loading external modules from FileSystem access API

service_worker.js

@@ -13,18 +13,51 @@ self.addEventListener('activate', function(event) {
 });
 */
 
-let data
+let dir_handle
 
 self.addEventListener('message', async function(e) {
   const msg = e.data
   let reply
   if(msg.type == 'SET') {
-    data = msg.data
+    dir_handle = msg.data
     reply = null
   } else if(msg.type == 'GET') {
-    reply = data
+    reply = dir_handle
   } else {
     throw new Error('unknown message type: ' + msg.type)
   }
   e.ports[0].postMessage(reply)
 })
+
+// Fake URL base prepended by code responsible for module loading
+const FAKE_URL_BASE = 'https://leporello.import/'
+
+self.addEventListener("fetch", event => {
+  if(event.request.url.startsWith(FAKE_URL_BASE)) {
+    if(dir_handle != null) {
+      const headers = new Headers([
+        ['Content-Type', 'text/javascript']
+      ])
+      const path = event.request.url.replace(FAKE_URL_BASE, '')
+      const response = read_file(dir_handle, path).then(file => 
+        new Response(file, {headers})
+      )
+      event.respondWith(response)
+    }
+  }
+})
+
+const read_file = async (dir_handle, filename) => {
+  if(typeof(filename) == 'string') {
+    filename = filename.split('/')
+  }
+  const [first, ...rest] = filename
+  if(rest.length == 0) {
+    const fhandle = await dir_handle.getFileHandle(first)
+    const file_data = await fhandle.getFile()
+    return await file_data.text()
+  } else {
+    const nested_dir_handle = await dir_handle.getDirectoryHandle(first)
+    return read_file(nested_dir_handle, rest)
+  }
+}