import paytm from 'react-native-paytm';
import { ..., Platform, DeviceEventEmitter, NativeModules, NativeEventEmitter, ... } from 'react-native';

....

// Daat received from PayTM
const paytmConfig = {
  MID: '...',
  WEBSITE: '...',
  CHANNEL_ID: '...',
  INDUSTRY_TYPE_ID: '...',
  CALLBACK_URL: 'https://securegw.paytm.in/theia/paytmCallback?ORDER_ID='
}

componentWillMount(){
    ...
	if(Platform.OS == 'ios'){
      const { RNPayTm } = NativeModules
      const emitter = new NativeEventEmitter(RNPayTm)
      emitter.addListener('PayTMResponse', this.onPayTmResponse)
    }else{
      DeviceEventEmitter.addListener('PayTMResponse', this.onPayTmResponse)
    }
    ...
}

onPayTmResponse(response) {
  // Process Response
  // response.response in case of iOS
  // reponse in case of Android
  console.log(response);
}

runTransaction(amount, customerId, orderId, mobile, email, checkSum) {
    const callbackUrl = `${paytmConfig.CALLBACK_URL}${orderId}`;
    const details = {
      mode: 'Staging', // 'Staging' or 'Production'
      mid: paytmConfig.MID,
      industryType: paytmConfig.INDUSTRY_TYPE_ID,
      website: paytmConfig.WEBSITE,
      channel: paytmConfig.CHANNEL_ID,
      amount: `${amount}`, // String
      orderId: orderId, // String
      email: email, // String
      phone: mobile, // String
      custId: customerId, // String
      checksumhash: checkSum, //From your server using PayTM Checksum Utility 
      callback: callbackUrl
    };
    paytm.startPayment(details);
}