# Babel
- JavaScript Compiler
- https://babeljs.io
- ES2015 and beyond
- JSX and React
- Type Annotations (Flow and TypeScript)
- Pluggable
- Debuggable
- Spec Compliant
- Compact

```js
// Babel Input: ES2015 arrow function
[1, 2, 3].map((n) => n + 1);

// Babel Output: ES5 equivalent
[1, 2, 3].map(function(n) {
  return n + 1;
});
```

## install

```
npm init -y
npm install --save-dev @babel/core @babel/cli @babel/preset-env
npm install @babel/polyfill
```

- `babel.config.js`

```js
const presets = [
  [
    "@babel/env",
    {
      targets: {
        edge: "17",
        firefox: "60",
        chrome: "67",
        safari: "11.1",
      },
      useBuiltIns: "usage",
    },
  ],
];

module.exports = { presets };
```

## run

```
./node_modules/.bin/babel src --out-dir lib
```

## babel-jest

- jest command
  - `npm i -g jest`

- package.json
  - `npm i -D @babel/cli @babel/core @babel/preset-env babel-jest`

```json
  "devDependencies": {
    "@babel/cli": "^7.8.4",
    "@babel/core": "^7.8.4",
    "@babel/preset-env": "^7.8.4",
    "babel-jest": "^25.1.0"
  }
```

- babel.config.js

```js
module.exports = {
  "presets": ["@babel/preset-env"]
}
```