How to register JavaScript in a back-office (in admin pages)?

In most cases to add any asset (JavaScript or CSS) to a back-office (admin pages) you should use the hook actionAdminControllerSetMedia(). Full steps to register correctly a JavaScript file are:

Step 1. Register the hook on a module install:

public function install()
{
    if (!parent::install()) {
        return false;
    }

    // After a module installation, register the hook
    if (!$this->registerHook('actionAdminControllerSetMedia')) {
        return false;
    }

    return true;
}

Step 2. Then, add your JavaScript asset:

public function hookActionAdminControllerSetMedia()
{
    // Adds jQuery and some it's dependencies for PrestaShop
    $this->context->controller->addJquery();

    // Adds your's JavaScript from a module's directory
    $this->context->controller->addJS($this->_path . 'views/js/example.js');
}

There are different ways and several methods that can be used to register assets in a back-office (admin pages) (they are listed in the order of the execution):

1. The hook hookDisplayBackOfficeHeader()
2. The controller's method AdminControllerCore::setMedia()
3. The hook actionAdminControllerSetMedia()
4. A module's method Module::getContent()
5. The hook hookDisplayBackOfficeFooter()

To add an inline code, the best way is use the hook hookDisplayBackOfficeFooter(). For example:

public function hookDisplayBackOfficeFooter()
{
    return '
        <script type="text/javascript">
            var EXAMPLE_VARIABLE = "Hello, Zapalm!";
        </script>
    ';
}

Yet another example for a case when you need to add a JavaScript asset but in a child class of AdminController of your module (for PrestaShop 1.7):

public function setMedia($isNewTheme = false)
{
    parent::setMedia($isNewTheme);

    $this->addCSS(_MODULE_DIR_ . $this->module->name . '/views/css/example.css');
    $this->addJS(_MODULE_DIR_ . $this->module->name . '/views/js/example.js');
}

For PrestaShop 1.6 and 1.5 you can do also like in this example but you need to remove $isNewTheme parameter from the method definition and the parent method call.

References:

1. Asset management in PrestaShop 1.7.

2. How to add an external asset to a front-office page in PrestaShop 1.7.

3. The explanation of which methods to use: addJS or registerJavascript.