纯前端JS压缩图片：基于Canvas的实现方案

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>压缩图片</title>
</head>
<body>
 <input id='file' type="file">
 <script>
  var eleFile = document.querySelector('#file')
  var file;
  var render = new FileReader(), img = new Image();
  render.onload = function(e) {
   img.src = e.target.result
  }
  // 获取图片文件
  eleFile.addEventListener('change', function(e) {
   file = e.target.files[0];
   if(file.type.indexOf('image') === 0) {
    //读取文件，并返回一个URL格式的Base64字符串
    render.readAsDataURL(file)
   }
  })
 
  //使用canvas把图片画出来
  var canvas = document.createElement('canvas');
  var context = canvas.getContext('2d');
 
  img.onload = function() {
 
   //原始尺寸
   var originWidth = this.width;
   var originHeight = this.height;
 
   //最大尺寸限制
   var maxWidth = 200, maxHeight = 200
 
   // 目标尺寸
   var targetWidth = originWidth, targetHeight = originHeight;
 
   //当原始尺寸大于200*200时候
   if(originWidth > maxWidth || originHeight > maxHeight) {
    if(originWidth / originHeight > maxWidth / maxHeight) {
     //更宽
     targetWidth = maxWidth;
     targetHeight = Math.round(maxWidth * (originHeight / originWidth))
    }else {
     targetHeight = maxHeight;
     targetWidth = Math.round(maxHeight * (originWidth / originHeight))
    } 
   }
 
   //画图
   canvas.width = targetWidth;
   canvas.height = targetHeight;
   //清除画布
   context.clearRect(0,0,targetWidth, targetHeight)
   //图片压缩
   context.drawImage(img, 0, 0, targetWidth, targetHeight);
   //canvas 转为blob并上传
   canvas.toBlob(function(blob) {
    try {
     var xhr = new XMLHttpRequest();
     xhr.onreadystatechange = function() {{
      if(xhr.status == 200) {
 
      }
     }}
     //开始上传
     xhr.open('POST','upload.php', true);
     xhr.send(blob)
    } catch (error) {
     console.log(error)
    }
    
   }, file.type || 'image/png')
   //在页面预览原图片
   var div1 = document.createElement('div')
   div1.innerText = '原图：'
   document.body.appendChild(div1)
   document.body.appendChild(img)
   //canvas预览
   var div2 = document.createElement('div')
   div2.innerText = 'canvas图：'
   document.body.appendChild(div2)
   document.body.appendChild(canvas)
  }
  
 </script>
</body>
</html>