Js+Canvas做出图片压缩

/* 
 * 图片压缩
 * img 原始图片
 * width 压缩后的宽度
 * height 压缩后的高度
 * ratio 压缩比率 
 */
 function compress(img, width, height, ratio) { 
 var canvas, ctx, img;
 canvas = document.createElement('canvas'); 
 canvas.width = width;
 canvas.height = height; 
 ctx = canvas.getContext("2d"); 
 img = canvas.toDataURL("image/jpeg", ratio); 
 return img;
 }

var image = new Image();
image.src = "/img/test.jpg";
 
image.onload = function(){
 var img = compress(image, 500, 400, 0.7);
 document.getElementById("test").src = img;
}