I implementing a scanner app using react native but when I wish to implement the app with api POST data to the mysql server it shown an error undefined is not an object (evaluating '_this.state.userid') I am new to react native and might done some simply error or mistake pls help me point out

below is my code

  export default function Scanner () {
  
  const [hasPermission, setHasPermission] = useState(null);
  const [scanned, setScanned] = useState(true);
  const [userid, setText] = useState('Not yet scanned')
  const [currentDate, setCurrentDate] = useState('');
  const navigation = useNavigation();


  const askForCameraPermission = () => {
    (async () => {
      const { status } = await BarCodeScanner.requestPermissionsAsync();
      setHasPermission(status === 'granted');
    })()
  }

  // Request Camera Permission
  useEffect(() => {
    askForCameraPermission();
  }, []);

  useEffect(() => {
    var date = new Date().getDate(); //Current Date
    var month = new Date().getMonth()   1; //Current Month
    var year = new Date().getFullYear(); //Current Year
    var hours = new Date().getHours(); //Current Hours
    var min = new Date().getMinutes(); //Current Minutes
    var sec = new Date().getSeconds(); //Current Seconds
    setCurrentDate(
      date   '/'   month   '/'   year 
        ' '   hours   ':'   min   ':'   sec
    );
  }, []);
  // What happens when we scan the bar code

  const handleBarCodeScanned = ({ type, data }) => {

   
    setScanned(true);
     
     
     setText(data )
     
   
  };
 
  // Check permissions and return the screens
  if (hasPermission === null) {
    return (
      <View style={styles.container}>
        <Text>Requesting for camera permission</Text>
      </View>)
  }
  if (hasPermission === false) {
    return (
      <View style={styles.container}>
        <Text style={{ margin: 10 }}>No access to camera</Text>
        <Button title={'Allow Camera'} onPress={() => askForCameraPermission()} />
      </View>)
  }

   
   const Register = () => {

    let userid = this.state.userid;
   
    let InsertAPIURL = "https://localhost/api/insert.php";

      let headers = {
        
        'Accept': 'application/json',
        'Content-Type': 'application/json',
        'Access-Control-Allow-Methods': 'POST, PUT, PATCH, GET, DELETE, OPTIONS',

        'Access-Control-Allow-Headers': 'Origin, X-Api-Key, X-Requested-With, Content-Type, Accept, Authorization',
      }
      let Data = {
        userid: userid,
        
      };

      fetch(InsertAPIURL, {
        mode:'no-cors',
        method: 'POST',
        headers: headers,
        body: JSON.stringify(Data)
      })
      try{
        ((response) =>response.json()),
      ((response)=>{
        alert(response[0].Message);
      })
      }
      
      catch{
        ((error) => {
        alert("Error" error);
      })
    }
}
  // Return the View
  return (
     
    <View style={styles.container}>
      <View style={styles.barcodebox}>
        <BarCodeScanner
          onBarCodeScanned={scanned ? undefined : handleBarCodeScanned}
          style={{ height: 400, width: 400 }} />
      </View>
      
      <Text style={styles.maintext}>{userid   '\n' currentDate}
    
      </Text>

      
      {
        scanned && <Button title={'Scan again?'} onPress={() => setScanned(false)} color='tomato' />
         
      }
     
      {
        scanned && <Button title={'OK'} onPress={()=>{navigation.navigate('Home,{userid}')},Register()} /> 
         
      }
    </View>
  );
}

CodePudding user response：

You are mixing class component approach with functional component in this statement:

let userid = this.state.userid;

You already have userid defined in useState and it's available for nested functions.

BTW: the React coding convention is to name setter function using "set" state variable, so you should have

const [userid, setUserid] = useState('Not yet scanned')