Point class

const Point = function(x, y){
    this.x = x;
    this.y = y;
}

Line class

const Line = function(from, to){
    this.from = from;
    this.to = to;
}

Circle

const Circle = function(x, y, r){
    this.cx = x;
    this.cy = y;
    this.r = r;
}

Geometry

const Geometry = function(){
    return {
        Line: {
            intersect: function(l1, l2){
                let ax = l1.from.x, ay = l1.from.y;
                let bx = l1.to.x, by = l1.to.y;
                let cx = l2.from.x, cy = l2.from.y;
                let dx = l2.to.x, dy = l2.to.y;
                var denominator = (by-ay) * (dx-cx) - (ax-bx) * (cy-dy);
                if (denominator == 0) {
                    return false;
                }
                var x = +((bx-ax) * (dx-cx) * (cy-ay)
                    + (by-ay) * (dx-cx) * ax
                    - (dy-cy) * (bx-ax) * cx ) / denominator;
                var y = -((by-ay) * (dy-cy) * (cx-ax)
                    + (bx-ax) * (dy-cy) * ay
                    - (dx-cx) * (by-ay) * cy ) / denominator;
                if ((x-ax) * (x-bx) <= 0 && (y-ay) * (y-by) <= 0 && (x-cx) * (x-dx) <= 0 && (y-cy) * (y-dy) <= 0){
                    return new Point(x, y);
                }
                return false;
            },
            middle: function(line){
                let p0 = line.from, p1 = line.to;
                let x0 = p0.x, y0 = p0.y;
                let x1 = p1.x, y1 = p1.y;
                return new Point((x0+x1)/2, (y0+y1)/2);
            }
        },
        Circle: {
            tengent: function(c1, c2, D=50){
                let x1 = c1.cx, y1 = c1.cy;
                let x2 = c2.cx, y2 = c2.cy;
                let r1 = c1.r, r2 = c2.r;
                let dx = x1-x2, dy = y1-y2;
                let d = Math.abs(Math.sqrt(dx*dx + dy*dy) - (r1+r2));
                let x0, y0;
                if( d < D) {
                    x0 = x1 + (x2-x1) * (r1/(r1+r2));
                    y0 = y1 + (y2-y1) * (r1/(r1+r2));
                    return new Point(x0, y0);
                }
                return false;
            },
            intersect: function(o1, o2){
                let x1 = o1.cx, y1 = o1.cy;
                let x2 = o2.cx, y2 = o2.cy;
                let r1 = o1.r, r2 = o2.r;
                let L = Geometry.distance(x1, y1, x2, y2);
                //console.log('L: %d', L);
                let k1 = (y2-y1)/(x2-x1), k2 = -1/k1;
                //console.log(k1, k2);
                let sum = r1 + r2, diff = Math.abs(r1 - r2);
                if (L > sum) {
                    return 1;
                }
                else if(L < sum) {
                    if(L < diff){
                        return -1;
                    }
                    else{
                        //http://blog.csdn.net/zx3517288/article/details/53326420
                        let d = L;
                        let a = (r1*r1 - r2*r2 + d*d) / (2*d);
                        //console.log(d, a);
                        let x0 = x1+a*(x2-x1)/d;
                        let y0 = y1+a*(y2-y1)/d;
                        let h = Math.sqrt(Math.abs(r1*r1 - a*a));
                        //console.log(h);
                        let x3 = x0 - h*(y2-y1)/d, y3 = y0 + h*(x2-x1)/d;
                        let x4 = x0 + h*(y2-y1)/d, y4 = y0 - h*(x2-x1)/d;
                        return [new Point(x3, y3), new Point(x4, y4)];
                    }
                }
                else{
                    //console.log('d = (r1 + r2)');
                    return 0;
                }
            }
        },
        distance: function(x0, y0, x1, y1){
            let dx = (x1-x0), dy = (y1-y0);
            return Math.sqrt(dx*dx + dy*dy);
        }
    }
}();