base64图片压缩

//压缩base64
function dealImage(base64, w, callback) {
    var newImage = new Image();
    var quality = 0.5;    //压缩系数0-1之间
    newImage.src = base64;
    newImage.setAttribute("crossOrigin", 'Anonymous');  //url为外域时需要
    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); //压缩语句
        // 如想确保图片压缩到自己想要的尺寸,如要求在50-150kb之间，请加以下语句，quality初始值根据情况自定
         while (base64.length / 1024 > 50) {
          quality -= 0.01;
          base64 = canvas.toDataURL("image/jpeg", quality);
         }
        // 防止最后一次压缩低于最低尺寸，只要quality递减合理，无需考虑
        while (base64.length / 1024 < 30) {
          quality += 0.001;
          base64 = canvas.toDataURL("image/jpeg", quality);
        }
        callback(base64);//必须通过回调函数返回，否则无法及时拿到该值
    }
}