Question

我正在尝试从单独的函数获取var newtotal的值，然后通过Ajax将值传递给PHP脚本。

这是我的代码：`

function customerCost(){

        var newtotal;

        var start = document.getElementById('source').value;
        var end = document.getElementById('destination').value;
        var via = document.getElementById('via').value;

        if(via != ""){
        var waypts = [];

        waypts.push({
            location:via,
            stopover:false});

        var newrequest = {
          origin: start,
          destination: end,
          waypoints: waypts,
          travelMode: google.maps.DirectionsTravelMode.DRIVING
        };
        directionsService.route(newrequest, function(newresponse, newstatus) {
          if (newstatus == google.maps.DirectionsStatus.OK) {

            var newtotal = 0;
            var mynewroute = newresponse.routes[0];
             for (i = 0; i < mynewroute.legs.length; i++) {
                 newtotal += mynewroute.legs[i].distance.value;
                }
                            return newtotal;
            }

        });

        }else{

            var newrequest = {
          origin: start,
          destination: end,
          travelMode: google.maps.DirectionsTravelMode.DRIVING
        };
        directionsService.route(newrequest, function(newresponse, newstatus) {
          if (newstatus == google.maps.DirectionsStatus.OK) {

            var newtotal = 0;
            var mynewroute = newresponse.routes[0];
             for (i = 0; i < mynewroute.legs.length; i++) {
                 newtotal += mynewroute.legs[i].distance.value;
                }
                return newtotal;
            }

        });

        }



    }

然后我在一个单独的函数中使用以下函数调用该值：

customertotal = customerCost();

我只收到“未定义”错误。但是，如果我将return newtotal;替换为alert(newtotal);，则会输出正确的值。

我哪里错了？

Answer 1

directionsService.route()调用是异步的。这意味着当你调用它时，它只会启动操作，你的customerCost()函数会继续并在.route() ajax调用完成之前很久就结束并调用它的回调。

您正尝试使用异步操作以同步方式进行编程。你不能这样做。您的customerCost()函数调用无法返回newtotal值，因为在函数返回时尚未知道它。

相反，您需要学习如何以异步方式编程。任何需要newtotal值的代码都需要位于该值可用的回调中，或者您可以从那里调用函数并将newtotal值传递给它。

开发类似这样的东西的典型方法可能是你自己的回调：

function customerCost(cb){

        var newtotal;

        var start = document.getElementById('source').value;
        var end = document.getElementById('destination').value;
        var via = document.getElementById('via').value;

        if(via != ""){
        var waypts = [];

        waypts.push({
            location:via,
            stopover:false});

        var newrequest = {
          origin: start,
          destination: end,
          waypoints: waypts,
          travelMode: google.maps.DirectionsTravelMode.DRIVING
        };
        directionsService.route(newrequest, function(newresponse, newstatus) {
          if (newstatus == google.maps.DirectionsStatus.OK) {

            var newtotal = 0;
            var mynewroute = newresponse.routes[0];
             for (i = 0; i < mynewroute.legs.length; i++) {
                 newtotal += mynewroute.legs[i].distance.value;
              }
              // call the callback and pass it the newtotal
              cb(newtotal);
            }

        });

        }else{

            var newrequest = {
          origin: start,
          destination: end,
          travelMode: google.maps.DirectionsTravelMode.DRIVING
        };
        directionsService.route(newrequest, function(newresponse, newstatus) {
          if (newstatus == google.maps.DirectionsStatus.OK) {

            var newtotal = 0;
            var mynewroute = newresponse.routes[0];
             for (i = 0; i < mynewroute.legs.length; i++) {
                 newtotal += mynewroute.legs[i].distance.value;
                }
                // call the callback and pass it the newtotal
                cb(newtotal);
            }

        });

    }
}


// call the asynchronous cost function and process the result
// in a callback function
customerCost(function(newtotal) {
    // the newtotal value is available here
});

Answer 2

你应该使用回调函数来处理异步函数中的变量。

您可以添加回调函数作为参数：

customerCost(callback){

  // your original code
}

然后将所有行return newtotal替换为callback(newtotal);

然后调用函数customerCost，如：

customerCost(function(customertotal){
  // work with customertotal
});

从函数返回值并在单独的函数中放入变量

2 个答案: