parse(this.response);
if (resp.type === "error") {
notify.add(resp.type, "Error", resp.message, 3000, true);
} else {
notify.add(resp.type, "Success", resp.message, 3000);
}
}
;
};
xhr.send(formData);
PHP
$documentName = $_POST["documentDescription"];
$fileName = $_FILES['afile']['name'];
$fileType = $_FILES['afile']['type'];
$ext = pathinfo($fileName, PATHINFO_EXTENSION);
$fileName = pathinfo($fileName, PATHINFO_FILENAME);
$fileContent = file_get_contents($_FILES['afile']['tmp_name']);
You will now have the Binary data on the server.
You should be able to make this work for multiple files by looping through the file.files[0] in JavaScript.
Hope you can apply this to your problem.

javascript xpcom component to download weather underground weather data

I am building a javascript component for Firefox that will take in a zip code, and will return the current weather conditions.
The sample code that weather underground uses jQuery, but as I understand it, I cannot include this code in my javascript component, as javascript does not have the functionality to include other javascript files.
At any rate, I have built up my skeleton code. It takes in the zip code and builds up the url
(example: http://api.wunderground.com/api/e17115d7e24a448e/geolookup/conditions/q/22203.json)
I have tried downloading the data from that url, via the following method:
getWeatherByUrl: function(url)
{
var persist = Components.classes["#mozilla.org/embedding/browser/nsWebBrowserPersist;1"].createInstance(Components.interfaces.nsIWebBrowserPersist);
var file = Components.classes["#mozilla.org/file/directory_service;1"].getService(Components.interfaces.nsIProperties).get("ProfD",Components.interfaces.nsILocalFile);
file.append("weather-forecaster.dat");
var urlURI = Components.classes["#mozilla.org/network/io-service;1"].getService(Components.interfaces.nsIIOService).newURI(url, null, null);
persist.saveURI(urlURI,null,null,null,"",file);
return url;
}
This should download the file to the user's profile directory. It indeed does create the file there. However, it does not look like it contains the json data from weather underground.
What exactly is going on? How would I download the file? I believe that there is a query going on when that url is passed to weather underground, but that shouldn't matter as the .json page is what gets spit out from them, right?
Is there a way to do this without downloading the file, but by streaming it and parsing it?

You can simply use XMLHttpRequest to download this data:
var request = Components.classes["#mozilla.org/xmlextras/xmlhttprequest;1"]
.createInstance(Components.interfaces.nsIXMLHttpRequest);
request.open("GET", "http://api.wunderground.com/api/Your_Key/geolookup/conditions/q/IA/Cedar_Rapids.json");
request.addEventListener("load", function(event)
{
var data = JSON.parse(request.responseText);
alert(data.response.version);
}, false);
request.send(null);

JavaScript/jQuery to download file via POST with JSON data

I have a jquery-based single-page webapp. It communicates with a RESTful web service via AJAX calls.
I'm trying to accomplish the following:
Submit a POST that contains JSON data to a REST url.
If the request specifies a JSON response, then JSON is returned.
If the request specifies a PDF/XLS/etc response, then a downloadable binary is returned.
I have 1 & 2 working now, and the client jquery app displays the returned data in the web page by creating DOM elements based on the JSON data. I also have #3 working from the web-service point of view, meaning it will create and return a binary file if given the correct JSON parameters. But I'm unsure the best way to deal with #3 in the client javascript code.
Is it possible to get a downloadable file back from an ajax call like this? How do I get the browser to download and save the file?
$.ajax({
type: "POST",
url: "/services/test",
contentType: "application/json",
data: JSON.stringify({category: 42, sort: 3, type: "pdf"}),
dataType: "json",
success: function(json, status){
if (status != "success") {
log("Error loading data");
return;
}
log("Data loaded!");
},
error: function(result, status, err) {
log("Error loading data");
return;
}
});
The server responds with the following headers:
Content-Disposition:attachment; filename=export-1282022272283.pdf
Content-Length:5120
Content-Type:application/pdf
Server:Jetty(6.1.11)
Another idea is to generate the PDF and store it on the server and return JSON that includes a URL to the file. Then, issue another call in the ajax success handler to do something like the following:
success: function(json,status) {
window.location.href = json.url;
}
But doing that means I would need to make more than one call to the server, and my server would need to build downloadable files, store them somewhere, then periodically clean up that storage area.
There must be a simpler way to accomplish this. Ideas?
EDIT: After reviewing the docs for $.ajax, I see that the response dataType can only be one of xml, html, script, json, jsonp, text, so I'm guessing there is no way to directly download a file using an ajax request, unless I embed the binary file in using Data URI scheme as suggested in the #VinayC answer (which is not something I want to do).
So I guess my options are:
Not use ajax and instead submit a form post and embed my JSON data into the form values. Would probably need to mess with hidden iframes and such.
Not use ajax and instead convert my JSON data into a query string to build a standard GET request and set window.location.href to this URL. May need to use event.preventDefault() in my click handler to keep browser from changing from the application URL.
Use my other idea above, but enhanced with suggestions from the #naikus answer. Submit AJAX request with some parameter that lets web-service know this is being called via an ajax call. If the web service is called from an ajax call, simply return JSON with a URL to the generated resource. If the resource is called directly, then return the actual binary file.
The more I think about it, the more I like the last option. This way I can get information back about the request (time to generate, size of file, error messages, etc.) and I can act on that information before starting the download. The downside is extra file management on the server.
Any other ways to accomplish this? Any pros/cons to these methods I should be aware of?

letronje's solution only works for very simple pages. document.body.innerHTML += takes the HTML text of the body, appends the iframe HTML, and sets the innerHTML of the page to that string. This will wipe out any event bindings your page has, amongst other things. Create an element and use appendChild instead.
$.post('/create_binary_file.php', postData, function(retData) {
var iframe = document.createElement("iframe");
iframe.setAttribute("src", retData.url);
iframe.setAttribute("style", "display: none");
document.body.appendChild(iframe);
});
Or using jQuery
$.post('/create_binary_file.php', postData, function(retData) {
$("body").append("<iframe src='" + retData.url+ "' style='display: none;' ></iframe>");
});
What this actually does: perform a post to /create_binary_file.php with the data in the variable postData; if that post completes successfully, add a new iframe to the body of the page. The assumption is that the response from /create_binary_file.php will include a value 'url', which is the URL that the generated PDF/XLS/etc file can be downloaded from. Adding an iframe to the page that references that URL will result in the browser promoting the user to download the file, assuming that the web server has the appropriate mime type configuration.

I've been playing around with another option that uses blobs. I've managed to get it to download text documents, and I've downloaded PDF's (However they are corrupted).
Using the blob API you will be able to do the following:
$.post(/*...*/,function (result)
{
var blob=new Blob([result]);
var link=document.createElement('a');
link.href=window.URL.createObjectURL(blob);
link.download="myFileName.txt";
link.click();
});
This is IE 10+, Chrome 8+, FF 4+. See https://developer.mozilla.org/en-US/docs/Web/API/URL.createObjectURL
It will only download the file in Chrome, Firefox and Opera. This uses a download attribute on the anchor tag to force the browser to download it.

I know this kind of old, but I think I have come up with a more elegant solution. I had the exact same problem. The issue I was having with the solutions suggested were that they all required the file being saved on the server, but I did not want to save the files on the server, because it introduced other problems (security: the file could then be accessed by non-authenticated users, cleanup: how and when do you get rid of the files). And like you, my data was complex, nested JSON objects that would be hard to put into a form.
What I did was create two server functions. The first validated the data. If there was an error, it would be returned. If it was not an error, I returned all of the parameters serialized/encoded as a base64 string. Then, on the client, I have a form that has only one hidden input and posts to a second server function. I set the hidden input to the base64 string and submit the format. The second server function decodes/deserializes the parameters and generates the file. The form could submit to a new window or an iframe on the page and the file will open up.
There's a little bit more work involved, and perhaps a little bit more processing, but overall, I felt much better with this solution.
Code is in C#/MVC
public JsonResult Validate(int reportId, string format, ReportParamModel[] parameters)
{
// TODO: do validation
if (valid)
{
GenerateParams generateParams = new GenerateParams(reportId, format, parameters);
string data = new EntityBase64Converter<GenerateParams>().ToBase64(generateParams);
return Json(new { State = "Success", Data = data });
}
return Json(new { State = "Error", Data = "Error message" });
}
public ActionResult Generate(string data)
{
GenerateParams generateParams = new EntityBase64Converter<GenerateParams>().ToEntity(data);
// TODO: Generate file
return File(bytes, mimeType);
}
on the client
function generate(reportId, format, parameters)
{
var data = {
reportId: reportId,
format: format,
params: params
};
$.ajax(
{
url: "/Validate",
type: 'POST',
data: JSON.stringify(data),
dataType: 'json',
contentType: 'application/json; charset=utf-8',
success: generateComplete
});
}
function generateComplete(result)
{
if (result.State == "Success")
{
// this could/should already be set in the HTML
formGenerate.action = "/Generate";
formGenerate.target = iframeFile;
hidData = result.Data;
formGenerate.submit();
}
else
// TODO: display error messages
}

There is a simplier way, create a form and post it, this runs the risk of resetting the page if the return mime type is something that a browser would open, but for csv and such it's perfect
Example requires underscore and jquery
var postData = {
filename:filename,
filecontent:filecontent
};
var fakeFormHtmlFragment = "<form style='display: none;' method='POST' action='"+SAVEAS_PHP_MODE_URL+"'>";
_.each(postData, function(postValue, postKey){
var escapedKey = postKey.replace("\\", "\\\\").replace("'", "\'");
var escapedValue = postValue.replace("\\", "\\\\").replace("'", "\'");
fakeFormHtmlFragment += "<input type='hidden' name='"+escapedKey+"' value='"+escapedValue+"'>";
});
fakeFormHtmlFragment += "</form>";
$fakeFormDom = $(fakeFormHtmlFragment);
$("body").append($fakeFormDom);
$fakeFormDom.submit();
For things like html, text and such, make sure the mimetype is some thing like application/octet-stream
php code
<?php
/**
* get HTTP POST variable which is a string ?foo=bar
* #param string $param
* #param bool $required
* #return string
*/
function getHTTPPostString ($param, $required = false) {
if(!isset($_POST[$param])) {
if($required) {
echo "required POST param '$param' missing";
exit 1;
} else {
return "";
}
}
return trim($_POST[$param]);
}
$filename = getHTTPPostString("filename", true);
$filecontent = getHTTPPostString("filecontent", true);
header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=\"$filename\"");
echo $filecontent;

It is been a while since this question was asked but I had the same challenge and want to share my solution. It uses elements from the other answers but I wasn't able to find it in its entirety. It doesn't use a form or an iframe but it does require a post/get request pair. Instead of saving the file between the requests, it saves the post data. It seems to be both simple and effective.
client
var apples = new Array();
// construct data - replace with your own
$.ajax({
type: "POST",
url: '/Home/Download',
data: JSON.stringify(apples),
contentType: "application/json",
dataType: "text",
success: function (data) {
var url = '/Home/Download?id=' + data;
window.location = url;
});
});
server
[HttpPost]
// called first
public ActionResult Download(Apple[] apples)
{
string json = new JavaScriptSerializer().Serialize(apples);
string id = Guid.NewGuid().ToString();
string path = Server.MapPath(string.Format("~/temp/{0}.json", id));
System.IO.File.WriteAllText(path, json);
return Content(id);
}
// called next
public ActionResult Download(string id)
{
string path = Server.MapPath(string.Format("~/temp/{0}.json", id));
string json = System.IO.File.ReadAllText(path);
System.IO.File.Delete(path);
Apple[] apples = new JavaScriptSerializer().Deserialize<Apple[]>(json);
// work with apples to build your file in memory
byte[] file = createPdf(apples);
Response.AddHeader("Content-Disposition", "attachment; filename=juicy.pdf");
return File(file, "application/pdf");
}

In short, there is no simpler way. You need to make another server request to show PDF file. Al though, there are few alternatives but they are not perfect and won't work on all browsers:
Look at data URI scheme. If binary data is small then you can perhaps use javascript to open window passing data in URI.
Windows/IE only solution would be to have .NET control or FileSystemObject to save the data on local file system and open it from there.

Not entirely an answer to the original post, but a quick and dirty solution for posting a json-object to the server and dynamically generating a download.
Client side jQuery:
var download = function(resource, payload) {
$("#downloadFormPoster").remove();
$("<div id='downloadFormPoster' style='display: none;'><iframe name='downloadFormPosterIframe'></iframe></div>").appendTo('body');
$("<form action='" + resource + "' target='downloadFormPosterIframe' method='post'>" +
"<input type='hidden' name='jsonstring' value='" + JSON.stringify(payload) + "'/>" +
"</form>")
.appendTo("#downloadFormPoster")
.submit();
}
..and then decoding the json-string at the serverside and setting headers for download (PHP example):
$request = json_decode($_POST['jsonstring']), true);
header('Content-Type: application/csv');
header('Content-Disposition: attachment; filename=export.csv');
header('Pragma: no-cache');

$scope.downloadSearchAsCSV = function(httpOptions) {
var httpOptions = _.extend({
method: 'POST',
url: '',
data: null
}, httpOptions);
$http(httpOptions).then(function(response) {
if( response.status >= 400 ) {
alert(response.status + " - Server Error \nUnable to download CSV from POST\n" + JSON.stringify(httpOptions.data));
} else {
$scope.downloadResponseAsCSVFile(response)
}
})
};
/**
* #source: https://github.com/asafdav/ng-csv/blob/master/src/ng-csv/directives/ng-csv.js
* #param response
*/
$scope.downloadResponseAsCSVFile = function(response) {
var charset = "utf-8";
var filename = "search_results.csv";
var blob = new Blob([response.data], {
type: "text/csv;charset="+ charset + ";"
});
if (window.navigator.msSaveOrOpenBlob) {
navigator.msSaveBlob(blob, filename); // #untested
} else {
var downloadContainer = angular.element('<div data-tap-disabled="true"><a></a></div>');
var downloadLink = angular.element(downloadContainer.children()[0]);
downloadLink.attr('href', window.URL.createObjectURL(blob));
downloadLink.attr('download', "search_results.csv");
downloadLink.attr('target', '_blank');
$document.find('body').append(downloadContainer);
$timeout(function() {
downloadLink[0].click();
downloadLink.remove();
}, null);
}
//// Gets blocked by Chrome popup-blocker
//var csv_window = window.open("","","");
//csv_window.document.write('<meta name="content-type" content="text/csv">');
//csv_window.document.write('<meta name="content-disposition" content="attachment; filename=data.csv"> ');
//csv_window.document.write(response.data);
};

I think the best approach is to use a combination, Your second approach seems to be an elegant solution where browsers are involved.
So depending on the how the call is made. (whether its a browser or a web service call) you can use a combination of the two, with sending a URL to the browser and sending raw data to any other web service client.

Found it somewhere long time ago and it works perfectly!
let payload = {
key: "val",
key2: "val2"
};
let url = "path/to/api.php";
let form = $('<form>', {'method': 'POST', 'action': url}).hide();
$.each(payload, (k, v) => form.append($('<input>', {'type': 'hidden', 'name': k, 'value': v})) );
$('body').append(form);
form.submit();
form.remove();

I have been awake for two days now trying to figure out how to download a file using jquery with ajax call. All the support i got could not help my situation until i try this.
Client Side
function exportStaffCSV(t) {
var postData = { checkOne: t };
$.ajax({
type: "POST",
url: "/Admin/Staff/exportStaffAsCSV",
data: postData,
success: function (data) {
SuccessMessage("file download will start in few second..");
var url = '/Admin/Staff/DownloadCSV?data=' + data;
window.location = url;
},
traditional: true,
error: function (xhr, status, p3, p4) {
var err = "Error " + " " + status + " " + p3 + " " + p4;
if (xhr.responseText && xhr.responseText[0] == "{")
err = JSON.parse(xhr.responseText).Message;
ErrorMessage(err);
}
});
}
Server Side
[HttpPost]
public string exportStaffAsCSV(IEnumerable<string> checkOne)
{
StringWriter sw = new StringWriter();
try
{
var data = _db.staffInfoes.Where(t => checkOne.Contains(t.staffID)).ToList();
sw.WriteLine("\"First Name\",\"Last Name\",\"Other Name\",\"Phone Number\",\"Email Address\",\"Contact Address\",\"Date of Joining\"");
foreach (var item in data)
{
sw.WriteLine(string.Format("\"{0}\",\"{1}\",\"{2}\",\"{3}\",\"{4}\",\"{5}\",\"{6}\"",
item.firstName,
item.lastName,
item.otherName,
item.phone,
item.email,
item.contact_Address,
item.doj
));
}
}
catch (Exception e)
{
}
return sw.ToString();
}
//On ajax success request, it will be redirected to this method as a Get verb request with the returned date(string)
public FileContentResult DownloadCSV(string data)
{
return File(new System.Text.UTF8Encoding().GetBytes(data), System.Net.Mime.MediaTypeNames.Application.Octet, filename);
//this method will now return the file for download or open.
}
Good luck.

I liked Frank's idea and decided to do my own twist to it. As trying to do it in one post is very complicated, I'm using the two post method but only hitting the database once and no need to save the file or clean up file when completed.
First I run the ajax request to retrieve the data but instead of returning the data from the controller I will return a GUID that is tied to a TempData storage of the records.
$.get("RetrieveData", { name: "myParam"} , function(results){
window.location = "downloadFile?id=" + results
});
public string RetrieveData(string name)
{
var data = repository.GetData(name);
string id = Guid.NewGuid().ToString();
var file = new KeyValuePair<string, MyDataModel>(name, data);
TempData[id]=file;
return id;
}
Then when I call the window.location I pass the Guid to the new method and get the data from TempData. After this method is executed TempData will be free.
public ActionResult DownloadFile(string id)
{
var file = (KeyValuePair<string,MyDataModel>)TempData[id];
var filename = file.Key;
var data = file.Value;
var byteArray = Encoding.UTF8.GetBytes(data);
...
return File(byteArray, "text/csv", "myFile.csv");
}

Another approach instead of saving the file on the server and retrieving it, is to use .NET 4.0+ ObjectCache with a short expiration until the second Action (at which time it can be definitively dumped). The reason that I want to use JQuery Ajax to do the call, is that it is asynchronous. Building my dynamic PDF file takes quite a bit of time, and I display a busy spinner dialog during that time (it also allows other work to be done). The approach of using the data returned in the "success:" to create a Blob does not work reliably. It depends on the content of the PDF file. It is easily corrupted by data in the response, if it is not completely textual which is all that Ajax can handle.

Solution
Content-Disposition attachment seems to work for me:
self.set_header("Content-Type", "application/json")
self.set_header("Content-Disposition", 'attachment; filename=learned_data.json')
Workaround
application/octet-stream
I had something similar happening to me with a JSON, for me on the server side I was setting the header to
self.set_header("Content-Type", "application/json")
however when i changed it to:
self.set_header("Content-Type", "application/octet-stream")
It automatically downloaded it.
Also know that in order for the file to still keep the .json suffix you will need to it on filename header:
self.set_header("Content-Disposition", 'filename=learned_data.json')

The Problems with Making your own events
Many of the solutions proposed on this article have the JavaScript run asynchronously and create a link element then calling
const a = documet.createElement("a")
a.click()
or creating a mouse event
new MouseEvent({/* ...some config */})
This would seem fine right? What could be wrong with this?
What is an Event-Sourcing?
Event sourcing has a bunch of meanings across computing such as a system of pub sub in a cloud based architecture, or the browser api EventSource. In the context of a browser
all events have a source and that source has hidden property that says who initiated this event (the user or the site).
Knowing this we can start to understand why two click events might not be treated the same
user click* new MouseEvent()
----------- -----------
| Event 1 | | Event 2 |
----------- -----------
| |
|----------------------|
|
|
----------------------
| Permissions Policy | Available in chrome allows the server to control
---------------------- what features are going to be used by the JS
|
|
----------------------------
| Browser Fraud Protection | The Browser REALLY doesnt like being told to pretend
---------------------------- to be a user. If you will remember back to the early
| 2000s when one click spun off 2000 pop ups. Well here
| is where popups are blocked, fraudulent ad clicks are
\ / thrown out, and most importantly for our case stops
v fishy downloads
JavaScript Event Fires
So I just Can't Download off A POST That's Dumb
No, of course you can. You just need to give the user a chance to create the event. Here are a number of patterns that you can use to create user flows that are obvious and convectional and will not be flagged as fraud. (using jsx sorry not sorry)
A Form can be used to navigate to a url with a post action.
const example = () => (
<form
method="POST"
action="/super-api/stuff"
onSubmit={(e) => {/* mutably change e form data but don't e.preventDetfault() */}}
>
{/* relevant input fields of your download */}
</form>
)
Preloading If your download is non-configurable you may want to consider preloading the download into resp.blob() or new Blob(resp) this tells the browser that this is a file and we wont be doing any string operations on it. As with the other answers you can use window.URL.createObjectURL what is not mentioned is that
createObjectURL CAN MAKE A MEMORY LEAK IN JAVASCRIPTsource
If you don't want the C++ bully's to come make fun of you you must free this memory. Ahh but I'm just a hobbiest who loves his garbage collector. Have no fear this is very simple if you are working in most frameworks (for me react) you just register some sort of clean up effect on your component and your right as rain.
const preload = () => {
const [payload, setPayload] = useState("")
useEffect(() => {
fetch("/super-api/stuff")
.then((f) => f.blob())
.then(window.URL.createObjectURL)
.then(setPayload)
return () => window.URL.revokeObjectURL(payload)
}, [])
return (<a href={payload} download disabled={payload === ""}>Download Me</a>)
}

I think I got close, but something is corrupting the file (Image), any way, maybe some one can disclose the problem of this approach
$.ajax({
url: '/GenerateImageFile',
type: 'POST',
cache: false,
data: obj,
dataType: "text",
success: function (data, status, xhr) {
let blob = new Blob([data], { type: "image/jpeg" });
let a = document.createElement('a');
a.href = window.URL.createObjectURL(blob);
a.download = "test.jpg";
document.body.appendChild(a);
a.click();
document.body.removeChild(a);
window.URL.removeObjectURL(a.href);
},
complete: function () {
},
beforeSend: function () {
}
});

With HTML5, you can just create an anchor and click on it. There is no need to add it to the document as a child.
const a = document.createElement('a');
a.download = '';
a.href = urlForPdfFile;
a.click();
All done.
If you want to have a special name for the download, just pass it in the download attribute:
const a = document.createElement('a');
a.download = 'my-special-name.pdf';
a.href = urlForPdfFile;
a.click();