How to use types with typescript on highcharts.js

I've read the docs regarding typescript on Highcharts but I still don't have it very clear how to use types in highcharts.js functions.

For example, I have a formatter function on my tooltip options and it admits a parameter this. How do I type it?

Right now I'm using any but that's just to bypass the typescript errors. I don't want to manually type it in case Highcharts updates its content in future versions, so I assume there's a proper way of doing it?

const options = {
    tooltip: {
      outside: true,
      formatter: function (this: any): string {  // <---- need to replace `any` here
      }
    }
} 

What type should I use there? And how do I find out exactly which of them should I use?

I've tried using Highcharts.TooltipFormatterCallbackFunction as that's what seems to be indicated in the callback docs.

formatter: function (this: Highcharts.TooltipFormatterCallbackFunction): string {

But it doesn't seem to recognize the property y inside the this object. this.y. Perhaps that's the return type? Where do I look for the one for this?

Same problem with plotOptions.column.point.events.mouseOver:

plotOptions: {
    column: {
        point: {
            events: {
                mouseOver: function (this: any) { //<-- here
                    this.graphic.attr({
                       fill: this.y < 0 ? "red" : "blue",
                    });
                }
            }
        }
    }
}