我认为你看起来像这样〜

I think you are looking something like this~

更新此实现有两种方法可以减小图像的大小。一种方法调整图像大小，另一种方法压缩图像保持相同的大小，但会降低质量。

Update This implementation has two methods to reduce the size of the image. One method resize the image the other method compress the image maintaining the same size but it reduces the quality.

//Console logging (html)
if (!window.console)
  console = {};

var console_out = document.getElementById('console_out');
var output_format = "jpg";

console.log = function(message) {
  console_out.innerHTML += message + '<br />';
  console_out.scrollTop = console_out.scrollHeight;
}

var encodeButton = document.getElementById('jpeg_encode_button');
var encodeQuality = document.getElementById('jpeg_encode_quality');


function previewFile() { 
  var preview = document.getElementById('source_image');
  var previewCompress = document.getElementById('result_compress_image');
  var file   = document.querySelector('input[type=file]').files[0]; 
  var reader  = new FileReader();
  reader.addEventListener("load", function(e) {
    preview.src = e.target.result; 
    preview.onload = function() {
      resizeFile(this, preview);
      compressFile(this, previewCompress)
    };

    // preview.src = reader.result; 
  }, false);
  
  if (file) {  
    reader.readAsDataURL(file);
  }
}

function resizeFile(loadedData, preview) { 
  console.log(loadedData.width + ' ' + loadedData.height);
  var canvas = document.createElement('canvas'),
    ctx;
  canvas.width = Math.round(loadedData.width / 2);
  canvas.height = Math.round(loadedData.height / 2);
  var resizedImage = document.getElementById('result_resize_image');
  resizedImage.appendChild(canvas);
  // document.body.appendChild(canvas);
  ctx = canvas.getContext('2d');
  ctx.drawImage(preview, 0, 0, canvas.width, canvas.height);
}


function compressFile(loadedData, preview) { 
  console.log('width: ' + loadedData.width + ' height: ' + loadedData.height);

  var result_image = document.getElementById('result_compress_image');
  var quality = parseInt(encodeQuality.value);
  console.log("Quality >>" + quality);

  console.log("process start...");
  var time_start = new Date().getTime();

  var mime_type = "image/jpeg";
  if (typeof output_format !== "undefined" && output_format == "png") {
    mime_type = "image/png";
  }

  var cvs = document.createElement('canvas');
  cvs.width = loadedData.width;
  cvs.height = loadedData.height;
  var ctx = cvs.getContext("2d").drawImage(loadedData, 0, 0);
  var newImageData = cvs.toDataURL(mime_type, quality / 100);
  var result_image_obj = new Image();
  result_image_obj.src = newImageData;
  result_image.src = result_image_obj.src;

  result_image.onload = function() {
  }
  var duration = new Date().getTime() - time_start;

  console.log("process finished...");
  console.log('Processed in: ' + duration + 'ms');
}

<input type="file" onchange="previewFile()"><br>
<div>
  <h3>Original Image<h3>
  <img id="source_image" alt="Image preview..." />
</div>
<div>
  <h3>Resized Image<h3>
  <div id="result_resize_image">
  </div>
</div>
<div>
  <h3>Compressed Image<h3>
  <img id="result_compress_image" class='img_container' />
</div>
<br><br>

<div>
  <fieldset>
    <legend>Compressor settings</legend>
    <div id='controls-wrapper'>
      Compression ratio : <input id="jpeg_encode_quality" size='3' readonly='true' type="text" value="30" /> %
    </div>
  </fieldset>
</div>


<div>
  <fieldset>
    <legend>Console output</legend>
    <div id='console_out'></div>
  </fieldset>
</div>