JS判斷座標點是否在給定的多邊形內

(function (window) {
        "use strict";
        /**
         * 判斷給出的座標點是否在多邊形內
         * */
        function pointInPolygon(targetPoint, targetPoints) {
            var leftPointCount = 0;
            var rightPointCount = 0;//左右點的個數
            var _points = [];
            //第一步：取出全部的點，並計算交點座標
            for (var i = 0, _length = targetPoints.length - 1; i < _length; i++) {
                var p1 = targetPoints[i], p2 = targetPoints[i + 1];//取出當前點和當前點的下一個點
                var point = _calcCrossoverPoint(targetPoint, p1, p2);
                //若是交點有效，則保存
                if (point) {
                    _points.push(point);
                }
            }
//            第二步：計算給定的座標點，左右兩邊的交點個數，奇數在範圍內，偶數則不在
            for (var j = 0, length = _points.length; j < length; j++) {
                var x = _points[j];
                if (x === targetPoint.x) {
                    return false;//在線上，直接返回不在範圍內
                } else {
                    (targetPoint.x !== x && targetPoint.x > x) ? leftPointCount++ : rightPointCount++;
                }
            }
            //判斷交點個數
            return (leftPointCount % 2 !== 0 && rightPointCount % 2 !== 0);
        }

        /**
         * 算交點座標，座標點在掃描行上或者上方時，交點無效
         * @param targetPoint
         * @param startPoint
         * @param endPoint
         * @returns {*}
         */
        function _calcCrossoverPoint(targetPoint, startPoint, endPoint) {
            var crossoverPointX = startPoint.x - ((startPoint.y - targetPoint.y) * (startPoint.x - endPoint.x) / (startPoint.y - endPoint.y));
//            判斷交點座標是否有效，即交點在startPoint，endPoint構成的線段範圍內
            if ((startPoint.y < targetPoint.y && endPoint.y >= targetPoint.y) || (endPoint.y < targetPoint.y && startPoint.y >= targetPoint.y)) {
                if ((crossoverPointX >= startPoint.x && crossoverPointX <= endPoint.x) || (crossoverPointX <= startPoint.x && crossoverPointX >= endPoint.x)) {
                    return crossoverPointX;
                } else {
                    return false;
                }
            } else {
                return false;
            }

        }

        var isIn = pointInPolygon({x: 4, y: 4}, [{x: 2, y: 1}, {x: 1, y: 3}, {x: 3, y: 5}, {x: 5, y: 4}, {
            x: 4,
            y: 1
        }, {x: 3, y: 2}, {
            x: 2,
            y: 1
        }]);
        console.log(isIn);
    })(window);