JavaScript的变量分配一个回调函数返回未定义

Question

里面我有一个简单的脚本来播放音频，如：

 var audio = new Audio(music) 
     var music_duration 
     audio.addEventListener('loadedmetadata', function() { 
      var duration = audio.duration 
      console.log(duration) 
      music_duration = duration 
     }) 

     console.log("checking duration") 
     console.log(music_duration) 

这里未定义music_duration回报..

而duration内addEventListener给出了音乐的持续时间。

我是javascript新手......这里有什么问题？被发射的等待loadedmetadata事件之前

Answer 1

console.log("checking duration") 
console.log(music_duration) 

将被执行。

您可以查看编辑脚本这样：

var audio = new Audio(music) 
    var music_duration 
    audio.addEventListener('loadedmetadata', function() { 
     console.log('here') 
     var duration = audio.duration 
     console.log(duration) 
     music_duration = duration 
    }) 
    console.log('there') 
    console.log("checking duration") 
    console.log(music_duration) 

你会看到there是here之前打印。