I correctly inserted a javascript file into the wordPress admin with :

```
 wp_enqueue_script()

```

I know that the script is loading. I tested it with an `alert()`.
I then found that I am unable to access the DOM. When I tried :

```
document.getElementsByTagName('body')[0].addEventListener('load', function() {});

```

This error was:

```
Uncaught TypeError: Cannot call method 'addEventListener' of undefined

```

This is the first time I have used 'settings api' or inserted scripts into the WordPress admin.