Make setTopic() remove the topic if newTopic is falsy (#367)

* Make setTopic() remove the topic if newTopic is falsy

* add clearTopic function

* Improve documentation for setTopic

docs/clientapi.md

@@ -109,7 +109,10 @@ Join a channel, optionally with a key/password.
 Part/leave a channel with an optional parting message.
 
 ##### `.setTopic(channel, newTopic)`
-Set the topic of a channel
+Set the topic of a channel, if newTopic is falsy or only whitespace then `.clearTopic()` will be called.
+
+##### `.clearTopic(channel)`
+Remove the topic of a channel.
 
 ##### `.ctcpRequest(target, type [, paramN])`
 Send a CTCP request to target with any number of parameters.

src/client.js

@@ -611,9 +611,23 @@ module.exports = class IrcClient extends EventEmitter {
     }
 
     setTopic(channel, newTopic) {
+        if (!newTopic || !newTopic.trim()) {
+            // If newTopic is undefined or empty, remove the existing topic
+            // this check is to prevent unexpectedly requesting the current topic
+            // when trying to clear the topic
+            this.clearTopic(channel);
+            return;
+        }
+
         this.raw('TOPIC', channel, newTopic);
     }
 
+    clearTopic(channel) {
+        // The trailing `:` is required otherwise it would be requesting the topic
+        // and not clearing it
+        this.raw(`TOPIC ${channel} :`);
+    }
+
     ctcpRequest(target, type /*, paramN */) {
         const params = Array.prototype.slice.call(arguments, 1);