function nanoid(size = 21) {
  let id = '';
  // A compact alternative for `for (var i = 0; i < step; i++)`.
  let i = size;
  while (i--) {
    // `| 0` is more compact and faster than `Math.floor()`.
    id += urlAlphabet[(Math.random() * 64) | 0];
  }
  return id;
}
Generate URL-friendly unique ID. This method uses the non-secure
predictable random generator with bigger collision probability.
Based on https://github.com/ai/nanoid

```js
model.id = nanoid() //=> "Uakgb_J5m9g-0JDMbcJqL"
```

@param size Size of the ID. The default size is 21.
@returns A random string.