Typescript and Array.map with thisArg results in an error

In Javascript, I can pass a thisArg into the Array.map function as a second argument. Here is a minimal example.

const a = [1,2,3,4,5,6];
a.map(function(e) { this.sum += e; return this.sum; }, {sum: 0});

The result is the array [1, 3, 6, 10, 15, 21]. When I try the same in Typescript, I get the following error.

'this' implicitly has type 'any' because it does not have a type annotation. ts(2683)

How can I annotate the code so that this inside the callback has the correct type?

I have tried giving everything a type annotation like this,

const a: number[] = [1,2,3,4,5,6];
const S: {sum: number} = {sum: 0};
a.map(function(e: number): number { this.sum += e; return this.sum; }, S);

But this doesn’t get rid of the error because none of these annotations specifies the type of this explicitly.