JavaScript/jQuery to download file via POST with JSON data

Question

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:

1. Submit a POST that contains JSON data to a REST url.
2. If the request specifies a JSON response, then JSON is returned.
3. 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:

1. 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.

2. 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.

3. 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?

Permanency

possible duplicate of Handle file download from ajax post – Dimidiate

this one was "a bit" earlier, so how is it a duplicate – Iolenta

url = 'http://localhost/file.php?file='+ $('input').val(); window.open(url); The easy way to get the same results. Put the header in php file. No need to send any ajax or get requests – Fluorene

Answer 1

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.

Parturition

I agree, this is better than using += and I'll update my application accordingly. Thanks! – Permanency

I like this concept, however Chrome has this message in the console: Resource interpreted as Document but transferred with MIME type application/pdf. Then it also warns me the file may be dangerous. If I visit the retData.url directly, no warnings or issues. – Vankirk

Looking around the internet, it looks like you can have issues with PDFs in iFrames if your server is not setting Content-Type and Content-Disposition properly. I haven't actually used this solution myself, just clarified a previous solution, so don't have any other advice I'm afraid. – Parturition

@Tauren: If you agree that this is a better answer, note that you can switch the accepted answer at any time - or even remove the accept mark entirely. Simply check off the new answer and the accept mark will be transferred. (Old question, but it was brought up in flags recently.) – Bering

this answer could have probably just been a comment on the original answer, which could have then been modified to avoid the specific issue you mentioned. – Portillo

what is the retData.url suppose to be? – Marijo

Could someone be as kind as to explain what exactly does this code? The only thing that I understand is the Post call – Cabanatuan

Does this work on IE? IE only allows few elements to be assigned to SRC. – Gauche

Can I somehow detect, that the iframe was processed? I want to remove an iframe from dom after download ( it's not needed anymore ), I solved it like this for now: setTimeout(() => $iframe.remove(), 2000); – Shontashoo

Answer 2

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.

Elagabalus

No support for download attribute on IE and Safari :( (caniuse.com/#feat=download). You can open a new window and put content there, but not sure about a PDF or Excel... – Siddur

You should free up browser memory after calling click: window.URL.revokeObjectURL(link.href); – Stichomythia

@Elagabalus This is old but it works on my end. How can I get it to open up my directory immediately to save rather than saving to my browser? Also is there a way to determine the name of the incoming file? – Longs

It will corrupt binary files, because they get converted to strings using encoding, and result is not very close to original binary... – Gravely

May use mimetypes to not corrupt the information as used here with PDFs: alexhadik.com/blog/2016/7/7/l8ztp8kr5lbctf5qns4l8t3646npqh – Leventis

Answer 3

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
    }

Skill

I haven't had a good look at this solution, but it's worth noting that nothing about the solution using create_binary_file.php requires files to be saved to disk. It would be entirely feasible to have create_binary_file.php generate binary files in memory. – Parturition

Also for the security concern, the URL that /create_binary_file.php returns could have access control applied to it if need be. – Parturition

Answer 4

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;

Epizoon

Answer 5

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");
}

Emlynn

I think I like this solution best. In my case though I am generating the file, so I think that I will try keeping the file in memory until it is accessed and then free up the memory after it's downloaded so I never have to write to disk. – Rett

Answer 6

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:

1. Look at data URI scheme. If binary data is small then you can perhaps use javascript to open window passing data in URI.
2. 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.

Sweetie

Thanks, I was not aware of the Data URI scheme. It looks like that may be the only way to do it in a single request. But it isn't really the direction I want to go. – Permanency

Due to client requirements I ended up solving this problem using the server headers in the response (Content-Disposition: attachment;filename="file.txt"), but I really want to try this approach next time. I'd used URI data for thumbnails before, but it never occurred to me to use it for downloads - thanks for sharing this nugget. – Zwart

Answer 7

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');

Dichromatism

I like this solution. In the OP's question it sounds like the requester knows whether to expect a file download or JSON data, so he could decide at the client end and post to a different URL, rather than deciding on the server end. – Slaughterhouse

Answer 8

$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);
};

Aurochs

Answer 9

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.

Hostel

My second approach is looking more appealing to me now too. Thanks for confirming it is a worthwhile solution. Are you suggesting that I pass an additional value in the json object that indicates this request is being made from a browser as an ajax call instead of a web service call? There are several ways to accomplish this that I can think of, but what technique you would use? – Permanency

can't that be determined by the User-Agent http header? Or any other http header? – Hostel

Answer 10

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();

Potato

Answer 11

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.

Bohner

This will work, for small files. But if you have a large file you are going to run into the problem of the url being too long. Try it with 1,000 records, it will break and only export part of the data. – Adversity

Answer 12

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");
}

Honeysuckle

Answer 13

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.

Antimacassar

Answer 14

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')

Brown

Answer 15

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>)
}

Driedup

Answer 16

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 () {

            }
        });

Corselet

Answer 17

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();

Baker

the question is about POST method – Gibbons