Passing data to Web Service getting - 500 (Internal Server Error)

Question

I have an issue with fetching and passing data to ASP.NET web service from ReactJS.

Other fetch function is working when only getting data from the database. Why am I getting these errors ? what am I doing here wrong ? I believe its something in my signUp fetch function.

That response I think is that the fetch can't read the response from the server -

main.chunk.js:2417 POST http://localhost:50496/WebService.asmx/SignUp 500 (Internal Server Error)

SyntaxError: Unexpected token T in JSON at position 0  at JSON.parse (<anonymous>)

ReactJS

userData = {
    fName: 'some',
    lName: 'two one',
    email: 'someonw@gmail.com',
    password: 'se123456'  }

My fetch function in ReactJs -

   const signUp = (signUpData) => {
 
    let data = {
      firstName: signUpData.fName,
      lastName: signUpData.lName,
      emailAddress: signUpData.email,
      password: signUpData.password,
    };
    fetch('http://localhost:50496/WebService.asmx/SignUp', {
      body: JSON.stringify(data),
      method: 'POST',
      headers: {
        'Content-Type': 'application/json; charset=UTF-8',
      },
    })
      .then((response) => response.text())
      .then(
        (xml) =>
          new window.DOMParser().parseFromString(xml, 'text/xml')
            .documentElement.firstChild.textContent
      )
      .then((jsonStr) => JSON.parse(jsonStr))
      .then((userData) => {
        if (userData.d  && userData.d.length > 0) {
          setUser(userData);
        } else {
          console.log('error no data');
        }
      })
      .catch((err) => {
        console.log(err);
      });
  };

Error from Chrome -

System.InvalidOperationException: Missing parameter : firstName. System.Web.Services.Protocols.ValueCollectionParameterReader.Read(NameValueCollection collection) System.Web.Services.Protocols.UrlParameterReader.Read(HttpRequest request) System.Web.Services.Protocols.HttpServerProtocol.ReadParameters() System.Web.Services.Protocols.WebServiceHandler.CoreProcessRequest()

SignUp WebMethod is working in the WebService.

[WebMethod]
public string SignUp(string firstName, string lastName, string emailAddress, string password)
{
   return BLL.SignUp(firstName, lastName, emailAddress, password);
}

Answer 1

I found What I needed to do,
another error was -

InvalidOperationException: Only web services with the [ScriptService] attribute in the class definition can be read from a script.

So I added to WebService.cs the Attribute Class[ScriptService]

 [WebService(Namespace = "http://tempuri.org/")]
 [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
 [ScriptService]

And Since this completely eliminates the need to parse XML as you must when using standard SOAP (Simple Object Access Protocol) web services,
I removed this code lines -

.then((response) => response.text())
      .then(
        (xml) =>
          new window.DOMParser().parseFromString(xml, 'text/xml')
             .documentElement.firstChild.textContent 
      )
      .then((jsonStr) => JSON.parse(jsonStr))

And changed to this -

 .then((response) => response.json())
  .then((userData) => {
    if (userData.d && userData.d.length > 0) {
      let user = JSON.parse(userData.d);
      setUserData(user);
      }