JavaScript的變量分配一個回調函數返回未定義

Question

裏面我有一個簡單的腳本來播放音頻，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) 

這裏未定義music_duration回報..

而duration內addEventListener給出了音樂的持續時間。

我是javascript新手......這裏有什麼問題？被髮射的等待loadedmetadata事件之前

Answer 1

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

將被執行。

您可以查看編輯腳本這樣：

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) 

你會看到there是here之前打印。