http://wordpress.stackexchange.com/questions/103693/ajax-issue-uncaught-syntaxerror-when-processing-zip-file

se_question_103693.php

public function zip_file()
{
    $zipFile = $this->_plugin_path . 'assets/temp_files' . date("Y-m-d-h-i-s-"). 'test.zip';

    $zip = new ZipArchive;
    $opened = $zip->open($zipFile, ZIPARCHIVE::CREATE);

    if( $opened !== true ){
        // return early
        die("cannot open {$zipFile} for writing.");            
    }

    // DO STUFF
    $zip->close();

   
    return $zip;
}

I removed the logic for setting the content headers, to limit the points of failure in this method. Don't forget to re-add them once confirming the zip file is created and returning correctly.