Gtk.Editable.prototype.select_region

function select_region(start_pos:Number, end_pos:Number):void {
    // Gjs wrapper for gtk_editable_select_region()
}

Selects a region of text. The characters that are selected are those characters at positions from start_pos up to, but not including end_pos. If end_pos is negative, then the characters selected are those characters from start_pos to the end of the text.

Note that positions are specified in characters, not bytes.