js：静态文件转换成base64

背景：上传图片需要是base64格式
代码：第一次转换得到base64为空值

 var imgFile = new Image()
      imgFile.src="./ceshi.jpeg"
      //转base64
      var canvas = document.createElement( "canvas" );
      canvas.width = imgFile.width;
      canvas.height = imgFile.height;
      var ctx = canvas.getContext( "2d" );
      ctx.drawImage(imgFile, 0 , 0 , imgFile.width, imgFile.height);
      var ext = imgFile.src.substring(imgFile.src.lastIndexOf( "." )+ 1 ).toLowerCase();
      var dataURL = canvas.toDataURL( "image/" +ext);

为了解决第一次转换就成功就用了onload监听是否成功

      var imgFile = new Image()
            imgFile.src="./ceshi.jpeg"
            imgFile.onload=()=>{
                console.log(imgFile,11)
    
                //转base64
                var canvas = document.createElement( "canvas" );
                console.log('canvas---',canvas)
                canvas.width = imgFile.width;
                canvas.height = imgFile.height;
                var ctx = canvas.getContext( "2d" );
                ctx.drawImage(imgFile, 0 , 0 , imgFile.width, imgFile.height);
                var ext = imgFile.src.substring(imgFile.src.lastIndexOf( "." )+ 1 ).toLowerCase();
                var dataURL =  canvas.toDataURL( "image/" +ext);
            }

但是这里要注意一下：img.onload方法是一个异步函数，所以在外部取onload方法里面的值会有拿不到的情况。