socket.io: Factor out client connection logic

src/static/js/socketio.js

@@ -0,0 +1,29 @@
+'use strict';
+
+/**
+ * Creates a socket.io connection.
+ * @param etherpadBaseUrl - Etherpad URL. If relative, it is assumed to be relative to
+ *     window.location.
+ * @param namespace - socket.io namespace.
+ * @param options - socket.io client options. See
+ *     https://socket.io/docs/v2/client-api/#new-Manager-url-options
+ * @return socket.io Socket object
+ */
+const connect = (etherpadBaseUrl, namespace = '/', options = {}) => {
+  // The API for socket.io's io() function is awkward. The documentation says that the first
+  // argument is a URL, but it is not the URL of the socket.io endpoint. The URL's path part is used
+  // as the name of the socket.io namespace to join, and the rest of the URL (including query
+  // parameters, if present) is combined with the `path` option (which defaults to '/socket.io', but
+  // is overridden here to allow users to host Etherpad at something like '/etherpad') to get the
+  // URL of the socket.io endpoint.
+  const baseUrl = new URL(etherpadBaseUrl, window.location);
+  const socketioUrl = new URL('socket.io', baseUrl);
+  const namespaceUrl = new URL(namespace, new URL('/', baseUrl));
+  return io(namespaceUrl.href, Object.assign({path: socketioUrl.pathname}, options));
+};
+
+if (typeof exports === 'object') {
+  exports.connect = connect;
+} else {
+  window.socketio = {connect};
+}