JavaScript - 未捕获的ReferenceError：谷歌没有定义？

Question

我复制了this question的代码，尝试获取两点的路线。显示了点的标记和方向。但是，我在控制台Uncaught ReferenceError: google is not defined中收到错误，只是想知道如何摆脱它。

以下是我从上述链接问题的接受答案中获得的JavaScript代码。

function initMap() {
    var pointA = new google.maps.LatLng(51.7519, -1.2578),
        pointB = new google.maps.LatLng(50.8429, -0.1313),
        myOptions = {
            zoom: 7,
            center: pointA
        },
        map = new google.maps.Map(document.getElementById('map-canvas'), myOptions),
        // Instantiate a directions service.
        directionsService = new google.maps.DirectionsService,
        directionsDisplay = new google.maps.DirectionsRenderer({
            map: map
        }),
        markerA = new google.maps.Marker({
            position: pointA,
            title: "point A",
            label: "A",
            map: map
        }),
        markerB = new google.maps.Marker({
            position: pointB,
            title: "point B",
            label: "B",
            map: map
        });

    // get route from A to B
    calculateAndDisplayRoute(directionsService, directionsDisplay, pointA, pointB);

}

function calculateAndDisplayRoute(directionsService, directionsDisplay, pointA, pointB) {
    directionsService.route({
        origin: pointA,
        destination: pointB,
        travelMode: google.maps.TravelMode.DRIVING
    }, function (response, status) {
        if (status == google.maps.DirectionsStatus.OK) {
            directionsDisplay.setDirections(response);
        } else {
            window.alert('Directions request failed due to ' + status);
        }
    });
}

initMap();

错误屏幕截图：

谢谢

Answer 1

Google async defer标记上的script正在异步（非即时）加载Google脚本。

当您的脚本正在加载并立即执行时。因此，您的脚本甚至在加载到文档之前就引用了Google API。

删除async defer会在您之前正常加载Google脚本，因此您可以确定它可用。