jQuery延迟并承诺 [英] Jquery deferred and promises

问题描述

我是jquery延迟和承诺的新手.我正在尝试这样做

I am new to jquery deferred and promises. I am trying to do this

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);
                                       });

我不断收到此错误: 未被捕获的TypeError:对象函数(a){return null！= a?n.extend(a，d):d}没有方法'done'

I keep getting this error: Uncaught TypeError: Object function (a){return null!=a?n.extend(a,d):d} has no method 'done'

有人可以帮忙吗?

推荐答案

Deferred.promise()是一种方法，您需要调用它并返回从.promise()

Deferred.promise() is a method, you need to invoke it and return the value returned from .promise()

所以

 return deferred.promise();

---

同样，不要使用闭包/全局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，例如

---

Again don't use closure/global objects to pass values from the async method to the callback. You can pass values to the done callback like

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:小提琴

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