### once

Ensures a function is called only once.

Utilizing a closure, use a flag, `called`, and set it to `true` once the function is called for the first time, preventing it from being called again. 
Allow the function to be supplied with an arbitrary number of arguments using the spread (`...`) operator.

```js
const once = fn => {
  let called = false;
  return (...args) => {
    if (!called) {
      fn(...args);
      called = true;
    }
  };
};
```

```js
const startApp = event => {
  // initializes the app
  console.log(event); // access to any arguments supplied
};
document.addEventListener('click', once(startApp)); // only runs `startApp` once upon click
```