add lint check

.eslintrc.json

 {
+  "parser": "@typescript-eslint/parser",
+  "extends": ["plugin:prettier/recommended"],
+  "plugins": ["@typescript-eslint"],
   "rules": {
-    "eqeqeq": 0
+    "eqeqeq": 0,
+    "no-var": "error",
+    "@typescript-eslint/consistent-type-definitions": [
+      "error",
+      "interface"
+    ]
   }
 }

.gitlab-ci.yml

 stages:
+  - lint
   - build
   - test
   - deploy
 
-variables:
-  GIT_DEPTH: "1"
+before_script:
+  - npm ci
+
+npm_lint:
+  stage: lint
+  tags:
+    - linux
+  script: npm run lint
 
 npm_build:
   stage: build
   tags:
     - linux
-  script:
-    - npm ci
-    - npm run build
+  script: npm run build
 
 # npm_test:
 #   stage: test

package.json

@@ -21,13 +21,13 @@
     "react-scripts": "^5.0.1",
     "socket.io-client": "^4.5.1",
     "three": "^0.145.0",
-    "typescript": "^4.7.4",
     "web-vitals": "^2.1.4"
   },
   "scripts": {
     "dev": "vite",
     "build": "tsc && vite build",
     "preview": "vite preview",
+    "lint": "eslint --ext .ts --ext .tsx src",
     "test": "react-scripts test",
     "eject": "react-scripts eject"
   },
@@ -52,7 +52,14 @@
   "devDependencies": {
     "@types/google-protobuf": "^3.15.6",
     "@types/three": "^0.144.0",
+    "@typescript-eslint/eslint-plugin": "^5.40.1",
+    "@typescript-eslint/parser": "^5.40.1",
     "@vitejs/plugin-react": "^2.1.0",
+    "eslint": "^8.26.0",
+    "eslint-config-prettier": "^8.5.0",
+    "eslint-plugin-prettier": "^4.2.1",
+    "prettier": "^2.7.1",
+    "typescript": "^4.8.4",
     "vite": "^3.1.0"
   }
 }

src/reducers/playerSlice.ts

 import { createSlice, PayloadAction } from "@reduxjs/toolkit";
 import { RootState } from "../store";
 
-export type Player = {
+export interface Player {
   name?: string;
   state?: string;
   isHost?: boolean;
-};
+}
 
 export interface playerState {
   player0: Player;