getCentroid of Polygon

interface Point {
    readonly lat: number;
    readonly lng: number;
}

function getArea(points: ReadonlyArray<Point>): number {
    let area = 0,
        i,
        j,
        point1,
        point2;

    for (i = 0, j = points.length - 1; i < points.length; j=i,++i) {
        point1 = points[i];
        point2 = points[j];
        area += point1.lng * point2.lat
        area -= point1.lat * point2.lng;
    }
    area /= 2;

    return area;
}

function getCentroid(points: ReadonlyArray<Point>): Point {
    let x = 0,
        y = 0,
        i,
        j,
        f,
        point1,
        point2;

    for (i = 0, j = points.length - 1; i < points.length; j=i,++i) {
        point1 = points[i];
        point2 = points[j];
        f = point1.lng * point2.lat - point2.lng * point1.lat;
        x += (point1.lng + point2.lng) * f;
        y += (point1.lat + point2.lat) * f;
    }

    f = getArea(points) * 6;

    return {lng:x / f, lat: y / f};
}