Centroid of a polygon

centroid.js

export function default (polygon) {
    let cx = 0, cy = 0, a = 0;

    for (let i  = 0, l = polygon.length; i < l; i++) {
        let x0 = polygon[i].x, 
            x1 = polygon[(i + 1) % l].x,
            y0 = polygon[i].y, 
            y1 = polygon[(i + 1) % l].y;

        let m = (x0 * y1 - x1 * y0);

        cx += (x0 + x1) * m;
        cy += (y0 + y1) * m;

        a += m;
    }

    cx /= 3 * a;
    cy /= 3 * a;

    return {
        x: cx,
        y: cy
    }
}