Download File Using JavaScript/jQuery

javascript

prompt-engineering

download-attribute

fetch-api

byAnton Shumikhin·Oct 26, 2024

Invoking immediate file download in JavaScript is achieved by crafting an invisible <a> element with href that points to the file URL followed by simulating an onclick() event. Check the code snippet:

function downloadFile(url, fileName) {
  const a = document.createElement('a');  // Hey Presto! A magical <a> appears!
  a.href = url;
  a.download = fileName;
  a.style.display = 'none';
  document.body.appendChild(a);  // Adding <a> to the DOM family 
  a.click();  // Click, click and away we go! 
  document.body.removeChild(a);  // Cleaning after ourselves, Mum would be proud!
}

// Usage: downloadFile('path/to/file', 'aspired-filename.ext');

Don't overlook the URL's accessibility to evade CORS issues and guarantee broader browser compatibility.

Adding flair with IFrames

The <a> element might be simple and effective, but adapting an <iframe> could be smart, especially managing convoluted download scenarios or preserving the current page state.

function downloadUsingIframe(url) { 
  $('<iframe>', {  
    style: 'display: none;',
    src: url  // I'm like a spy-cam for downloads🕵️‍♀️
  }).appendTo('body').on('load', function() {
    $(this).remove();  // Done with spying, self-destructing in 5, 4, 3 ...
  });
}

For downloading, the server should set Content-Disposition or MIME Type as application/octet-stream.

Harnessing fetch with blobs

For deadweight files or streams, the superhero-like fetch API swings into rescue. Cooperative with window.URL.createObjectURL, it effectively manages blobs.

fetch('file_url').then(response => {
  if (!response.ok) throw new Error('Network response was not ok');  // Well, nobody is perfect!
  return response.blob();
}).then(blob => {
  const url = window.URL.createObjectURL(blob);  // Bonding with the blob
  downloadFile(url, 'filename.ext');  // Fetch, Good Boy! 🐕
  window.URL.revokeObjectURL(url);  // Break-up with blob
});

By releasing the blob URL after download, this ensures efficient memory usage and a smooth page experience.

Background downloads: Smooth Operators

To offer a smooth user experience, directly opening a new tab or window can be a let-down. In such cases, go for either jQuery File Download plugin for better management & callbacks, or download attribute on 'a' elements to specify the filename while maintaining the browser context.

function downloadInBackground(url, fileName) {
  $('<a>', {
    href: url,
    download: fileName
  }).appendTo(document.body)[0].click();
  $('a[download]').remove();  // Hide evidence
}

Prioritize compatibility with the download attribute by checking on caniuse.com.

The silent download agents

Keep the current page state intact by performing downloads in a new context. Inform users about the download status using a modal or a new tab to avoid vandalism to their current activity.

React to different outcomes of the download action by utilizing success and fail callbacks within fetch or AJAX operations.

Browsers and attention to detail

The download attribute garners support across most modern browsers. However, vintage browsers need to be taken into account:

Vintage browser support

For browsers that defy the download attribute approach, a server-side script or a JavaScript fallback can assist:

// PHP script version of Gandalf: "You shall not pass!" (without a download request)
header('Content-Disposition: attachment; filename="filename.ext"');
readfile('/path/to/the/file.ext');

Or a JavaScript fallback:

function fallbackDownload(url) {
  if (!'download' in document.createElement('a')) {
    window.location.href = url;  // You don't want the file, eh? Well here, enjoy the redirect!
  }
}

// Usage: fallbackDownload('path/to/file');

Server-side considerations

Creating a smooth download experience heavily depends on setting correct MIME types and headers server-side. Use Content-Type: application/octet-stream and Content-Disposition: attachment; filename="example.ext" to prompt browsers to download rather than display content.

Bear in mind that you can turn to Axios or jQuery Ajax for more intricate download scenarios to suit your needs.

Managing AJAX downloads

Downloading files via AJAX is a tricky puzzle since XMLHttpRequest and fetch don't natively support it. Here's a solution:

$.ajax({
  url: 'server_endpoint',
  method: 'GET',
  xhrFields: {
    responseType: 'blob' // I got you covered for 'pdf', 'csv', 'doc', etc.
  },
  success: function(data) {
    const url = window.URL.createObjectURL(data);
    downloadFile(url, 'report.pdf');  // Here's your report, Sir! 🧐
    window.URL.revokeObjectURL(url);  // Blob breakup time again! 💔
  },
  error: function(jqXHR, textStatus, errorThrown) {
    console.error('Download failed:', textStatus, errorThrown);  // Spreading the bad news
  }
});