箭頭不是畫的線段，是貼的圖示，再按方向旋轉一下。

程式碼：

//新增箭頭線
function addLineDirection(polylinePointArr, source, target) {
    var lineDirection = {};

    var polyline1 = L.polyline(polylinePointArr, { stroke: true, color: "#009922", opacity: 0.3, weight: 14, pane: drawPaneBelow });
    vectorsLayer.addLayer(polyline1);
    lineDirection.polyline1 
 
= polyline1;
    var polyline2 = L.polyline(polylinePointArr, { stroke: true, color: "#009922", weight: 8, opacity: 0.6, pane: drawPaneBelow });
    vectorsLayer.addLayer(polyline2);
    lineDirection.polyline2 = polyline2;

    var distanceSum = 0;
    var count = 0;
    for (var i = 0; i < polylinePointArr.length - 1; i++) {
        point1 
 
= polylinePointArr[i];
        point2 = polylinePointArr[i + 1];
        var angle = calcLineAngle(point1, point2);

        var distance = Math.pow(Math.pow(point2[1] - point1[1], 2) + Math.pow(point2[0] - point1[0], 2), 0.5);
        distanceSum += distance;
        var arrowCount = Math.floor(distanceSum / 0.01) - count;
        count 
 
+= arrowCount;

        for (var j = 1; j <= arrowCount; j++) {
            var lng = point1[1] + (point2[1] - point1[1]) * ((j - 0.2) / arrowCount);
            var lat = point1[0] + (point2[0] - point1[0]) * ((j - 0.2) / arrowCount);

            var width = 14;
            var height = 14;
            var html = '<img src="images/arrow.png" style="width:' + width + 'px; height:' + width + 'px; transform:rotate(' + angle + 'deg) scale(0.6); " />';

            var icon = L.divIcon({
                html: html,
                className: 'draw-vectors-label',
                iconSize: [width, height],
                iconAnchor: [width / 2.0, height / 2.0]
            });

            lineDirection.marker = addMarker(icon, lng, lat);
            lineDirection.source = source;
            lineDirection.target = target;
            lineDirectionArr.push(lineDirection);
        }
    }

}

//計算箭頭圖示角度
function calcLineAngle(point1, point2) {
    var h = point2[0] - point1[0];
    var w = point2[1] - point1[1];

    var angle = Math.atan(Math.abs(h) / Math.abs(w)) * 180 / Math.PI;
    if (w == 0) {
        if (h > 0) {
            return -90;
        }
        if (h < 0) {
            return 90;
        }
    }
    if (h == 0) {
        if (w > 0) {
            return 0;
        }
        if (w < 0) {
            return 180;
        }
    }
    if (h > 0) {
        if (w > 0) {
            return 0 - angle;
        }
        if (w < 0) {
            return angle - 180;
        }
    }
    if (h < 0) {
        if (w > 0) {
            return angle;
        }
        if (w < 0) {
            return 180 - angle;
        }
    }

    return 0;
}

效果圖：