add: unit and integration testing

app/node_modules/babel-preset-jest/README.md

@@ -0,0 +1,33 @@
+# babel-preset-jest
+
+> Babel preset for all Jest plugins. This preset is automatically included when using [babel-jest](https://github.com/jestjs/jest/tree/main/packages/babel-jest).
+
+## Install
+
+```sh
+$ npm install --save-dev babel-preset-jest
+```
+
+## Usage
+
+### Via `babel.config.js` (Recommended)
+
+```js
+module.exports = {
+  presets: ['jest'],
+};
+```
+
+### Via CLI
+
+```sh
+$ babel script.js --presets jest
+```
+
+### Via Node API
+
+```javascript
+require('@babel/core').transform('code', {
+  presets: ['jest'],
+});
+```