### isBrowser

Determines if the current runtime environment is a browser so that front-end modules can run on the server (Node) without throwing errors.

Use `Array.prototype.includes()` on the `typeof` values of both `window` and `document` (globals usually only available in a browser environment unless they were explicitly defined), which will return `true` if one of them is `undefined`.
`typeof` allows globals to be checked for existence without throwing a `ReferenceError`.
If both of them are not `undefined`, then the current environment is assumed to be a browser.

```js
const isBrowser = () => ![typeof window, typeof document].includes('undefined');
```

```js
isBrowser(); // true (browser)
isBrowser(); // false (Node)
```
