javascript开启网页录制功能ES6语法 async

var body = document.body;    
body.addEventListener("click",async function(){        
    var stream = await navigator.mediaDevices.getDisplayMedia({video: true});         
    var mime = MediaRecorder.isTypeSupported("video/webm; codecs=vp9") ?"video/webm; codecs=vp9" :"video/webm";        
    var mediaRecorder = new MediaRecorder(stream, {mimeType: mime});         
    //录制        
     var chunks = [];        
      mediaRecorder.addEventListener('dataavailable', function(e) {chunks.push(e.data)})         
       //停止        
       mediaRecorder.addEventListener('stop', function(){            
           var blob = new Blob(chunks, {type: chunks[0].type});            
           var url = URL.createObjectURL(blob);            
           var a = document.createElement('a');            
           a.href = url;            
           a.download = 'video.webm';            
           a.click();       
       })       
    //手动启动       
    mediaRecorder.start()   
});