Question

我是jquery deferred和promises的新手。我正在努力做到这一点

var maxRes=function()
                {
                    var deferred=$.Deferred();
                      $("<img/>").attr("src", newurl).load(function(){
                             s = {w:this.width, h:this.height};
                             imgHeight =  this.height ;  
                             deferred.resolve();


                          }); 
                    return deferred.promise;
                }

                    maxRes().done(function(){


             if(imgHeight >=720)
                 {
                temp="...[HD]"
                 }
             else
                 {
                 temp = "...";
                 }
            console.log(temp);
                                       });

我一直收到这个错误：未捕获的TypeError：对象函数（a）{return null！= a？n.extend（a，d）：d}没有方法'done'

请帮忙吗？

Answer 1

Deferred.promise()是一个方法，您需要调用它并返回从.promise()返回的值

所以

 return deferred.promise();

再次不要使用closure / global对象将值从异步方法传递给回调。您可以将值传递给完成的回调，如

var newurl = '//placehold.it/256';
var maxRes = function () {
    var deferred = $.Deferred();
    $("<img/>").attr("src", newurl).load(function () {
        var s = {
            w: this.width,
            h: this.height
        };
        //pass the dimensions are arguments to the done callback
        deferred.resolve(s);
    });
    return deferred.promise();
}

maxRes().done(function (s) {
    //recieve the dimension as a parameter and use it instead of the shared variable

    //declare temp as a local variable instead of messing up the global scope
    var temp;
    console.log(s)

    if (s.height >= 720) {
        temp = "...[HD]"
    } else {
        temp = "...";
    }
    console.log(temp);
});

演示：Fiddle

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