var bili = 0.5;//压缩后的图片尺寸,0.7就是70%
var quality = 0.2;//压缩后图片的质量,数字越小图片越模糊
var img = new Image();
img.src = imagebase64;
img.onload = function () {
	var newWidth = img.width*bili;//压缩后图片的宽度
	var newHeight = img.height*bili;//压缩后图片的高度
	//开始画压缩图
	var canvas = document.createElement("canvas");
	var ctx = canvas.getContext("2d");
	canvas.width = newWidth;//压缩图的宽度
	canvas.height = newHeight;//压缩图的高度
	ctx.drawImage(img,0,0,newWidth,newHeight);
	var newBase64 = canvas.toDataURL("image/jpeg",quality);
}