Closes #54

- New option for matrix-send-message node that allows replacing existing message if enabled and `msg.eventId` is passed in.

src/matrix-send-message.html

@@ -12,6 +12,7 @@
             roomId: { value: null },
             messageType: { value: 'm.text' },
             messageFormat: { value: '' },
+            replaceMessage : { value: false }
         },
         label: function() {
             return this.name || "Send Message";
@@ -60,6 +61,17 @@
             <option value="msg.format">msg.format input</option>
         </select>
     </div>
+
+    <div class="form-row">
+        <input
+                type="checkbox"
+                id="node-input-replaceMessage"
+                style="width: auto; margin-left: 105px; vertical-align: top"
+        />
+        <label for="node-input-replaceMessage" style="width: auto;max-width:50%;">
+            Update existing message if <code>msg.eventId</code> is set
+        </label>
+    </div>
 </script>
 
 <script type="text/html" data-help-name="matrix-send-message">
@@ -78,10 +90,15 @@
         </dt>
         <dd> the message text. </dd>
 
+        <dt>msg.replace
+            <span class="property-type">bool</span>
+        </dt>
+        <dd> If true and <code>msg.eventId</code> is present it will update an existing message. Posts a new message if false or <code>msg.eventId</code> is missing. </dd>
+
         <dt class="optional">msg.formatted_payload
             <span class="property-type">string</span>
         </dt>
-        <dd> the formatted HTML message (uses msg.payload if not defined). This only affects HTML messages.</dd>
+        <dd> the formatted HTML message (uses <code>msg.payload</code> if not defined). This only affects HTML messages.</dd>
 
         <dt class="optional">msg.type
             <span class="property-type">string | null</span>

src/matrix-send-message.js

@@ -1,3 +1,5 @@
+const {RelationType} = require("matrix-js-sdk");
+
 module.exports = function(RED) {
     function MatrixSendImage(n) {
         RED.nodes.createNode(this, n);
@@ -9,6 +11,7 @@ module.exports = function(RED) {
         this.roomId = n.roomId;
         this.messageType = n.messageType;
         this.messageFormat = n.messageFormat;
+        this.replaceMessage = n.replaceMessage;
 
         // taken from https://github.com/matrix-org/synapse/blob/master/synapse/push/mailer.py
         this.allowedTags = [
@@ -116,6 +119,25 @@ module.exports = function(RED) {
                         : msg.payload.toString();
             }
 
+            if((node.replaceMessage || msg.replace) && msg.eventId) {
+                content['m.new_content'] = {
+                    msgtype: content.msgtype,
+                    body: content.body
+                };
+                if('format' in content) {
+                    content['m.new_content']['format'] = content['format'];
+                }
+                if('formatted_body' in content) {
+                    content['m.new_content']['formatted_body'] = content['formatted_body'];
+                }
+
+                content['m.relates_to'] = {
+                    rel_type: RelationType.Replace,
+                    event_id: msg.eventId
+                };
+                content['body'] = ' * ' + content['body'];
+            }
+
             node.server.matrixClient.sendMessage(msg.topic, content)
                 .then(function(e) {
                     node.log("Message sent: " + msg.payload);