javascript - 如何让 jquery 在执行另一个函数之前等待一个函数完成？

Question

function test(){
var distance=null;
       first();
       second();
       third();
alert(distance);//it shows null always because it take 2 second to complete.

}
 function first(tolat, tolon, fromlat,fromlon){

// calulating road distance between two points on the map using any other distance caluculating apis. 

distance=dis;  // update the value of distance but it takes 2 second to complete.

}
 function second(){}
 function third(){}

我的代码中有这种情况，现在在第一次和第二次完成执行之前调用了很多时间函数 3，并且距离值没有更新。

Answer 1

使用回调：

function first(tolat, tolon, fromlat, fromlon, callback) {
     if (typeof(callback) == 'function') {
        callback(distance);
     }
}

function second() { }
function third() { }

first("vartolat", "vartolon", "varfromlat", "varfromlon", function(distance) {
    alert(distance);
    second();
    third();
});