Add comments to stream example

api/server.js

@@ -31,15 +31,27 @@ const storeUpload = async upload => {
 
   // Store the file in the filesystem.
   await new Promise((resolve, reject) => {
-    stream
+    // Create a stream to which the upload will be written.
-      .on('error', error => {
+    const writeStream = createWriteStream(path)
+
+    // When the upload is fully written, resolve the promise.
+    writeStream.on('finish', resolve)
+
+    // If there's an error writing the file, remove the partially written file
+    // and reject the promise.
+    writeStream.on('error', error => {
       unlink(path, () => {
         reject(error)
       })
     })
-      .pipe(createWriteStream(path))
+
-      .on('error', reject)
+    // In node <= 13, errors are not automatically propagated between piped
-      .on('finish', resolve)
+    // streams. If there is an error receiving the upload, destroy the write
+    // stream with the corresponding error.
+    stream.on('error', error => writeStream.destroy(error))
+
+    // Pipe the upload into the write stream.
+    stream.pipe(writeStream)
   })
 
   // Record the file metadata in the DB.