How to Trigger File Downloads from AJAX POST Responses?

How to Handle File Download with AJAX POST

When sending AJAX POST requests in a JavaScript application, developers may encounter scenarios where the server responds with a file instead of JSON data. In such cases, triggering a file download in the client's browser becomes necessary. This article explores a solution for this challenge.

AJAX requests typically specify the Content-Type and inspect the Content-Disposition header to identify the response type. Upon determining that the response contains a file, the question arises: how to initiate a download?

The solution lies in utilizing portions of the FileAPI in modern browsers. Here's how it works:

1. Configure an XMLHttpRequest:

   • Set the responseType to 'blob' to receive the response as a binary data blob.
   • Specify the Content-type header as 'application/x-www-form-urlencoded' for POST requests.

2. Handle Onload Event:

   • On a successful response (status 200), extract the blob from the response.
   • Obtain the Content-Disposition header to check for a suggested filename.

3. Initiate Download:

   • For Internet Explorer, use window.navigator.msSaveBlob to save the blob with the suggested filename.
   • For other browsers, leverage window.URL or window.webkitURL to create a downloadable URL for the blob.
   • Use the HTML5 a[download] attribute to specify the filename (if available) or redirect the user to the downloadable URL.
   • Revoke the temporary downloadable URL after a short delay for cleanup.

4. jQuery.ajax Alternative:

   • Use the jQuery.ajax method with similar configurations and handle the blob in the success callback.

In summary, by employing the FileAPI, it is possible to initiate file downloads in the browser even when the server response is triggered by AJAX POST requests. This technique is supported in modern browsers and provides a convenient and user-friendly experience for downloading files from web applications.

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