linesIntersect.js

/**
     * checks if 2 lines intersect TESTED
     * coincidentReturn: what to return if lines are coincident (exactly overlaping)
     * parallelReturn: what to return if lines are exactly parallel
     */
    linesIntersect =  function (a1, a2, b1, b2, coincidentReturn, parallelReturn){

        var result,
        coincidentReturn = coincidentReturn || false,
        parallelReturn = parallelReturn || false,
        ua_t = (b2.x - b1.x) * (a1.y - b1.y) - (b2.y - b1.y) * (a1.x - b1.x),
        ub_t = (a2.x - a1.x) * (a1.y - b1.y) - (a2.y - a1.y) * (a1.x - b1.x),
        u_b  = (b2.y - b1.y) * (a2.x - a1.x) - (b2.x - b1.x) * (a2.y - a1.y);

        if ( u_b != 0 ) {
            var ua = ua_t / u_b;
            var ub = ub_t / u_b;

            if ( 0 <= ua && ua <= 1 && 0 <= ub && ub <= 1 ) {
                //Intersection
                return true;
            } else {
                //No Intersection
                return false;
            }
        } else {
            if ( ua_t == 0 || ub_t == 0 ) {
                //Coincident
                return coincidentReturn;
            } else {
                //Parallel
                return parallelReturn;
            }
        }
        return result;
    }

pointInPolygon.js

/**
     * checks if a point is inside of a concave or convex polygon TESTED
     * polygon: array of {x:1, y: 1} objects
     */
    pointInPolygon = function (point, polygon){
        var i,
            j,
            c = 0,
            pointX = point.x,
            pointY = point.y;
            
        for (i = 0, j = polygon.length - 1, len = polygon.length; i < len; j = i++){
            
            if (((polygon[i].y > pointY) != (polygon[j].y > pointY)) && (pointX < (polygon[j].x - polygon[i].x) * (pointY - polygon[i].y) / (polygon[j].y - polygon[i].y) + polygon[i].x)){
                c = !c;
            }
        }
        return c;
    }