starting file download with JavaScript

Let's say I have download links for files on my site.

When clicked these links send an AJAX request to the server which returns the URL with the location of the file.

What I want to do is direct the browser to download the file when the response gets back. Is there a portable way to do this?

We do it that way: First add this script.

<script type="text/javascript">
function populateIframe(id,path) 
{
    var ifrm = document.getElementById(id);
    ifrm.src = "download.php?path="+path;
}
</script>

Place this where you want the download button(here we use just a link):

<iframe id="frame1" style="display:none"></iframe>
<a href="javascript:populateIframe('frame1','<?php echo $path; ?>')">download</a>

The file 'download.php' (needs to be put on your server) simply contains:

<?php 
   header("Content-Type: application/octet-stream");
   header("Content-Disposition: attachment; filename=".$_GET['path']);
   readfile($_GET['path']);
?>

So when you click the link, the hidden iframe then gets/opens the sourcefile 'download.php'. With the path as get parameter. We think this is the best solution!

It should be noted that the PHP part of this solution is a simple demonstration and potentially very, very insecure. It allows the user to download any file, not just a pre-defined set. That means they could download parts of the source code of the site itself, possibly containing API credentials etc.

I have created an open source jQuery File Download plugin (Demo with examples) (GitHub) which could also help with your situation. It works pretty similarly with an iframe but has some cool features that I have found quite handy:

• User never leaves the same page they initiated a file download from. This feature is becoming crucial for modern web applications
• Tested cross browser support (including mobile!)
• It supports POST and GET requests in a manner similar to jQuery's AJAX API
• successCallback and failCallback functions allow for you to be explicit about what the user sees in either situation
• In conjunction with jQuery UI a developer can easily show a modal telling the user that a file download is occurring, disband the modal after the download starts or even inform the user in a friendly manner that an error has occurred. See the Demo for an example of this.

Just call window.location.href = new_url from your javascript and it will redirect the browser to that URL as it the user had typed that into the address bar

Reading the answers - including the accepted one I'd like to point out the security implications of passing a path directly to readfile via GET.

It may seem obvious to some but some may simply copy/paste this code:

<?php 
   header("Content-Type: application/octet-stream");
   header("Content-Disposition: attachment; filename=".$_GET['path']);
   readfile($_GET['path']);
?>

So what happens if I pass something like '/path/to/fileWithSecrets' to this script? The given script will happily send any file the webserver-user has access to.

Please refer to this discussion for information how to prevent this: https://stackoverflow.com/questions/456546/how-do-i-make-sure-a-file-path-is-within-a-given-subdirectory

If this is your own server application then i suggest using the following header

Content-disposition: attachment; filename=fname.ext

This will force any browser to download the file and not render it in the browser window.

Try this lib https://github.com/PixelsCommander/Download-File-JS it`s more modern than all solutions described before because uses "download" attribute and combination of methods to bring best possible experience.

Explained here - http://pixelscommander.com/en/javascript/javascript-file-downliading-ignore-content-type/

Seems to be ideal piece of code for starting downloading in JavaScript.

A agree with the methods mentioned by maxnk, however you may want to reconsider trying to automatically force the browser to download the URL. It may work fine for binary files but for other types of files (text, PDF, images, video), the browser may want to render it in the window (or IFRAME) rather than saving to disk.

If you really do need to make an Ajax call to get the final download links, what about using DHTML to dynamically write out the download link (from the ajax response) into the page? That way the user could either click on it to download (if binary) or view in their browser - or select "Save As" on the link to save to disk. It's an extra click, but the user has more control.

To get around the security flaw in the top-voted answer, you can set the iframe src directly to the file you want (instead of an intermediate php file) and set the header information in an .htaccess file:

<Files *.apk>
     ForceType application/force-download
     Header set Content-Disposition attachment
     Header set Content-Type application/vnd.android.package-archive
     Header set Content-Transfer-Encoding binary
</Files>

I suggest to make an invisible iframe on the page and set it's src to url that you've received from the server - download will start without page reloading.

Or you can just set the current document.location.href to received url address. But that's can cause for user to see an error if the requested document actually does not exists.

In relation to the top answer I have a possible solution to the security risk.

<?php
     if(isset($_GET['path'])){
	     if(in_array($_GET['path'], glob("*/*.*"))){
		     header("Content-Type: application/octet-stream");
		     header("Content-Disposition: attachment; filename=".$_GET['path']);
		     readfile($_GET['path']);
	     }
     }
?>

Using the glob() function (I tested the download file in a path one folder up from the file to be downloaded) I was able to make a quick array of files that are "allowed" to be downloaded and checked the passed path against it. Not only does this insure that the file being grabbed isn't something sensitive but also checks on the files existence at the same time.

Note: Javascript / HTML

HTML:

<iframe id="download" style="display:none"></iframe>

and

<input type="submit" value="Download" onclick="ChangeSource('document_path');return false;">

JavaScript:

<script type="text/javascript">
    <!--
        function ChangeSource(path){
            document.getElementByID('download').src = 'path_to_php?path=' + document_path;
        }
    -->
</script>

I'd suggest window.open() to open a popup window. If it's a download, there will be no window and you will get your file. If there is a 404 or something, the user will see it in a new window (hence, their work will not be bothered, but they will still get an error message).

Why are you making server side stuff when all you need is to redirect browser to different window.location.href?

Here is code that parses ?file= QueryString (taken from this question) and redirects user to that address in 1 second (works for me even on Android browsers):

<script type="text/javascript">
    var urlParams;
    (window.onpopstate = function () {
        var match,
        pl = /\+/g,  // Regex for replacing addition symbol with a space
        search = /([^&=]+)=?([^&]*)/g,
        decode = function (s) { return decodeURIComponent(s.replace(pl, " ")); },
        query = window.location.search.substring(1);

        urlParams = {};
        while (match = search.exec(query))
            urlParams[decode(match[1])] = decode(match[2]);
    })();

    (window.onload = function() {
        var path = urlParams["file"];
        setTimeout(function() { document.location.href = path; }, 1000);
    });
</script>

If you have jQuery in your project definitely remove those window.onpopstate & window.onload handlers and do everything in $(document).ready(function () { } );