// outside of your componet
import BraintreeCardClient from 'react-native-braintree-card';

// in your componentDidMount() block
BraintreeCardClient.initWithAuthorization(<token>);

// handle form submit event after user inputs card detail
BraintreeCardClient.getCardNonce( card, expMonth, expYear, cvv, (error, nonce) => {
   // if error handle accordingly or pass nonce to your server
});

// Or with object, more info on object schema here http://cocoadocs.org/docsets/Braintree/4.7.4/Classes/BTCard.html
BraintreeCardClient.getCardNonce(
  {
    number,
    expirationMonth,
    expirationYear,
    cvv,
    cardholderName,
    billing_address: {
      street_address, postal_code, locality, region,
      country_code_alpha2,
      country_name,
    },
    options: {
      validate: false,
    }
  },
   (error, nonce) => {
     // if error handle accordingly or pass nonce to your server
});