JavaScript

componentDidMount() {
        console.log(clientConfiguration)
        
        fetch(clientConfiguration['communitiesApi.local'])
            .then((response) => {
                return response.json();                
            })
            .then(data => {
                console.log(data);
                let communitiesFromApi = data.map(community => { return { value: community, display: community } });
                this.setState({ communities: [{ value: '', display: 'Select a Community...' }].concat(communitiesFromApi) });    
                            
            }).catch(error => {
                console.log(error);
            });
    };

handleDownload = (e) => {
        e.preventDefault();

        var formData = new FormData();
        formData.append('communityname', this.state.selectedCommunity);
        formData.append('files', JSON.stringify(this.state['checkedFiles']));

        let url = clientConfiguration['filesApi.local'];
        let tempFiles = clientConfiguration['tempFiles.local'];

        axios({
            method: 'post',
            responseType: 'application/zip',
            contentType: 'application/zip',
            url: url,
            data: formData
        })
            .then(res => {       
                var fileName = `${this.state['selectedCommunity']}.zip`;
                saveAs(`https://localhost:44352/TempFiles/${res.data}`, fileName);
            });
    };

[HttpGet("{communityName}")]
public string Get(string communityName)
{
    string rootPath = Configuration.GetValue<string>("ROOT_PATH");
    string communityPath = rootPath + "\\" + communityName;

    string[] files = Directory.GetFiles(communityPath);

    List<string> strippedFiles = new List<string>();
    foreach (string file in files)
    {
        strippedFiles.Add(file.Replace(communityPath + "\\", ""));
    }

    return JsonConvert.SerializeObject(strippedFiles);
}

[HttpPost]
public string Post([FromForm] string communityName, [FromForm] string files) //FileContentResult
{
    var removedInvalidCharsFromFileName = removeInvalidCharsFromFileName(files);
    var tFiles = removedInvalidCharsFromFileName.Split(',');
    string rootPath = Configuration.GetValue<string>("ROOT_PATH");
    string communityPath = rootPath + "\\" + communityName;

    byte[] theZipFile = null;

    using (MemoryStream zipStream = new MemoryStream())
    {
        using (ZipArchive zip = new ZipArchive(zipStream, ZipArchiveMode.Create, true))
        {
            foreach (string attachment in tFiles)
            {
                var zipEntry = zip.CreateEntry(attachment);

                using (FileStream fileStream = new FileStream(communityPath + "\\" + attachment, FileMode.Open))
                using (Stream entryStream = zipEntry.Open())
                {
                    fileStream.CopyTo(entryStream);
                }
            }
        }

        theZipFile = zipStream.ToArray();
    }

    ////return File(theZipFile, "application/zip", communityName + ".zip");

    string tempFilesPath = Configuration.GetValue<string>("Temp_Files_Path");

    if (!System.IO.Directory.Exists(tempFilesPath))
        System.IO.Directory.CreateDirectory(tempFilesPath);

    System.IO.File.WriteAllBytes($"{tempFilesPath}\\{communityName}.zip", theZipFile);

    //return System.IO.File.ReadAllBytes($"{tempFilesPath}\\Test.zip");

    //return $"{tempFilesPath}\\{communityName}.zip";
    return $"{communityName}.zip";
}

Access to fetch at 'https://localhost:44368/api/communities' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

[HttpPost]
        public FileContentResult Post([FromForm] string communityName, [FromForm] string files)
        {
           
            var removedInvalidCharsFromFileName = removeInvalidCharsFromFileName(files);
               
            var tFiles = removedInvalidCharsFromFileName.Split(',');
            string rootPath = Configuration.GetValue<string>("ROOT_PATH");
            string communityPath = rootPath + "\\" + communityName;

            byte[] theZipFile = null;

            using (MemoryStream zipStream = new MemoryStream())
            {
                using (ZipArchive zip = new ZipArchive(zipStream, ZipArchiveMode.Create, true))
                {
                    foreach (string attachment in tFiles)
                    {
                        var zipEntry = zip.CreateEntry(attachment);

                        using (FileStream fileStream = new FileStream(communityPath + "\\" + attachment, FileMode.Open))
                        using (Stream entryStream = zipEntry.Open())
                        {
                            fileStream.CopyTo(entryStream);
                        }
                    }
                }
                theZipFile = zipStream.ToArray();
            }

            return File(theZipFile, "application/zip", communityName + ".zip");
        }

handleDownload = (e) => {
        e.preventDefault();

        var formData = new FormData();
        formData.append('communityname', this.state.selectedCommunity);
        formData.append('files', JSON.stringify(this.state['checkedFiles']));        
     
        let url = clientConfiguration['filesApi.local'];

        axios({
            method: 'post',
            url: url,
            data:  formData
        });

        window.open(url, '_blank');
    }

handleDownload = (e) => {
        e.preventDefault();

        var formData = new FormData();
        formData.append('communityname', this.state.selectedCommunity);
        formData.append('files', this.state['files']);
        
        alert(this.state['files'].join(','));
        let url = clientConfiguration['filesApi.local'];

        axios({
            method: 'post',
            url: url,
            data:  formData
        });
    }

handleDownload(e) {
        e.preventDefault();
        alert('Hi');
        let communityName = this.state['selectedCommunity'];
        let files = this.state[files];

        fetch(clientConfiguration['filesApi.local'], {
            method: 'POST',
            headers: new Headers(),
            body: JSON.stringify({ communityName: communityName, body: files })
        }).then((res) => res.json())
            .then((data) => console.log(data))
            .catch((err) => console.log(err))
    };

I have a button as below:

        renderDownloadButton() {
        if (this.state.files && this.state.files.filter(i => i.checked).length) {
            return (
                <button id="download" styles="display: none;" onClick={this.handleDownload} >
                    Download
        </button>
            );
        }
    };

let communityName = this.state['selectedCommunity'];

Can not read property state undefined