Javascript Fetch Blob Download [patched]

Downloading files using the JavaScript and Blobs is the standard way to programmatically trigger file saves in the browser, especially when you need to handle authentication headers, POST requests, or dynamic file generation. The Standard Implementation

: The HTML download attribute often fails on different origins. Fetching the resource and creating a local Blob URL bypasses this restriction. javascript fetch blob download

: You can validate the server response (e.g., check response.ok ) before attempting to save a file, preventing the download of error pages. Best Practices & Memory Management Downloading files using the JavaScript and Blobs is

: Unlike a standard , fetch allows you to send JWT tokens or custom headers in the request. : You can validate the server response (e

The process involves fetching the data as a Blob (Binary Large Object), creating a temporary URL for that data, and using a hidden anchor ( ) tag to trigger the browser's download mechanism. javascript

async function downloadFile(url, fileName) { try { // 1. Fetch the data const response = await fetch(url, { method: 'GET', // Can also be POST if required by your API headers: { 'Authorization': 'Bearer YOUR_TOKEN' // Useful for protected files } }); if (!response.ok) throw new Error('Download failed'); // 2. Convert response to a Blob const blob = await response.blob(); // 3. Create a temporary object URL const downloadUrl = window.URL.createObjectURL(blob); // 4. Create a hidden anchor element const link = document.createElement('a'); link.href = downloadUrl; link.download = fileName; // The name the file will be saved as // 5. Append, click, and cleanup document.body.appendChild(link); link.click(); // Cleanup: remove the link and revoke the URL to free memory document.body.removeChild(link); window.URL.revokeObjectURL(downloadUrl); } catch (error) { console.error('Error downloading file:', error); } } Use code with caution. Why Use Fetch + Blob Instead of a Direct Link?

: If your server requires a JSON payload to generate a report (e.g., a PDF export), fetch can handle the POST body.