Add CORS support and improve error responses in fetch handler

src/index.js

@@ -14,15 +14,24 @@ export default {
 	async fetch(request, env, ctx) {
 		const url = new URL(request.url);
 		const rawKey = url.pathname.slice(1); // remove leading slash
+		const corsHeaders = {
+            'Access-Control-Allow-Origin': request.headers.get('Origin'),
+            'Access-Control-Allow-Methods': '*',
+            'Access-Control-Allow-Headers': '*',
+        };
 		
 		if (url.hostname !== '127.0.0.1' && url.protocol === 'http:') {
 			return new Response(null, { status: 301, headers: {
 				Location: url.toString().replace(/^(http:)/, 'https:')
 			}});
 		}
+
+		if (request.method === 'OPTIONS') {
+			return new Response(null, { status: 200, headers: corsHeaders });
+		}
 		
 		if (!rawKey) {
-			return new Response('Invalid key', { status: 400 });
+			return new Response('Invalid key', { status: 400, headers: corsHeaders });
 		}
 		
 		let endpoint, accessKeyId, secretAccessKey, region, bucket;
@@ -57,7 +66,7 @@ export default {
 		const pat = /^[a-zA-Z0-9]{1,64}$/;
 		if (!expId || !participantId || expId.includes('/') || !participantId.match(pat)) {
 			// prevent path traversal
-			return new Response('Invalid expId or participantId', { status: 400 });
+			return new Response('Invalid parameters', { status: 400, headers: corsHeaders });
 		}
 		
 		const client = new S3Client({
@@ -73,6 +82,6 @@ export default {
 			Key: `${expId}/${participantId}/${rawKey}`
 		});
   		const signedUrl = await getSignedUrl(client, command, { expiresIn: 12*60*60 });
-		return new Response(signedUrl);
+		return new Response(signedUrl, { headers: corsHeaders });
 	},
 };