js压缩图片base64长度

var myCanvas=$(".img-container > img").cropper("getCroppedCanvas");
(function (base64){
    var image = new Image(),newBase64,MAX_HEIGHT = 160;
    image.onload = function(){
        var canvas = document.getElementById("myCanvas");
        if(image.height > MAX_HEIGHT) {
            image.width *= MAX_HEIGHT / image.height;
            image.height = MAX_HEIGHT;
        }
        var ctx = canvas.getContext("2d");
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        canvas.width = image.width;
        canvas.height = image.height;
        ctx.drawImage(image, 0, 0, image.width, image.height);
        newBase64=canvas.toDataURL("image/jpeg",0.4);
　　　　　　//上传base64编码
        $.ajax({
            type: "post",
            url: "地址",
            data: {
                
            },
            success: function (msg) {
               42             },
            error: function (msg) {
                tips.error("上传图片服务出错，请稍候再试。")
            }
        });
        console.log("base64Size",newBase64.length/1024);50     };
    image.src = base64;
})(myCanvas.toDataURL("image/jpeg"));