stache.registerHelper(name, helper)

Registers a helper function. Pass the name of the helper followed by the function to which stache should invoke. See Helpers for more details on using helpers and addHelper to avoid converting computes;

stache.registerHelper( "upper", function( str ) {
    if ( str.isComputed ) {
        str = str();
    }
    return str.toUpperCase();
} );