var buffer = new ArrayBuffer(8);
// buffer 是接收到后台的流

function _arrayBufferToBase64( buffer ) {

    var binary = '';

    var bytes = new Uint8Array( buffer );

    var len = bytes.byteLength;

    for (var i = 0; i < len; i++) {

        binary += String.fromCharCode( bytes[ i ] );

    }

    return 'data:application/pdf;base64,'+ window.btoa( binary );

}

如果是图片类型，可以拼接 data:image/jpeg;base64,或data:image/png;base64,