Javascript Javascript中嵌套函数的返回值

Return value from nested function in Javascript

提问by Chris Armstrong

I have a function that is set up as follows

我有一个功能设置如下

function mainFunction() {
      function subFunction() {
            var str = "foo";
            return str;
      }
}

var test = mainFunction();
alert(test);

To my logic, that alert should return 'foo', but instead it returns undefined. What am I doing wrong?

按照我的逻辑，该警报应该返回“foo”，但它返回未定义。我究竟做错了什么？

UPDATE: Here's my actual code (it's a function for reverse-geocoding with the Google API)

更新：这是我的实际代码（它是使用 Google API 进行反向地理编码的函数）

function reverseGeocode(latitude,longitude){
    var address = "";
    var country = "";
    var countrycode = "";
    var locality = "";

    var geocoder = new GClientGeocoder();
    var latlng = new GLatLng(latitude, longitude);

     return geocoder.getLocations(latlng, function(addresses) {
     address = addresses.Placemark[0].address;
     country = addresses.Placemark[0].AddressDetails.Country.CountryName;
     countrycode = addresses.Placemark[0].AddressDetails.Country.CountryNameCode;
     locality = addresses.Placemark[0].AddressDetails.Country.AdministrativeArea.SubAdministrativeArea.Locality.LocalityName;
     return country;
    });   
   }

回答by z33m

you have to call a function before it can return anything.

您必须先调用函数才能返回任何内容。

function mainFunction() {
      function subFunction() {
            var str = "foo";
            return str;
      }
      return subFunction();
}

var test = mainFunction();
alert(test);

Or:

或者：

function mainFunction() {
      function subFunction() {
            var str = "foo";
            return str;
      }
      return subFunction;
}

var test = mainFunction();
alert( test() );

for your actual code. The return should be outside, in the main function. The callback is called somewhere inside the getLocationsmethod and hence its return value is not recieved inside your main function.

对于您的实际代码。返回应该在外部，在主函数中。回调在getLocations方法内部的某处被调用，因此它的返回值不会在您的主函数中收到。

function reverseGeocode(latitude,longitude){
    var address = "";
    var country = "";
    var countrycode = "";
    var locality = "";

    var geocoder = new GClientGeocoder();
    var latlng = new GLatLng(latitude, longitude);

    geocoder.getLocations(latlng, function(addresses) {
     address = addresses.Placemark[0].address;
     country = addresses.Placemark[0].AddressDetails.Country.CountryName;
     countrycode = addresses.Placemark[0].AddressDetails.Country.CountryNameCode;
     locality = addresses.Placemark[0].AddressDetails.Country.AdministrativeArea.SubAdministrativeArea.Locality.LocalityName;
    });   
    return country
   }

回答by Mark Bessey

Right. The function you pass to getLocations() won't get called until the data is available, so returning "country" before it's been set isn't going to help you.

对。您传递给 getLocations() 的函数在数据可用之前不会被调用，因此在设置之前返回“国家/地区”对您没有帮助。

The way you need to do this is to have the function that you pass to geocoder.getLocations() actually do whatever it is you wanted done with the returned values.

您需要这样做的方法是让您传递给 geocoder.getLocations() 的函数实际上执行您想要对返回值执行的任何操作。

Something like this:

像这样的东西：

function reverseGeocode(latitude,longitude){
  var geocoder = new GClientGeocoder();
  var latlng = new GLatLng(latitude, longitude);

  geocoder.getLocations(latlng, function(addresses) {
    var address = addresses.Placemark[0].address;
    var country = addresses.Placemark[0].AddressDetails.Country.CountryName;
    var countrycode = addresses.Placemark[0].AddressDetails.Country.CountryNameCode;
    var locality = addresses.Placemark[0].AddressDetails.Country.AdministrativeArea.SubAdministrativeArea.Locality.LocalityName;
    do_something_with_address(address, country, countrycode, locality);
  });   
}

function do_something_with_address(address, country, countrycode, locality) {
  if (country==="USA") {
     alert("USA A-OK!"); // or whatever
  }
}

---

---

If you might want to do something different every time you get the location, then pass the function as an additional parameter to reverseGeocode:

如果您可能想在每次获取位置时做一些不同的事情，那么将该函数作为附加参数传递给 reverseGeocode：

function reverseGeocode(latitude,longitude, callback){
  // Function contents the same as above, then
  callback(address, country, countrycode, locality);
}
reverseGeocode(latitude, longitude, do_something_with_address);

If this looks a little messy, then you could take a look at something like the Deferred feature in Dojo, which makes the chaining between functions a little clearer.

如果这看起来有点乱，那么您可以看看 Dojo 中的 Deferred 特性之类的东西，它使函数之间的链接更加清晰。

回答by efwjames

Just FYI, Geocoder is asynchronous so the accepted answer while logical doesn't really work in this instance. I would prefer to have an outside object that acts as your updater.

仅供参考，地理编码器是异步的，因此在这种情况下，逻辑上的公认答案实际上不起作用。我希望有一个外部对象作为您的更新程序。

var updater = {};

function geoCodeCity(goocoord) { 
    var geocoder = new google.maps.Geocoder();
    geocoder.geocode({
        'latLng': goocoord
    }, function(results, status) {
        if (status == google.maps.GeocoderStatus.OK) {
            updater.currentLocation = results[1].formatted_address;
        } else {
            if (status == "ERROR") { 
                    console.log(status);
                }
        }
    });
};