How to integrate a payment gateway using JavaScript?

Integrating a payment gateway using JavaScript mainly involves a frontend part where you can create a payment form and then submit the data to the gateway's API. You can integrate a payment gateway into your website using vanilla JavaScript, AJAX, etc. but we'll use Stripe as an example as it's a widely-used payment gateway.

You'll also need to create a backend endpoint that could be in any language. We won't cover this part here though since it's beyond the scope of the question.

The primary steps to integrate Stripe onto a web page are:

Step 1

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Include Stripe.js on your webpage:

<script src="https://js.stripe.com/v3/"></script>

Step 2

Create a Stripe instance with your publishable key. You get the key by registering for an account on Stripe's website.

<script>
  var stripe = Stripe('your-publishable-key');
</script>

Step 3

Now we create an instance of Elements which will help us to generate a credit card input form.

<script>
  var elements = stripe.elements();
</script>

Step 4

Create an instance of the card Element.

<script>
  var card = elements.create('card');
  card.mount('#card-element');
</script>

Here, #card-element is the id of a div where the form containing the card fields will be rendered.

Step 5

Now, we need to handle form submission. When user submits the form, we will prevent the default action of form submission and then confirm the card payment.

<script>
var form = document.getElementById('payment-form');

form.addEventListener('submit', function(event) {
  event.preventDefault();
  
  stripe.confirmCardPayment('{CLIENT_SECRET}', {
    payment_method: {
      card: card,
    }
  }).then(function(result) {
    if (result.error) {
      // Show error to your customer (e.g., insufficient funds)
      console.log(result.error.message);
    } else {
      // The payment has been processed!
      if (result.paymentIntent.status === 'succeeded') {
        alert("Payment Successful!")
      }
    }
  });
});
</script>

Now, let's put it all together:

<html>
  <head>
    <title>Website Title</title>
  </head>
  <body>
    <form id="payment-form">
      <div id="card-element">
        <!-- a Stripe Element will be inserted here. -->
      </div>

      <!-- Used to display form errors -->
      <div id="card-errors" role="alert"></div>

      <button type="submit">Submit Payment</button>
    </form>

    <script src="https://js.stripe.com/v3/"></script>
    <script>
      var stripe = Stripe('your-publishable-key');
      var elements = stripe.elements();
      
      var card = elements.create('card');
      card.mount('#card-element');

      var form = document.getElementById('payment-form');

      form.addEventListener('submit', function(event) {
        event.preventDefault();
      
        stripe.confirmCardPayment('{CLIENT_SECRET}', {
          payment_method: {
            card: card,
          }
        }).then(function(result) {
          if (result.error) {
            console.log(result.error.message);
          } else {
            if (result.paymentIntent.status === 'succeeded') {
              alert("Payment Successful!")
            }
          }
        });
      });
    </script>
  </body>
</html>

Note: Replace 'your-publishable-key' with your Stripe Public Key and '{CLIENT_SECRET}' should be obtained from your server. In a backend server, you have to use your secret key to create PaymentIntent and get client secret.

A full playground is available in Stripe's documentation. You can find it here: https://stripe.com/docs/js

This is just one way to integrate a payment gateway using JavaScript, different gateways will have a different set of APIs and SDKs and steps will obviously vary. Also, remember to have the amount and currency of the transaction secured server-side, to avoid fraudulent attempts to change the price or currency.

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