Javascipt 上传图片之前修改尺寸和预览

render(){
  return(
    ...
    <label>Image1
    <input type="file" id="img1" on Change={this.handleChange} />
    </label>
    <br/>
    <img id="img1_preview" src="" height="100" />
    <label>Image2
    <input type="file" id="img2" on Change={this.handleChange} />
    </label>
    <img id="img2_preview" src="" height="100" />
    ...
  )
}

...
handleChange(event) {
  ...
  /* handle all image input change */
  if (event.target.id.includes('img')) {
      var field = event.target.id;
      var preview = document.getElementById(field + '_preview');
      var file = event.target.files[0];
      var reader = new FileReader();
      reader.onload = function(readerEvent) {
      var image = new Image();
      image.onload = function(imageEvent) {
          var canvas = document.createElement('canvas');
          var max_size = 800; /* max size */
          var width = image.width, height = image.height;
          /* alter image size */
          if (width > height) {
              if (width > max_size) {
                  height *= max_size / width;
                  width = max_size;
              }
          } else {
              if (height > max_size) {
                  width *= max_size / height;
                  height = max_size;
              }
          }
          /* draw image on canvas */
          canvas.width = width;
          canvas.height = height;
          canvas.getContext('2d').drawImage(image, 0, 0, width, height);
          var dataUrl = canvas.toDataURL('image/jpeg');
          /* canvas to blob */
          canvas.toBlob(function(blob){
              this.setState({[field]:blob});
              var reader1 = new FileReader();
              reader1.addEventListener("load",function(){
                  preview.src = dataUrl;
              }, false);
              reader1.readAsDataURL(blob);
          });
      }
      image.src = readerEvent.target.result;
    }
    reader.readAsDataURL(file);
  }