How to test api with dummy data within code in React native?

I am creating a SignUp Page for which used fetch to get data from an api. For which I want to test it with dummy data within the code itself.

const Signup: FC<AuthNavigationProps<'Signup'>> = () => {
  const {control, handleSubmit} = useForm();
  const {setUser} = useUser();
  const toast = useToast();

  const signup: SubmitHandler<ISignupValues> = async ({email, password}) => {
    try {
      const res = await fetch(`${config.apiUrl}/api/signup`, {
        method: 'POST',
        body: JSON.stringify({
          email,
          password,
        }),
      });
      if (res.ok) {
        await setGenericPassword(email, password, CREDENTIALS_STORAGE_OPTIONS);
        setUser({isLoggedIn: true, hasSessionExpired: false});
        toast.setToast({message: 'Signup has succeeded', visible: true});
      }
    } catch (error) {
      toast.setToast({message: 'Signup failed', visible: true});
    }
  };

  return (
    <View style={styles.container}>
      <Text style={styles.helloText}>Welcome Tayze User!</Text>
      <TextInput
        placeholder="Email"
        // inputProps={{keyboardType: 'email-address', autoCorrect: false}}
        autoCorrect
        style={styles.textInput}
      />
      <TextInput
        placeholder="Password"
        // inputProps={{secureTextEntry: true}}
        secureTextEntry
        style={styles.textInput}
      />
      <Button label="Sign up" onPress={() => handleSubmit(signup)()} />
    </View>
  );
};

export default Signup;

I want to create one mock data here and call it to check the login and signup pages.