getCompression = function (fileCompression, zipCompression) {

    var compressionName = fileCompression || zipCompression;
    var compression = compressions[compressionName];
    if (!compression) {
        throw new Error(compressionName + " is not a valid compression method !");
    }
    return compression;
}
Find the compression to use.
@param {String} fileCompression the compression defined at the file level, if any.
@param {String} zipCompression the compression defined at the load() level.
@return {Object} the compression object to use.