React component has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource

Question

I am calling the Web API from the my react component using fetch when I used to run it as one application, there was no problem, but when I am running the application react separate from API, I am getting the CORS error, my fetch call is as below,

    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);
        });
};

and my POST call using Axios as below also.

    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);
        });
};

Here is my server side api code:

        [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";
    }

And I am getting the error for Get as below: "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."

Answer 1

You'll need to modify your sever. You'll need to

1. Enable CORS on the server side and
2. Add the domain where you'll be hosting your front-end to your list of Allowed Origins.