Attempting to update component value inside the mouseOver event handler:

import * as Highcharts from 'highcharts';

export class AssetDetailComponent {

  highcharts = Highcharts;
  highchartsTooltip: string = '';
  chartOptions: Highcharts.Options = {};

  constructor() {
    this.chartOptions = {
      series: [{
        point: {
          events: { mouseOver: this.onChartMouseOver }
        }
      }]
    };
  }

  onChartMouseOver(event: any) { this.highchartsTooltip = `1234`; };

}

However, the component value isn't being updated and trying to use a component method instead throws a not a function error. Unfortunately, not finding very much documentation on this subject - All help is greatly appreciated. Thx!