The response.body is a base64 string of a file. How can I save it into a String variable?

fetchCustom(store.apiURL, {
            trtyp: 'file_download',
            seskey: store.userSessionKey,
            id: item.iddoc
        }).then(response => {
            const data = response.body as FetchData[];
            console.log('gotten base64', data);

            let base64File:string = data;
            
            Filesystem.writeFile({
                path: "filename.txt",
                data: base64File, //"base64 data",
                directory: Directory.Documents,
            }).then(response => {
            
            }).catch(error => {
                console.log('error a', error.message);
            });
        
        }).catch(error => {
            console.log('error b', error.message);
        });