base64图片压缩

/****压缩走的路线***/
function dealImage(re,w,callback){
	var newImage=new Image();
	var quality=0.7;
	newImage.src=re;
	var imgWidth,imgHeight;
	newImage.onload=function(){
	imgWidth=this.width;
	imgHeight=this.height;
	var canvas=document.createElement("canvas");
	var ctx=canvas.getContext("2d");
	if(Math.max(imgWidth,imgHeight)>w){
		if(imgWidth>imgHeight){
			canvas.width=w;
			canvas.height=w*imgHeight/imgWidth;
		}else{
			canvas.height=w;
			canvas.width=w*imgWidth/imgHeight;
		}
	}else{
		canvas.width=imgWidth;
		canvas.height=imgHeight;
		quality=0.6;
	}
	ctx.clearRect(0,0,canvas.width,canvas.height);
	ctx.drawImage(this,0,0,canvas.width,canvas.height);
	var base64=canvas.toDataURL("image/jpeg",quality);

	callback(base64);//必须通过毁掉函数返回，否则无法及时拿到改值

	}
}

//使用时

dealImage(e.target.result,800,next);

function next(base64){
//这里写一些业务逻辑，同时把base64赋值给后台接口
}