changed to handle 404 during download, also removing progress listener once percent completion is 100 to avoid manipulating DOM twice

frontend/src/download.js

@@ -15,6 +15,8 @@ let download = () => {
     progress.innerText = `Progress: ${percentComplete}%`;
 
     if (percentComplete === 100) {
+      fileReceiver.removeAllListeners('progress');
+
       let finished = document.createElement('p');
       finished.innerText = 'Your download has finished.';
       li.appendChild(finished);
@@ -28,7 +30,13 @@ let download = () => {
     }
   });
 
-  fileReceiver.download().then(([decrypted, fname]) => {
+  fileReceiver.download()
+  .catch((err) => {
+    console.log('The file has expired, or has already been deleted.');
+    document.getElementById('downloaded_files').removeChild(li);
+    return;
+  })
+  .then(([decrypted, fname]) => {
     name.innerText = fname;
     let dataView = new DataView(decrypted);
     let blob = new Blob([dataView]);

frontend/src/fileReceiver.js

@@ -20,6 +20,11 @@ class FileReceiver extends EventEmitter {
         };
 
         xhr.onload = function(e) {
+
+          if (xhr.status === 404) {
+            reject(new Error('The file has expired, or has already been deleted.'));
+          }
+
           let blob = new Blob([this.response]);
           let fileReader = new FileReader();
           fileReader.onload = function() {
@@ -52,7 +57,8 @@ class FileReceiver extends EventEmitter {
         true,
         ['encrypt', 'decrypt']
       )
-    ]).then(([fdata, key]) => {
+    ])
+    .then(([fdata, key]) => {
       let salt = this.salt;
       return Promise.all([
         window.crypto.subtle.decrypt(