✅ add basic unit tests

package.json

@@ -10,12 +10,14 @@
     "lint:js": "eslint --ext js,jsx,ts,tsx ./ --max-warnings=0",
     "cypress:open": "cypress open",
     "cypress:run": "cypress run",
+    "jest:start": "jest --watchAll",
     "start": "cd playground && node_modules/.bin/next start"
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^5.11.2",
     "@testing-library/react": "^10.4.8",
     "@testing-library/user-event": "^12.1.0",
+    "@types/jest": "^26.0.9",
     "@typescript-eslint/eslint-plugin": "^3.8.0",
     "@typescript-eslint/parser": "^3.8.0",
     "babel-eslint": "^10.1.0",
@@ -32,11 +34,13 @@
     "eslint-plugin-react-hooks": "^4.0.4",
     "eslint-plugin-simple-import-sort": "^5.0.3",
     "husky": "^4.2.5",
+    "jest": "^26.3.0",
     "lint-staged": "^10.2.9",
     "prettier": "^2.0.5",
     "stylelint": "^13.6.0",
     "stylelint-config-standard": "^20.0.0",
     "stylelint-order": "^4.1.0",
+    "ts-jest": "^26.1.4",
     "typescript": "^3.9.7"
   }
 }

playground/src/components/Counter/Counter.tsx

@@ -3,22 +3,24 @@ import React, { useState } from 'react';
 import { jsx } from '@emotion/core';
 import tw from 'twin.macro';
 
+import { add, subtract } from 'utils';
+
 const Counter = (): JSX.Element => {
   const [count, setCount] = useState(0);
 
   return (
     <div id="counter" tw="flex items-center justify-center w-full h-screen">
       <button
         type="button"
-        onClick={() => setCount(i => i - 1)}
+        onClick={() => setCount(i => subtract(i, 1))}
         tw="bg-blue-500 text-white rounded py-2 px-4"
       >
         -
       </button>
       <span tw="px-4 font-bold text-2xl">{count}</span>
       <button
         type="button"
-        onClick={() => setCount(i => i + 1)}
+        onClick={() => setCount(i => add(i, 1))}
         tw="bg-blue-500 text-white rounded py-2 px-4"
       >
         +

playground/tsconfig.json

@@ -5,7 +5,6 @@
   ],
   "compilerOptions": {
     "baseUrl": "src",
-    "types": [],
   },
   "include": [
     "next-env.d.ts",

tsconfig.json

@@ -18,7 +18,7 @@
     "skipLibCheck": true,
     "strict": true,
     "target": "esnext",
-    "types": ["cypress"],
+    "types": ["jest"],
   },
   "exclude": ["node_modules"],
   "include": [