The read-only methodDetails property of the PaymentMethodChangeEvent interface is an object containing any data the payment handler may provide to describe the change the user has made to their payment method. The value is null if no details are available.

An object containing any data needed to describe the changes made to the payment method. The contents vary depending on the actual payment method chosen, so you will need to refer to the methodName property first, then interpret the methodDetails after that.

The default value is null, indicating that no additional details are available.

This example uses the paymentmethodchange event to watch for changes to the payment method selected for Apple Pay, in order to compute a discount if the user chooses to use a Visa card as their payment method.

request.onpaymentmethodchange = (ev) => {
  const { type: cardType } = ev.methodDetails;
  const newStuff = {};
  if (ev.methodName === "https://apple.com/apple-pay") {
    switch (cardType) {
      case "visa": {
        // do Apple Pay specific handling for Visa card…
        // methodDetails contains the card information
        const discount = calculateDiscount(ev.methodDetails);
        Object.assign(newStuff, discount);
        break;
      }
    }
  }
  // finally…
  ev.updateWith(newStuff);
};
const response = await request.show();

Note that the methodDetails property is being used by the calculateDiscount() function to compute any payment discount, then updateWith() is called to update the event with the computed update.