I had an endpoint on .Net server

[HttpPost]
[Route("api/tagExportSelectedToExcel")]

and a React frontend with axios. The task was to add a button which downloads a file from this API. I spent several hours before found this solution. I hope it will be helpful for someone else.

This is what I did:

axios('/api/tagExportSelectedToExcel', {
    data: exportFilter,
    method: 'POST',
    responseType: 'blob'
}).then(res => resolveAndDownloadBlob(res));

Where resolveAndDownloadBlob:

/**
 * Resolved and downloads blob response as a file.
 * FOR BROWSERS ONLY
 * @param response
 */
function resolveAndDownloadBlob(response: any) {
    let filename = 'tags.xlsx';
    filename = decodeURI(filename);
    const url = window.URL.createObjectURL(new Blob([response.data]));
    const link = document.createElement('a');
    link.href = url;
    link.setAttribute('download', filename);
    document.body.appendChild(link);
    link.click();
    window.URL.revokeObjectURL(url);
    link.remove();
}

I think you could using blob, something like

var content=...the content of your request;
var mypdf = new Blob(content, {type : 'application/pdf'});

and check answer from "panzi" in this other question Using HTML5/Javascript to generate and save a file

(One character per element in the array seem pretty nice binary. Probably you don't need to transform it. https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Sending_and_Receiving_Binary_Data )

I found some examples in other posts that light the way.

You can do this using blob files. Here's an example:

function downloadReport(level, beginningYearmo, endingYearmo){
  var requestData = JSON.stringify({plevel: level, yearMonthStart: beginningYearmo, yearMonthEnd: endingYearmo});
  var url = 'http://localhost:8181/file/download';
  d3.request(url)
    .header("X-Requested-With", "XMLHttpRequest")
    .header("Content-Type", "application/json")
    .header("Accept","application/vnd.openxmlformats-officedocument.spreadsheetml.sheet")
    .header("Cache-Control", "no-cache")
    .header("Accept-Charset", "utf-8")
    .on("load", function(data){
      var blob = new Blob([data], {type: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"});
      var objectUrl = URL.createObjectURL(blob);
      window.open(objectUrl);
      console.log("Download request was successful.");
    })
    .on("error", function(error){ alert("Error: ", error) })
    .send("POST", requestData);
}

$('#your_button').click(function(){
    window.open('http://localhost:xxxx/xx.xml');
    return false;
});

I usually use AttachmentFiles endpoint to do this, it is provided when you create or select Item with API.

<link  rel="http://schemas.microsoft.com/ado/2007/08/dataservices/related/AttachmentFiles" type="application/atom+xml;type=feed" title="AttachmentFiles" href="Web/Lists(guid'12f9fd2c-0eea-4440-b9d3-a6e445839ba3')/Items(66)/AttachmentFiles" />

Main part is

href="Web/Lists(guid'12f9fd2c-0eea-4440-b9d3-a6e445839ba3')/Items(66)/AttachmentFiles" 

You can use both

To download      /Items(66)/AttachmentFiles('file.docx')
To upload        /Items(66)/AttachmentFiles/add(FileName='file.docx')

Then u can use a GET method to download file content :

executor.executeAsync(
    {
        url: UsePreviousEndPoint,
        type: "GET",
        contentType: "application/atom+xml;type=entry",
        headers: {
                "Accept": "application/atom+xml",
                "Authorization": "Bearer " + UserOrAppToken
            },      
        success: function (blobURL)
        {
            SaveToDisk_blob(blobURL, fileName)
        },
        error: function (xhr)
        {
            console.log(xhr);
            console.log("downloadFile" + xhr.status + ": " + xhr.statusText);

        }
    });

I guess Digest is only mandatory for POST request.

I am not sure if Restangular can do that, but I am using FileSaver script for stuff like that. Add Filesaver to your HTML head and then:

Restangular.one('download').get().then(function (res) {
    var file = new Blob([res], { type: 'text/csv' });
    saveAs(file, 'something.csv');
});

Advice is simple: you cannot download files via AJAX - it's a security policy. I mean you can download the data, but you can't save it to disk from JavaScript side.

If you want to download a file on click, then you can just add href to you a tag. Or open a new window with file's URL.