This Means you are getting Html response from the server probably a 404 or 500 error. Instead of response.json() use response.text() you will get the html in text.
fetch("http:/example.com", {method: "POST",
body: JSON.stringify(
{
uname: uname,
password: password
}
)
})
.then((response) => response.text())
.then((responseData) => {
AlertIOS.alert(
"POST Response",
"Response Body -> " + responseData
)
}).done();
this.props.navigation.navigate("Home")
};