Javascript 以函数为参数的问题，求大神指点！

Question

 //由坐标获取地名
    function getAddress(lng,lat) {
        var point = new BMap.Point(lng,lat);
        var gc = new BMap.Geocoder(); 
        var address = null;
         gc.getLocation(point, function(rs) {
            var addComp = rs.addressComponents;
            address = addComp.province+addComp.city + addComp.district + addComp.street + addComp.streetNumber;
            alert(address);
        }); 
        alert(address);
    }

    gc.getLocation中的address有值，而外部的address却为null，为什么?百思不得其解.

Answer 1

异步的，没有执行address = addComp.province+a。。。这句都已经执行你最后的alert语句了，你要得到正确的地址得以回掉形式来调用

   function getAddress(lng,lat,callback) {
        var point = new BMap.Point(lng,lat);
        var gc = new BMap.Geocoder(); 
        var address = null;
         gc.getLocation(point, function(rs) {
            var addComp = rs.addressComponents;
            address = addComp.province+addComp.city + addComp.district + addComp.street + addComp.streetNumber;
            callback(address);//////////////
        }); 
        //alert(address);
    }

        getAddress('xxxxxxx','xxxxxxx',function(addr){
        alert(addr)
        })