From the Microsoft HTC Reference🕗:
For Microsoft Internet Explorer 5 and
later, HTML Components (HTCs) provide
a mechanism to implement components in
script as Dynamic HTML (DHTML)
behaviors. An HTC is an HTML file that
contains script and a set of
HTC-specific elements that define the
component. The component is saved with
an .htc extension. This section lists
these HTC-specific elements and the
members they support.
Therefore, they are not just javascript .js files, they also contain a set of HTC-specific elements.
Edit:
Concerning the conversion from .htc to .js, I’m not sure if it can be done directly, but I’ve come across this set of .js tools to address the same issues that .htc files are trying to fix in IE.