js: make use of implicit casts

Some methods like `setAttribute` are automatically casting their parameters to the designed types:

```js
e.setAttribute("tabindex","0")
```

can be simplified to

```js
e.setAttribute("tabindex",0)
```

As the [documentation says](https://developer.mozilla.org/en-US/docs/Web/API/Element/setAttribute):

> Any non-string value specified is converted automatically into a string.
