why is it null

    function handleMousemove(vmap){
        var self = this;

        self.event = null;

        self.highlight = function(e){
            console.log(self.event);
            if (self.event != e){
                return;
            }

            var features = vmap.queryRenderedFeatures(e.point, {
                layers: ['regions']
            });

            // Change the cursor style as a UI indicator.
            vmap.getCanvas().style.cursor = features.length ? 'pointer' : '';

            // Remove things if no feature was found.
            if (!features.length) {
                return;
            }
            var feature = features[0];
            vmap.setFilter(
                'regions-highlight',
                ['==', 'id', feature.properties.id]
            );
        }

        self.handler = function(e){
            self.event = e;
            setTimeout(function(){
                self.highlight(e);
            }, 1000);
        }

        return self.handler;
    }
    
    map.on('mousemove', handleMousemove(map));