base64与blob 互相转换

//**dataURL to blob**
    function dataURLtoBlob(dataurl) {
        var arr = dataurl.split(','), mime = arr[0].match(/:(.*?);/)[1],
            bstr = atob(arr[1]), n = bstr.length, u8arr = new Uint8Array(n);
        while (n--) {
            u8arr[n] = bstr.charCodeAt(n);
        }
        return new Blob([u8arr], { type: mime });
    }

    //**blob to dataURL**
    function blobToDataURL(blob, callback) {
        var a = new FileReader();
        a.onload = function (e) { callback(e.target.result); }
        a.readAsDataURL(blob);
    }

    //test:
    //var blob = dataURLtoBlob('data:text/plain;base64,YWFhYWFhYQ==');
    //blobToDataURL(blob, function (dataurl) {
    //    console.log(dataurl);
    //});