Integrate CommercePro Checkout using Callback URL

<script src='https://jssdk.payu.in/bolt/bolt.min.js'></script>

// Create the express object. Refer to the table for a full list of parameters
const expressData = {
    key: '<merchant_key>',
    txnid: '<merchant_transaction_ID>',
    amount: '<total_transaction_amount>',
    phone: '<user_mobile_number>',
    firstname: '<user_first_name>',
    lastname: '<user_last_name>',
    email: '<user_email>',
    udf1: '<user_defined_parameter1>',
    udf2: '<user_defined_parameter2>',
    udf3: '<user_defined_parameter3>',
    udf4: '<user_defined_parameter4>',
    udf5: '<user_defined_parameter5>',
    isCheckoutExpress: true,
    icp_source: "express",
    productinfo: '<merchant_product_info>',
    surl: '<merchant_success_url>',
    furl: '<merchant_failure_url>',
    orderid: '<merchant_orider_ID>',
    cart_details: {
      'amount': '<total_transaction_amount>',
      'items': '<total_items_in_cart>',
      'sku_details': [
        {
          'offer_key': ['<offer_key1>', '<offer_key2>', '<offer_key3>', ... (do this for the offers configure for SKU)],
          'amount_per_sku': '<sku_amount>',
          'quantity': '<quantity_of_sku>',
          'sku_id': '<sku_ID>',
          'sku_name': '<sku_name>',
          'logo': '<url_of_sku_image>'
        },
        ... (do this for each SKU)
      ]
    },
    {
    "shipping_charges": '<shipping_charges>',
    "cod_fee": '<cod_fee>',
    "other_charges": '<other_charges>',
    "tax_info": {
        "breakup": {
            <tax_name> : '<tax_amount>'
        },
        "total": '<total_Amount_sum_all_charges>'
    }
}}

// Get date;
const date = new Date().toGMTString();

// Authentication hash supported
const AUTH_TYPE = 'sha512';

// Stringigy the express object
const stringifiedExpressData = JSON.stringify(expressData);

// Construct hash input on the backend by '|'(pipe) seperating parts
const hash_string = stringifiedExpressData + '|' + date + '|' + <merchant_salt>;

// Construct hash using SHA512 and encoding in hexadecimal
const hash = merchant_function_that_calculates_hash(hash_string)

// Construct authentication header with merchant key, authentication type, v2 hash received from the BE  
const  authHeader = 'hmac username="' + key + '", ' + 'algorithm="' + AUTH_TYPE + '", headers="date", signature="' + hash + '"';

bolt.launch({
  data: stringifiedExpressData,
  date: date,
  isCheckoutExpress: true,
  v2Hash: authHeader,
  mode: 'dropOut' // Parameter for callback URL method
}, {
  responseHandler: function(express) {
    if(express.response.txnStatus == "CANCEL"){
        console.log('Payment Cancelled.');
    } 
  },
  catchException: function(express) {
    console.log('Exception occured');
  }
})