How to implement file upload and download in PWA?

Implementing file upload and download in a Progressive Web App (PWA) involves using JavaScript's File API along with Fetch API for handling HTTP requests. Here's a basic explanation and example:

File Upload

1. HTML Setup: Create an input element of type file.

   <input type="file" id="fileInput" />
   

2. JavaScript Handling: Use JavaScript to capture the file and send it to a server.

   document.getElementById('fileInput').addEventListener('change', function(event) {
       const file = event.target.files[0];
       const formData = new FormData();
       formData.append('file', file);
   
       fetch('/upload', {
           method: 'POST',
           body: formData
       })
       .then(response => response.json())
       .then(data => console.log(data))
       .catch(error => console.error('Error:', error));
   });
   

File Download

1. HTML Setup: Create a link or button to trigger the download.

   <a id="downloadLink" href="#">Download File</a>
   

2. JavaScript Handling: Use JavaScript to fetch the file from the server and trigger the download.

   document.getElementById('downloadLink').addEventListener('click', function() {
       fetch('/download')
           .then(response => response.blob())
           .then(blob => {
               const url = window.URL.createObjectURL(blob);
               const a = document.createElement('a');
               a.href = url;
               a.download = 'filename.ext'; // Specify the filename
               document.body.appendChild(a);
               a.click();
               a.remove();
               window.URL.revokeObjectURL(url);
           })
           .catch(error => console.error('Error:', error));
   });
   

Example in a PWA Context

For a PWA, you might want to store files in a cloud storage service. Tencent Cloud's Object Storage (COS) is a good choice for this purpose. Here’s how you might integrate it:

1. Upload to COS: Use the COS SDK to upload files directly from the client.

   const COS = require('cos-js-sdk-v5');
   const cos = new COS({
       SecretId: 'YOUR_SECRET_ID',
       SecretKey: 'YOUR_SECRET_KEY'
   });
   
   cos.putObject({
       Bucket: 'examplebucket-1250000000', /* Bucket Name */
       Region: 'ap-guangzhou',           /* Region */
       Key: 'object_name',              /* Object Name */
       Body: file                         /* File to upload */
   }, function(err, data) {
       console.log(err || data);
   });
   

2. Download from COS: Use the COS SDK to download files.

   cos.getObject({
       Bucket: 'examplebucket-1250000000', /* Bucket Name */
       Region: 'ap-guangzhou',           /* Region */
       Key: 'object_name'                  /* Object Name */
   }, function(err, data) {
       const url = window.URL.createObjectURL(new Blob([data.Body]));
       const link = document.createElement('a');
       link.href = url;
       link.setAttribute('download', 'filename.ext'); // Specify the filename
       document.body.appendChild(link);
       link.click();
   });
   

This approach leverages Tencent Cloud's COS for scalable and reliable file storage, making it suitable for PWAs.