图片转base64并压缩大小

$("#file").change(function(){
var m_this = this;
      cutImageBase64(m_this,null,400,0.8);  
})
function cutImageBase64(m_this,id,wid,quality) {

    var file = m_this.files[0];
    var URL = window.URL || window.webkitURL;
    var blob = URL.createObjectURL(file);
    var base64;
var img = new Image();
    img.src = blob;
    img.onload = function() {
        var that = this;
        //生成比例
        var w = that.width,
            h = that.height,
            scale = w / h;
            w = wid || w;
            h = w / scale;
//生成canvas
        var canvas = document.createElement('canvas');
        var ctx = canvas.getContext('2d');
        $(canvas).attr({
            width: w,
            height: h
        });
        ctx.drawImage(that, 0, 0, w, h);
        // 生成base64            
        base64 = canvas.toDataURL('image/jpeg', quality || 0.8);
console.log(base64)
    };
}