Implementing Refund of Payment

  <button onclick="cancelPay()">Refund</button>
  <script
    src="https://code.jquery.com/jquery-3.3.1.min.js"
    integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
    crossorigin="anonymous"></script><!-- jQuery CDN --->
  <script>
    function cancelPay() {
      jQuery.ajax({
        "url": "http://www.myservice.com/payments/cancel",
        "type": "POST",
        "contentType": "application/json",
        "data": JSON.stringify({
          "merchant_uid": "mid_" + new Date().getTime(), // Order number
          "cancel_request_amount": 2000, // Refund amount
          "reason": "Testing payment refund" // Reason for the refund
          "refund_holder": "Jone Doe", // [required to refund a virtual account payment] The recipient’s account holder for refund
          "refund_bank": "88" // [required to refund a virtual account payment] The recipient’s account bank code for refund (e.g. Shinhan Bank is 88 for KG Inicis)
          "refund_account": "56211105948400" // [required to refund a virtual account payment] The recipient’s account number for refund
        }),
        "dataType": "json"
      });
    }
  </script>

  <!-- Node.js -->
  var express = require('express');
  var app = express();
  var axios = require('axios');
  /* ... code omitted here ... */
  app.post('/payments/cancel', async (req, res, next) => {
    try {
      /* Acquire access token */
      const getToken = await axios({
        url: "https://api.iamport.kr/users/getToken",
        method: "post", // POST method
        headers: { 
          "Content-Type": "application/json" 
        },
        data: {
          imp_key: "imp_apikey", // [Iamport Admin] REST API key
          imp_secret: "ekKoeW8RyKuT0zgaZsUtXXTLQ4AhPFW3ZGseDA6bkA5lamv9OqDMnxyeB9wqOsuO9W3Mx9YSJ4dTqJ3f" // [Iamport Admin] REST API Secret
        }
      });
      const { access_token } = getToken.data.response; // Access token
      /* Query payment Information */
      ...
    } catch (error) {
      res.status(400).send(error);
    }
  });

  /* ... model/payments.js ... */
  var mongoose = require('mongoose');
  var Schema = mongoose.Schema;
  ...
  var PaymentsSchema = new Schema({
    imp_uid: String, // Iamport unique ID (used as a unique number at refund request)
    merchant_uid: String, // order number (used when querying payment information)
    amount: { type: Number, default: 0 }, // Payment amount (used for calculating refundable amount)
    cancel_amount: { type: Number, default: 0 }, // Total amount refunded (used for calculating refundable amount)
    ...
  });
  ...
  module.exports = mongoose.model('Payments', PaymentsSchema);

  /* ... code omitted here ... */
  var Payments = require('./models/payments');
  app.post('/payments/cancel', async (req, res, next) => {
    try {
      /* Acquire access token */
      /* ... code omitted here ... */
      /* Query payment Information */
      const { body } = req;
      const { merchant_uid } = body; // Order number from the client
      Payments.find({ merchant_uid }, async function(err, payment) { 
        if (err) {
          return res.json(err);
        }
        const paymentData = payment[0]; // Retrieved payment information
        /* Request a refund to Iamport REST API */
        ...
      });
    } catch (error) {
      res.status(400).send(error);
    }
  });

  /* ... code omitted here ... */
  app.post('/payments/cancel', async (req, res, next) => {
    try {
      /* Acquire access token */
      /* ... code omitted here ... */
      /* Query payment Information */
      const { body } = req;
      const { merchant_uid, reason } = body; // Order number from the client, reason for the refund
      Payments.find({ merchant_uid }, async function(err, payment) { 
        /* ... code omitted here ... */
        const paymentData = payment[0]; // Retrieved payment information
        const { imp_uid } = paymentData; // Extract imp_uid from the retrieved payment information
        /* Request a refund to Iamport REST API */
        const getCancelData = await axios({
          url: "https://api.iamport.kr/payments/cancel",
          method: "post",
          headers: {
            "Content-Type": "application/json",
            "Authorization": access_token // The access token issued from the Iamport server
          },
          data: {
            reason, // The reason for the refund received from the merchant client
            imp_uid, // Use imp_uid as a unique refund number
            ...
          }
        });
        const { response } = getCancelData.data; // Refund result
        /* Synchronize refund results */
        ...
      });
    } catch (error) {
      res.status(400).send(error);
    }
  });

  /* ... code omitted here ... */
app.post('/payments/cancel', async (req, res, next) => {
  try {
    /* Acquire access token */
    /* ... code omitted here ... */
    /* Query payment Information */
    const { body } = req;
    const { merchant_uid, reason, cancel_request_amount } = body; // Order number from the client, reason for the refund, refund amount
    Payments.find({ merchant_uid }, async function(err, payment) { 
      /* ... code omitted here ... */
      const paymentData = payment[0]; // Retrieved payment information
      const { imp_uid } = paymentData; // Extract imp_uid from the retrieved payment information
      /* Request a refund to Iamport REST API */
      const getCancelData = await axios({
        url: "https://api.iamport.kr/payments/cancel",
        method: "post",
        headers: {
          "Content-Type": "application/json",
          "Authorization": access_token // The access token issued from the Iamport server
        },
        data: {
          reason, // The reason for the refund received from the merchant client
          imp_uid, // Use imp_uid as a unique refund number
          amount: cancel_request_amount, // The requested refund amount from the merchant client
          ...
        }
      });
      const { response } = getCancelData.data; // Refund result
      /* Synchronize refund results */
      ...
    });
  } catch (error) {
    res.status(400).send(error);
  }
});

  /* ... code omitted here ... */
  app.post('/payments/cancel', async (req, res, next) => {
    try {
      /* Acquire access token */
      /* ... code omitted here ... */
      /* Query payment Information */
      const { body } = req;
      const { merchant_uid, reason, cancel_request_amount } = body; // Order number from the client, reason for the refund, refund amount
      Payments.find({ merchant_uid }, async function(err, payment) { 
        /* ... code omitted here ... */
        const paymentData = payment[0]; // Retrieved payment information
        const { imp_uid, amount, cancel_amount } = paymentData; // Extract imp_uid, amount (payment amount), cancel_amount (the total refund amount) from the retrieved payment information
        const cancelableAmount = amount - cancel_amount; // The refundable amount (= payment amount - total refunded amount)
        if (cancelableAmount <= 0) { // if already refunded in full
          return res.status(400).json({ message: "This order has already been fully refunded." });
        }
        ...
        /* Request a refund to Iamport REST API */
        const getCancelData = await axios({
          url: "https://api.iamport.kr/payments/cancel",
          method: "post",
          headers: {
            "Content-Type": "application/json",
            "Authorization": access_token // The access token issued from the Iamport server
          },
          data: {
            reason, // The reason for the refund received from the merchant client
            imp_uid, // Use imp_uid as a unique refund number
            amount: cancel_request_amount, // The requested refund amount from the merchant client
            checksum: cancelableAmount // [Recommended] Specify refundable amount
          }
        });
        const { response } = getCancelData.data; // Refund result
        /* Synchronize refund results */
        ...
      });
    } catch (error) {
      res.status(400).send(error);
    }
  });

  /* ... code omitted here ... */
  app.post('/payments/cancel', async (req, res, next) => {
    try {
      /* Acquire access token */
      /* ... code omitted here ... */
      /* Query payment Information */
      const { body } = req;
      const { merchant_uid, reason, cancel_request_amount, refund_holder, refund_bank, refund_account } = body; // Order number from the client, reason for the refund, refund amount, recipient information (account holder, account number, bank code)
      Payments.find({ merchant_uid }, async function(err, payment) { 
        /* ... code omitted here ... */
        const paymentData = payment[0]; // Retrieved payment information
        const { imp_uid, amount, cancel_amount } = paymentData; // Extract imp_uid, amount (payment amount), cancel_amount (the total refund amount) from the retrieved payment information
        const cancelableAmount = amount - cancel_amount; // The refundable amount (= payment amount - total refunded amount)
        if (cancelableAmount <= 0) { /// if already refunded in ful
          return res.status(400).json({ message: "This order has already been fully refunded." });
        }
        ...
        /* Request a refund to Iamport REST API */
        const getCancelData = await axios({
          url: "https://api.iamport.kr/payments/cancel",
          method: "post",
          headers: {
            "Content-Type": "application/json",
            "Authorization": access_token // The access token issued from the Iamport server
          },
          data: {
            reason, // The reason for the refund received from the merchant client
            imp_uid, // Use imp_uid as a unique refund number
            amount: cancel_request_amount, // The requested refund amount from the merchant client
            checksum: cancelableAmount, // [Recommended] Specify refundable amount
            refund_holder, // [required to refund a virtual account payment] The recipient’s account holder for refund
            refund_bank, // [required to refund a virtual account payment] The recipient’s account bank code for refund (e.g. Shinhan Bank is 88 for KG Inicis)
            refund_account // [required to refund a virtual account payment] The recipient’s account number for refund
          }
        });
        const { response } = getCancelData.data; // Refund result
        /* Synchronize refund results */
        ...
      });
    } catch (error) {
      res.status(400).send(error);
    }
  });

  /* ... code omitted here ... */
  app.post('/payments/cancel', async (req, res, next) => {
    try {
      /* Acquire access token */
      /* ... code omitted here ... */
      /* Query payment Information */
      Payments.find({ merchant_uid }, async function(err, payment) { 
        /* ... code omitted here ... */
        /* Request a refund to Iamport REST API */
        /* ... code omitted here ... */
        const { response } = getCancelData.data; // Refund result
        /* Synchronize refund results */
        const { merchant_uid } = response; // Extract the order information from the refund result
        Payments.findOneAndUpdate({ merchant_uid }, response, { new: true }, function(err, payment) { // Extract and synchronize the payment information that matches the order information
          if (err) {
            return res.json(err);
          }
          res.json(payment); // response to the client with the refund result
        });
      });
    } catch (error) {
      res.status(400).send(error);
    }
  });

  <button onclick="cancelPay()">cancelPay</button>
  <script
    src="https://code.jquery.com/jquery-3.3.1.min.js"
    integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
    crossorigin="anonymous"></script><!-- jQuery CDN --->
  <script>
    function cancelPay() {
      jQuery.ajax({
        /* ... code omitted here ... */
      }).done(function(result) { // Refund is successful 
        alert("Refund succeeded");
      }).fail(function(error) { // Refund is failed
        alert("Refund failed");
      });
    }
  </script>