The appendMedium() method of the MediaList interface adds a media query to the list. If the media query is already in the collection, this method does nothing.

medium

A string containing the media query to add.

None (undefined).

The following adds the media query print into the MediaList associated with the first stylesheet applied to the current document.

const stylesheet = document.styleSheets[0];
stylesheet.media.appendMedium("print");