Next, learn how to use JavaScript to create a temporary downloadable URL for Azure Blob Storage.js

Generating Temporary URLs for Blob Downloads in Azure with Next.js

Front-end JavaScript solution using the Azure SDK and Blob Object URLs

// Import the Azure SDK and setup the blockBlobClient  
import { BlobServiceClient } from "@azure/storage-blob";  
const blobServiceClient = BlobServiceClient.fromConnectionString(process.env.AZURE_STORAGE_CONNECTION_STRING);  
const containerClient = blobServiceClient.getContainerClient("my-container");  
const blockBlobClient = containerClient.getBlockBlobClient("example-image.png");  

// Function to generate temporary downloadable URL from blob  
async function generateBlobDownloadURL() {  
  try {  
    const response = await blockBlobClient.download();  
    const blobData = await response.blobBody; // Retrieve the blob body  
    const tempUrl = URL.createObjectURL(blobData); // Create an object URL  
    console.log("Temporary URL:", tempUrl); // Log for testing  
    return tempUrl;  
  } catch (error) {  
    console.error("Error generating download URL:", error);  
    return null;  
  }  
}  

// Usage in React component within Next.js  
export default function BlobDownloader() {  
  const handleDownload = async () => {  
    const url = await generateBlobDownloadURL();  
    if (url) window.open(url, "_blank"); // Open URL in new tab  
  };  

  return (  
    <button onClick={handleDownload}>Download Image</button>  
  );  
}  

Handling Blob Data Download with Error Management

Back-end Node.js approach using Streams for efficient memory usage

// Import necessary Azure SDK modules  
const { BlobServiceClient } = require("@azure/storage-blob");  
const express = require("express");  
const app = express();  
const PORT = process.env.PORT || 3000;  

// Initialize Azure Blob Service Client  
const blobServiceClient = BlobServiceClient.fromConnectionString(process.env.AZURE_STORAGE_CONNECTION_STRING);  

app.get("/download", async (req, res) => {  
  try {  
    const containerClient = blobServiceClient.getContainerClient("my-container");  
    const blockBlobClient = containerClient.getBlockBlobClient("example-image.png");  

    // Stream the blob content to the response  
    const downloadBlockBlobResponse = await blockBlobClient.download();  
    downloadBlockBlobResponse.readableStreamBody.pipe(res);  
  } catch (error) {  
    console.error("Error downloading blob:", error);  
    res.status(500).send("Failed to download blob");  
  }  
});  

// Start Express server  
app.listen(PORT, () => {  
  console.log(`Server is running on port ${PORT}`);  
});  

Unit Tests for Blob Download Functionality

Unit testing using Jest to ensure correct download behavior

// Test for front-end blob download function using Jest  
import { generateBlobDownloadURL } from "../components/BlobDownloader";  

describe("generateBlobDownloadURL", () => {  
  test("should return a valid object URL", async () => {  
    const url = await generateBlobDownloadURL();  
    expect(url).toMatch(/^blob:/);  
  });  

  test("should handle errors gracefully", async () => {  
    jest.spyOn(console, "error").mockImplementation(() => {});  
    const url = await generateBlobDownloadURL();  
    expect(url).toBeNull();  
  });  
});  

// Test for back-end stream handling using Jest and Supertest  
const request = require("supertest");  
const app = require("../server"); // Assuming the server script is named server.js  

describe("GET /download", () => {  
  it("should return 200 and stream the blob content", async () => {  
    const response = await request(app).get("/download");  
    expect(response.status).toBe(200);  
  });  

  it("should return 500 on error", async () => {  
    jest.spyOn(console, "error").mockImplementation(() => {});  
    const response = await request(app).get("/download");  
    expect(response.status).toBe(500);  
  });  
});  

Common Questions and Solutions for Blob URL Issues in Next.js

1. Why is my blob URL not displaying the correct image?
2. Ensure you are using URL.createObjectURL() on the correct blob object and that the blob's content-type is set correctly in Azure Blob Storage.
3. How can I revoke a blob URL to prevent memory leaks?
4. Use URL.revokeObjectURL() after you are done with the blob to free up memory and avoid stale references.
5. Is it possible to secure blob URLs with expiration?
6. Yes, using Azure’s Shared Access Signatures (SAS), you can create URLs that expire after a certain time, providing secure access control.
7. What should I do if blob URLs don't work on mobile browsers?
8. Implement fallbacks such as redirecting using window.location.href or prompting users to manually save the file if blob URLs are not supported.
9. How do I manage large file downloads efficiently in Node.js?
10. Use readableStreamBody.pipe() to stream the content directly to the response, which prevents memory overload and ensures smooth file transfers.
11. Can I download files from Azure Blob Storage without using temporary URLs?
12. Yes, you can set up a backend route with Express and stream the blob content directly to the client using blockBlobClient.download().
13. Why is my blob download returning corrupted data?
14. Check if the blob’s encoding and content-type are correctly configured in Azure. Also, ensure the response body is correctly parsed using response.blobBody.
15. What is the best way to test blob downloads?
16. Use Jest and Supertest to simulate download requests and validate that your download logic works correctly across various conditions.
17. Can blob URLs be reused multiple times?
18. Yes, but keep in mind that browser sessions may cache these URLs. Use URL.revokeObjectURL() to release memory and avoid issues.
19. How do I open a blob URL in a new tab?
20. Use window.open() with the blob URL to open it in a new tab. Ensure the browser settings allow pop-ups if this doesn't work.
21. How do I display the blob content inline instead of downloading it?
22. Set the appropriate content-disposition header in Azure Blob Storage to display the file inline instead of forcing a download.

Key Takeaways from Managing Blob Downloads:

Handling blob downloads efficiently in a Next.js app involves converting binary data into temporary URLs using methods like URL.createObjectURL(). Proper memory management, such as revoking object URLs, is crucial to avoid leaks and performance issues.

Security is another key consideration, as temporary URLs can be shared. Implementing SAS tokens adds time-limited access control. Additionally, ensuring browser compatibility and providing fallbacks for devices that don't support blob URLs ensures an optimal user experience.