Force Download with JavaScript

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Force download scripts have been an important part of internet usability for a long time.  I can attest to that by the number of times I’ve implemented this feature on the server side and the popularity of my PHP Force Download post, even to this day.  With the web world having moved much more the client side, I started looking for a method to force download without the need of a server, and I found it….right in the Firefox DevTools Debugger!

The JavaScript

The function to do this is quite small and relies on URL.createObjectUrl:

function downloadFile(data, fileName, type="text/plain") {
  // Create an invisible A element
  const a = document.createElement("a");
  a.style.display = "none";
  document.body.appendChild(a);

  // Set the HREF to a Blob representation of the data to be downloaded
  a.href = window.URL.createObjectURL(
    new Blob([data], { type })
  );

  // Use download attribute to set set desired file name
  a.setAttribute("download", fileName);

  // Trigger the download by simulating click
  a.click();

  // Cleanup
  document.body.removeChild(a);
}

The function injects an <a> element into the body, sets it URL to a Blob value to the text content of the destination file, and clicks the element to trigger the download.  The element remains hidden during the process and is removed from the DOM immediately after the click() call.  As soon as the function is called, the browser’s download prompt is displayed.

I look forward to learning more about both createObjectURL and Blob; those two are the true magic of this technique!

Shout out to Sneha Jain for implementing this great technique within the Firefox DevTools debugger!

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