POST fails with ReadableNativeMap cannot be cast to String error - react-native

Question

I'm working in React Native, I use PHP for backend and when I use fetch POST request I get so strange error, and I dont know why it happens. I checked the url so it works no problem, also normal fetch() is working without POST but when I try to post it happens. When I try it in local server fetch POST works.. but in server, I get this error :
ERROR : com.facebook.react.bridge.ReadableNativeMap cannot be cast to
java.lang.String
React native codes :
fetch('http://xxx/react_test1', {
method: 'POST',
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json',
},
body: JSON.stringify({
name: inputName,
email: inputEmail,
phone: inputPhone
}),
}).then((response) => response.json())
.then((responseJson) => {
Alert.alert(responseJson);
}).catch((error) => {
alert(error);
});

Answer 1

Alert.alert receives an string, and what you're getting from the fetch response is internally a com.facebook.react.bridge.ReadableNativeMap object (which is what the native implementation for fetch returns).
You can try:
Alert.alert(JSON.stringify(responseJson))
If you were using iOS you'll get a completely different error:
Exception '-[_NSFrozenDictionaryM length]: unrecognized selector ...

Answer 2

Alert.alert only accepts string as input.
Use alert() instead to show the popup.
Example: alert("Response: ", responseJson)
Happy Coding. :)

Answer 3

So with fetch() in javascript you need headers: {} but when switching over to RNFetchBlob you should put the headers directly in the {}
import RNFetchBlob from "rn-fetch-blob";
const aPath = Platform.select({ ios: DocumentDir, android: DownloadDir });
const fPath = aPath + '/' + Math.floor(date.getTime() + date.getSeconds() / 2) + '.xls';
Also if you want to add body in the request then directly add it there.
For example:-
RNFetchBlob.config({
// response data will be saved to this path if it has access right.
path: fPath,
}).fetch('POST', URL, {
Accept: 'application/json',
'Content-Type': 'application/json',
Authorization: token,
},
JSON.stringify(reqBody)
)
.then(res => {
console.log("response body>>>",res);
})
.catch(function (err) {
console.log(err);
});
that's all enjoy your coding...