Download a file by jQuery.Ajax

Fetch API alternative

If XMLHttpRequest feels a bit 2005 to you, the Fetch API stands at the ready. The advantage of Fetch is that it's promises-based, and lets you handle downloads with a side-dish of user experience enhancements. Defeat won't be an option when we have failsafes wrapped around the command:

fetch('file_url')
  .then(response => response.blob()) // Blob–kill confirmed!
  .then(blob => {
    const downloadLink = document.createElement('a'); // Our secret weapon
    downloadLink.href = URL.createObjectURL(blob);
    downloadLink.download = 'desired_filename.ext'; // A name to go by
    document.body.appendChild(downloadLink);
    downloadLink.click(); // Fire away!
    document.body.removeChild(downloadLink); // Leave no trace behind
    URL.revokeObjectURL(downloadLink.href); // Pack up and head home
  })
  .catch(e => console.error('Operation failed:', e));


UX enhancement with jQuery and friends

If user experience was a 5-star restaurant, feedback would be the chef's special. Be it download completion or an unexpected error, plugins like jQuery File Download help deliver the message like an uppercut.

$(document).on('click', '#download-button', function() {
    $.fileDownload('file_url', {
        successCallback: function () {
            alert('Mission accomplished!');
        },
        failCallback: function () {
            alert('Abort mission! Abort!');
        }
    });
});


Also, check out FileSaver.js library. It's the Swiss army knife of client-side file saving with oodles of control.

Handling large fish (aka files)

Things often escalate quickly when you have large files at play. The sheer size of these beasts can punch holes in your application's memory and CPU performance, making it as fast as a napping turtle. Opt for Streams or seek refuge in the versatility of Blob, slicing and dicing as needed to manage these monstrous files.

Browser diversity and you

The path to universal browser support is treacherous. One wrong step and you have undefined is not a function breathing down your neck. When facing older browsers, it's best to adopt some time-honored techniques like:

• Deploy window.location or iframe
• If all else fails, turn Base64 encoded data into Blob. This shall maintain compatibility across the browser realm
• When dealing with archaic interfaces, don't hesitate to provide HTML4 fallbacks. They're not trendy, but they get the job done

Server considerations

Servers need love too! On the back-end, ensure you're setting the correct Content-Type, Content-Disposition, and file name in response headers. They are like your application's passport in the land of browsers, guiding them on how to handle file downloads.

If your back-ally is Struts2, tune your struts.xml to remove roadblocks and use <s:a> and <s:url> tags to create seamless download URLs.

Multi-part downloads: Divide and conquer

Giant files can be a pain, but not if you break 'em down. A multi-part download approach carves the file into manageable pieces allowing parallel downloads and client-side reassembly. If this was a heist movie, it's the part where everyone goes in together, steals bits of the file, and put it together as getaway music hits a crescendo.

Responsive feedback: Progress bars and beyond

A progress bar or something similar is like the comfort blanket your users need during file downloads. Trust me, nobody likes staring at a screen hoping something magically happens. Dish out real-time updates and a little bit of hand-holding with the onprogress event in XMLHttpRequest.

Security: Save the day

Before initiating downloads, sanitize and validate your file URLs. Secure your download endpoints, because exposing sensitive data is a villain's job. Always keep the good guy's hat on and use HTTPS, Access-Control headers, and other protocols best suited to protect your server and client.