How to find size of an image file from canvas?

Question

Here is a js function resizing an jpeg image . The original image is resized to width x height by the function. The image.size in alert returns undefined . Is mainCanvas.toDataURL.length the size of resized image file? If not, how to find the image file size after resizing?

    function resize(image, width, height) {
      var mainCanvas = document.createElement("canvas");
      mainCanvas.width = width;
      mainCanvas.height = height;
      var ctx = mainCanvas.getContext("2d");
      ctx.drawImage(image, 0, 0, width, height);
      $('#uploaded_file_hidden_file').val(mainCanvas.toDataURL("image/jpeg")); 
      $('#file_size').val(Math.ceil(image.size/1024));
      alert(image.size);
    };

Answer 1

If by size you mean file size in bytes, the image element will not have a size property like image.size . You would need to convert the canvas into a blob and then you can get the size:

 // canvas.toBlob() is not well supported, so here is the polyfill just in case.
 // https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob#Polyfill
 if (!HTMLCanvasElement.prototype.toBlob) {
 Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob', {
  value: function (callback, type, quality) {

    var binStr = atob( this.toDataURL(type, quality).split(',')[1] ),
        len = binStr.length,
        arr = new Uint8Array(len);

    for (var i=0; i<len; i++ ) {
     arr[i] = binStr.charCodeAt(i);
    }

    callback( new Blob( [arr], {type: type || 'image/png'} ) );
  }
 });
}

function resize(image, width, height) {
  var mainCanvas = document.createElement("canvas");
  mainCanvas.width = width;
  mainCanvas.height = height;
  var ctx = mainCanvas.getContext("2d");
  ctx.drawImage(image, 0, 0, width, height);
  $('#uploaded_file_hidden_file').val(mainCanvas.toDataURL("image/jpeg"));

  // Canvas to blob so we can get size.
  mainCanvas.toBlob(function(blob) {
    $('#file_size').val(Math.ceil(blob.size/1024));
    alert(blob.size);
  }, 'image/jpeg', 1);
};

Answer 2

要找出文件的最终大小（这当然是可能的），请检查包含指定格式的图像表示的数据 URI 的长度

  const imageFileSize = Math.round(mainCanvas.toDataURL('image/jpeg').length);