如何从JavaScript中的嵌套函数处理程序返回？

Question

我有这段代码：

function fetchSocialCount(type,fileSrc){
    var count = {likes: 0, dislikes: 0};
    var req = new XMLHttpRequest();
    req.onload = function(){
        if(req.status === 200 && req.readyState === 4){
            var countResponse = json_encode(req.responseText);
            count.likes = countResponse.likes;
            count.dislikes = countResponse.dislikes;
            return count;
        }
    }
}

所以，我想从count函数表达式返回req.onload，就好像它是从fetchSocialCount函数返回的一样。我怎样才能做到这一点？

Answer 1

req.onload是异步的。您需要使用回调，承诺或类似的东西来获得结果“计数”

function fetchSocialCount(type,fileSrc, cb){
    var count = {likes: 0, dislikes: 0};
    var req = new XMLHttpRequest();
    req.onload = function(){
        if(req.status === 200 && req.readyState === 4){
            var countResponse = json_encode(req.responseText);
            count.likes = countResponse.likes;
            count.dislikes = countResponse.dislikes;
            cb(count);
        }
    }
}

//calling fetchSocialCount
fetchSocialCount(my_type, my_file_src, function(count){
    //here it is
    console.log(count);
});