How can I use chart tooltip formatter?

Hi, how can I use chart tooltip formatter?
I have config like this:

const CHART_CONFIG = {
    ...
    tooltip: {
        formatter: (tooltip) => {
            var s = '<b>' + this.x + '</b>';
            _.each(this.points, () => {
                s += '<br/>' + this.series.name + ': ' + this.y + 'm';
            });
            return s;
        },
        shared: true
    },
    ...
}

But I can’t access chart scope using this keyword and also I can’t get point from tooltip param.
Thanks

[keithdmoore]

I found the solution to this problem for those using Typescript. I thought I would share it for other Google Sleuthers

tooltip: {
   formatter: function() {
       var self: any = this;
       return self.point.category; 
   }               
},

[viotti]

I think it should be:

const CHART_CONFIG = {
    ...
    tooltip: {
        formatter() {

Notice that the function name was missing parentheses.