diff --git a/.eslintrc.json b/.eslintrc.json
index 4b988e7..d87bd03 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -146,7 +146,8 @@
 				"allowArrowFunctions": true
 			}
 		],
-		"function-paren-newline": ["error", "multiline"],
+		"function-paren-newline": ["error", "multiline-arguments"],
+		"comma-dangle": ["warn", "always-multiline"],
 		"key-spacing": "error",
 		"keyword-spacing": "error",
 		"lines-around-comment": "error",
diff --git a/handlers/commands/leave.js b/handlers/commands/leave.js
index 03f2703..ffe9b81 100644
--- a/handlers/commands/leave.js
+++ b/handlers/commands/leave.js
@@ -1,30 +1,29 @@
+// @ts-check
 'use strict';
 
+const { isMaster } = require('../../utils/config');
 const { managesGroup, removeGroup } = require('../../stores/group');
 
 /** @param { import('../../typings/context').ExtendedContext } ctx */
 const leaveCommandHandler = async ctx => {
-	const { chat, message, telegram, state, replyWithHTML } = ctx;
-	const { isMaster } = state;
-	if (!isMaster) return null;
+	const { chat, message, telegram, replyWithHTML } = ctx;
+	if (!isMaster(ctx.from)) return null;
 
 	const groupName = message.text.split(' ').slice(1).join(' ');
 
 	if (groupName) {
 		const group = /^-?\d+/.test(groupName)
 			? { id: Number(groupName) }
-			:	{ title: groupName };
+			: { title: groupName };
 		const isGroup = await managesGroup(group);
 		if (!isGroup) {
-			// eslint-disable-next-line function-paren-newline
 			return replyWithHTML(
 				'ℹ️ <b>Couldn\'t find a group with that ID/name.</b>'
-			// eslint-disable-next-line function-paren-newline
 			);
 		}
 		await Promise.all([
 			removeGroup(isGroup),
-			telegram.leaveChat(isGroup.id)
+			telegram.leaveChat(isGroup.id),
 		]);
 		return replyWithHTML('✅ <b>I no longer manage that group.</b>');
 	}
diff --git a/handlers/middlewares/addedToGroup.js b/handlers/middlewares/addedToGroup.js
index fd3945f..7864e95 100644
--- a/handlers/middlewares/addedToGroup.js
+++ b/handlers/middlewares/addedToGroup.js
@@ -1,3 +1,4 @@
+// @ts-check
 'use strict';
 
 // Bot
@@ -7,7 +8,7 @@ const { admin } = require('../../stores/user');
 const { addGroup } = require('../../stores/group');
 const { isMaster } = require('../../utils/config');
 
-/** @param { import('telegraf').ContextMessageUpdate } ctx */
+/** @param { import('telegraf').Context } ctx */
 const addedToGroupHandler = async (ctx, next) => {
 	const msg = ctx.message;
 
@@ -19,14 +20,14 @@ const addedToGroupHandler = async (ctx, next) => {
 			? `https://t.me/${ctx.chat.username.toLowerCase()}`
 			: await ctx.exportChatInviteLink().catch(() => '');
 		if (!link) {
-			// eslint-disable-next-line function-paren-newline
 			await ctx.replyWithHTML(
 				'⚠️ <b>Failed to export chat invite link.</b>\n' +
 				'Group won\'t be visible in /groups list.\n' +
 				'\n' +
 				'If this isn\'t your intention, ' +
 				'make sure I am permitted to export chat invite link, ' +
-				'and then run /showgroup.');
+				'and then run /showgroup.'
+			);
 		}
 		const { id, title, type } = ctx.chat;
 		await addGroup({ id, link, title, type });
diff --git a/tsconfig.json b/tsconfig.json
index ea8d250..1f93c71 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -5,6 +5,7 @@
 		"noImplicitAny": false,
 		"noEmit": true,
 		"resolveJsonModule": true,
+		"strictNullChecks": false,
 		"strict": true,
 		"target": "ES2020"
 	}