Target: get the file length before the audio is played. 
 Terminal: ios Wechat version 6.7.1 
 phenomenon: the canplay event will only be triggered when the user manually clicks the play button. 
related codes
  var audio = document.getElementById("audio");
  audio.addEventListener("canplay", function(e) {
     console.log(audio.duration);
  });
? What are we going to do about it?
