[TYPO3] How to include javascript in page header from plugin code?

[Tapio Markula]

Mauro Lorenzutti wrote:
> Hi Chetan,
> 
> 
>>I would like to know how one can include javascript in page header from
>>plugin code.

How to put if I want to XCLASS some existing class.
Does

$GLOBALS['TSFE']->setJS($this->extKey, $js);
$GLOBALS['TSFE']->additionalHeaderData[] = '<script
type="text/javascript" src="pathtojsfile.js"></script>';
work there too and where to put these declarations?