arrayBuffer 转base64

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,