docs: retained message config example

examples/client/both-with-retain.js

@@ -0,0 +1,36 @@
+'use strict'
+
+var mqtt = require('../..')
+
+// Subscribe to a custom server with a URL string
+var client = mqtt.connect('mqtt://test.mosquitto.org:1883')
+
+client.subscribe('presence-retained', { qos: 1 })
+client.on('message', function (topic, message) {
+  console.log(`Received on '${topic}': ${message.toString()}`)
+})
+
+/* Sends the message 'hello!' to the topic 'presence'. The message will be delivered
+ * at least once to clients even if they subscribe later.
+ * If compiling TypeScript, the options object must include the qos property
+ * to be recognized as an instance of IClientPublishOptions (and not the callback).
+ * Remaining properties are optional.
+ */
+client.on('connect', function (topic, message) {
+  console.log('Connected')
+  client.publish('presence-retained', 'hello!', {qos: 1, retain: true},
+    (err) => {
+      if (err) {
+        console.log('There was an error sending the message.')
+        return
+      }
+      console.log('QoS handling completed')
+    }
+  )
+
+  // Clear the retained message when no longer needed:
+  setTimeout(() => {
+    client.publish('presence-retained', '', {qos: 0, retain: true})
+    client.end() // Exits this example
+  }, 5000)
+})