Gtk.Editable.prototype.insert_text

function insert_text(new_text:String, new_text_length:Number, position:Number):void {
    // Gjs wrapper for gtk_editable_insert_text()
}

Inserts new_text_length bytes of new_text into the contents of the widget, at position position.

Note that the position is in characters, not in bytes. The function updates position to point after the newly inserted text.