Question

下面是我的简单代码，它获取从A点到B点（地址数组）的距离。问题在于“alert(end[i]+distanceKM);”，尤其是变量distanceKM。

我认为我做对了。变量distanceKM在for循环之外声明，值在for循环中设置。我的警报在for循环中。

但我还是得到了

警报值：4700高速公路280未定义等等......

似乎distanceKM尚未设置，= undefined，我有：distanceKM = response.routes [0] .legs [0] .distance.value / 1000;

测试代码：

<html>
<head>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no"/>
    <title>Distance Calculator</title>
    <script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false"></script>

    <script type="text/javascript">
    var directionDisplay;
    var directionsService = new google.maps.DirectionsService();
    var map;

    function initialize() {
        directionsDisplay = new google.maps.DirectionsRenderer();
    }

    function calcRoute() {
        var start = document.getElementById("start").value;
        var end = document.getElementById("end").value.split('@@');
        var distanceKM;         
        for (i = 0; i < end.length; i++) 
        {
            var request = {
                origin:start, 
                destination:end[i],
                travelMode: google.maps.DirectionsTravelMode.DRIVING
            };

            directionsService.route(request, function(response, status) 
            {
                if (status == google.maps.DirectionsStatus.OK) 
                {
                    directionsDisplay.setDirections(response);
                    distanceKM=response.routes[0].legs[0].distance.value / 1000;
                }
            }); 
            alert(end[i]+distanceKM);   
        }

    }
    </script>
</head>
<body onload="initialize()">
    <div>
        <p>
            <label for="start">Start: </label>
            <input type="text" name="start" id="start" value="815010 NE 36th Steet, Redmond, WA, USA" />

            <label for="end">End: </label>
            <input type="text" name="end" id="end" value="4700 Highway 280, Birmingham, AL, USA@@6900 US Highway 90, Suite 2, Daphne, AL, USA@@549 Brookwood Village, Homewood, AL, USA@@4800 Whitesburg Dr., Huntsville, AL, USA" />

            <input type="submit" value="Calculate Route" onclick="calcRoute()" />
        </p>
        <p>
            <label for="distance">Distance (km): </label>
            <input type="text" name="distance" id="distance" readonly="true" />
        </p>
    </div>
</body>
</html>

Answer 1

路由请求不会同步返回结果，因此如果您尝试在请求之外立即访问distanceKM，它仍然是未定义的。

为了帮助您了解执行顺序，请参阅编号注释：

    // 1.) route request is sent, and the anonymous callback
    //     function is defined.
    directionsService.route(request, function(response, status) 
    {
        // 3.) Response from google is received and the anonymous 
        //     callback function is called.
        if (status == google.maps.DirectionsStatus.OK) 
        {
            directionsDisplay.setDirections(response);
            // 4.) distanceKM is set.
            distanceKM=response.routes[0].legs[0].distance.value / 1000;
        }
    });
    // 2.) Alert is shown
    alert(end[i]+distanceKM);

因此，为了让您的提醒工作，请在distanceKM=... / 1000;之后立即安排。您还需要对end [i]进行特殊处理。 e.g。

    directionsService.route(request, (function(destination){ 
        return function(response, status) {
            if (status == google.maps.DirectionsStatus.OK) {
                directionsDisplay.setDirections(response);
                distanceKM=response.routes[0].legs[0].distance.value / 1000;
                alert(destination+distanceKM);
            }
        };
    })(request.destination));

这使用Immediately-invoked function expression(IIFE)模式。

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