add Allow header to server responses with status code 405

Furthermore, a response to requests using the OPTIONS method is
implemented.

changelog.md

@@ -10,6 +10,17 @@ Since [Plotly.js](https://plotly.com/javascript/) is the main dependency of this
 application, major version changes in Plotly.js will also trigger a major
 version change in this application.
 
+## Version 4.5.4 (2023-11-30)
+
+* __[improvement]__
+Whenever the server responds to an unallowed method with HTTP status code 405,
+it will now also send an `Allow` HTTP header to indicate the allowed methods.
+This behaviour is required by [RFC 9110](https://httpwg.org/specs/rfc9110.html#status.405),
+so it should be implemented.
+
+Furthermore, any HTTP request to the server root using the OPTIONS method will
+now return the allowed options via an `Allow` HTTP header.
+
 ## Version 4.5.3 (2023-11-29)
 
 * __[bug fix]__

export-server/package.json

@@ -1,6 +1,6 @@
 {
   "name": "plotly-node-export-server",
-  "version": "4.5.3",
+  "version": "4.5.4",
   "description": "Plotly.js Node.js export server",
   "repository": {
     "url": "https://gitlab.com/striezel/plotly-node-export-server.git",

export-server/server.js

@@ -54,10 +54,17 @@ const server = http.createServer(function(req, res) {
 
   // ---- Handle plot generation requests ----
 
+  if (req.method === 'OPTIONS') {
+    res.statusCode = 204; // 204 == No content
+    res.setHeader('Allow', 'POST');
+    res.end();
+    return;
+  }
   // Only post request shall be allowed.
   if (req.method !== 'POST') {
     res.statusCode = 405; // 405 == Method not allowed
     res.setHeader('Content-Type', 'text/plain');
+    res.setHeader('Allow', 'POST');
     res.end('Only POST requests are allowed!\n');
     return;
   }