once : parameter

Parameter Description
type(string) The event type to listen for; one of 'mousedown' , 'mouseup' , 'click' , 'dblclick' , 'mousemove' , 'mouseenter' , 'mouseleave' , 'mouseover' , 'mouseout' , 'contextmenu' , 'touchstart' , 'touchend' , or 'touchcancel' . mouseenter and mouseover events are triggered when the cursor enters a visible portion of the specified layer from outside that layer or outside the map canvas. mouseleave and mouseout events are triggered when the cursor leaves a visible portion of the specified layer, or leaves the map canvas.
listener(Function) The function to be called when the event is fired.
layerId(string) (optional) The ID of a style layer. Only events whose location is within a visible feature in this layer will trigger the listener. The event will have a features property containing an array of the matching features.