fix(connection): avoid unhandled error on createConnection() if on('error') handler registered

Fix #14377

Author: vkarpov15

lib/connection.js

@@ -829,6 +829,30 @@ Connection.prototype.openUri = async function openUri(uri, options) {
   return this;
 };
 
+/*!
+ * Treat `on('error')` handlers as handling the initialConnection promise
+ * to avoid uncaught exceptions when using `on('error')`. See gh-14377.
+ */
+
+Connection.prototype.on = function on(event, callback) {
+  if (event === 'error' && this.$initialConnection) {
+    this.$initialConnection.catch(() => {});
+  }
+  return EventEmitter.prototype.on.call(this, event, callback);
+};
+
+/*!
+ * Treat `once('error')` handlers as handling the initialConnection promise
+ * to avoid uncaught exceptions when using `on('error')`. See gh-14377.
+ */
+
+Connection.prototype.once = function on(event, callback) {
+  if (event === 'error' && this.$initialConnection) {
+    this.$initialConnection.catch(() => {});
+  }
+  return EventEmitter.prototype.once.call(this, event, callback);
+};
+
 /*!
  * ignore
  */

test/connection.test.js

@@ -916,6 +916,21 @@ describe('connections:', function() {
     assert.equal(err.name, 'MongooseServerSelectionError');
   });
 
+  it('avoids unhandled error on createConnection() if error handler registered (gh-14377)', async function() {
+    const opts = {
+      serverSelectionTimeoutMS: 100
+    };
+    const uri = 'mongodb://baddomain:27017/test';
+
+    const conn = mongoose.createConnection(uri, opts);
+    await new Promise(resolve => {
+      conn.on('error', err => {
+        assert.equal(err.name, 'MongoServerSelectionError');
+        resolve();
+      });
+    });
+  });
+
   it('`watch()` on a whole collection (gh-8425)', async function() {
     this.timeout(10000);
     if (!process.env.REPLICA_SET) {