paymentRequestFullExample.js

function startPaymentRequestCheckout() {
  var methodData = [
    {     
      supportedMethods: ['basic-card'],     
      data: {          
        supportedNetworks: ['visa', 'mastercard', 'amex'],
        supportedTypes: ['credit']                
      }    
    }     
  ]; 
  
  var details =  {
    displayItems: [
      {
        label: "Sub-total",
        amount: { currency: "USD", value : "100.00" }, // US$100.00
      },
      {
        label: "Sales Tax",
        amount: { currency: "USD", value : "9.00" }, // US$9.00
      }
    ],
    total:  {
      label: "Total due",
      amount: { currency: "USD", value : "109.00" }, // US$109.00
    }
  };

  var options = {
    requestShipping: true 
  };

  var request = new PaymentRequest(methodData, details, options);

  //Show the Native UI
  request.show()
  //When the promise is fulfilled, pass the results to your server for processing
  .then(result => {
    return process(result).then(response => {
      if (response.status === 200) {
        //Show that the transaction was successful in the UI
        return result.complete('success');
      } else {
        //Show in the Native UI that the transaction failed
        return result.complete('fail');
      }
    })
  });

  request.addEventListener("shippingaddresschange", function (changeEvent) {
    // Process shipping address change
  });

  request.addEventListener("onshippingoptionchange", function (changeEvent) {
    // Process shipping option change (e.g. "one day shipping")
  });
}
document.getElementById('#checkout').addEventListener('click', startPaymentRequestCheckout)

document.getElementById('#checkout').addEventListener('click', startPaymentRequestCheckout)
Should be -
document.getElementById('checkout').addEventListener('click', startPaymentRequestCheckout)
(checkout instead of #checkout)

Also, function() startPaymentRequestCheckout { should be function startPaymentRequestCheckout() { (the parentheses should be after the name of the function...).

Did anyone even bother to run this code (or just pass it through a JavaScript interpreter for syntax checking)? :(