fix: failures of preview-links in Node.js runtime

* Only send one response in all cases.
* Process card click events before matched URL to avoid unreachable code.

node/preview-link/attach-card.js

@@ -26,6 +26,7 @@ exports.onMessage = (req, res) => {
   if (req.method === 'GET' || !req.body.message) {
     res.send(
       'Hello! This function is meant to be used in a Google Chat Space.');
+    return;
   }
 
   // Checks for the presence of event.message.matchedUrl and attaches a card
@@ -97,6 +98,7 @@ exports.onMessage = (req, res) => {
         },
       ],
     });
+    return;
   }
 
   // If the Chat app doesn’t detect a link preview URL pattern, it says so.

node/preview-link/preview-link.js

@@ -26,12 +26,7 @@ exports.onMessage = (req, res) => {
   if (req.method === 'GET' || !req.body.message) {
     res.send(
       'Hello! This function is meant to be used in a Google Chat Space.');
-  }
-
-  // Checks for the presence of event.message.matchedUrl and attaches a card
-  // if present
-  if (req.body.message.matchedUrl) {
-    res.json(createMessage());
+    return;
   }
 
   // Respond to button clicks on attached cards
@@ -45,9 +40,17 @@ exports.onMessage = (req, res) => {
 
     if (req.body.action.actionMethodName === 'assign') {
       res.json(createMessage(actionResponseType, 'You'));
+      return;
     }
   }
 
+  // Checks for the presence of event.message.matchedUrl and attaches a card
+  // if present
+  if (req.body.message.matchedUrl) {
+    res.json(createMessage());
+    return;
+  }
+
   // If the Chat app doesn’t detect a link preview URL pattern, it says so.
   res.json({'text': 'No matchedUrl detected.'});
 };

node/preview-link/simple-text-message.js

@@ -26,6 +26,7 @@ exports.onMessage = (req, res) => {
   if (req.method === 'GET' || !req.body.message) {
     res.send(
       'Hello! This function is meant to be used in a Google Chat Space.');
+    return;
   }
 
   // Checks for the presence of event.message.matchedUrl and responds with a
@@ -35,6 +36,7 @@ exports.onMessage = (req, res) => {
       'text': 'req.body.message.matchedUrl.url: ' +
         req.body.message.matchedUrl.url,
     });
+    return;
   }
 
   // If the Chat app doesn’t detect a link preview URL pattern, it says so.

node/preview-link/update-card.js

@@ -26,6 +26,7 @@ exports.onMessage = (req, res) => {
   if (req.method === 'GET' || !req.body.message) {
     res.send(
       'Hello! This function is meant to be used in a Google Chat Space.');
+    return;
   }
 
   // Respond to button clicks on attached cards