feat(writeapi): added group joining and deletion

src/middleware/assert.js

@@ -0,0 +1,20 @@
+'use strict';
+
+/**
+ * The middlewares here strictly act to "assert" validity of the incoming
+ * payload and throw an error otherwise.
+ */
+
+const groups = require('../groups');
+
+module.exports = function (middleware) {
+	middleware.assertGroup = async (req, res, next) => {
+		const name = await groups.getGroupNameByGroupSlug(req.params.slug);
+		const exists = await groups.exists(name);
+		if (!exists) {
+			throw new Error('[[error:no-group]]');
+		}
+
+		next();
+	};
+};