Authorizing a payment

function get_transaction_id () {
    // *Always* use this endpoint prefix for the Bango Payment API.
    const base_url = "https://api.bango.net/v5/";

    // This is what we're trying to find.
    let transaction_id = null;

    const request_body_params = {
        // The user's identity.
        // In production you would probably pass this as an argument
        // to the function.
        bangoUserId: "123456789",

        // An array of strings, one for each supported payment method
        // you would like the Bango Platform to consider.
        // For example:
        paymentMethods: [
            "OPERATORBILLING", // Direct Carrier Billing payment
            "INTERNET", // Internet-based payment
        ],

        // The item to be purchased.
        // (Always use an array of one item: multiple items might
        // be supported at a later date.)
        // In production you would probably pass this as an argument
        // to the function.
        paymentItems: [
            {
                itemName: "My item",
                itemDescription: "My item description",
                priceList: [
                    {
                        grossAmount: "0.99",
                        currencyIso3: "USD",
                        financialBreakdown: {
                            taxAmount: "0.00"
                        }

                    },
                    {
                        grossAmount: "0.89",
                        currencyIso3: "EUR",
                        financialBreakdown: {
                            taxAmount: "0.00"
                        }
                    },
                    {
                        grossAmount: "0.79",
                        currencyIso3: "GBP",
                        financialBreakdown: {
                            taxAmount: "0.00"
                        }
                    }
                ]
            }
        ],

        // Store's custom transaction id for correlation/idempotency, if necessary
        externalTransactionId: "my-unique-tx-id-12345678",

        // Store's custom URL: the Bango Platform POSTs asynchronously
        // to this URL on authorization, if the response can't be
        // returned synchronously due to REDIRECT action
        extensionData: {
            notificationUrl: "https://example.com/my-store/tx-create?my_param=abcd1234"
        }
    };

    try {
        // POST UTF-8 JSON to the Bango Payment API and parse the JSON response
        let response = rest_client_call({
            method: "POST",
            url: base_url + "transaction/",
            headers: {
                "Authorization": get_authorization_header(),
                "Content-Type": "application/json",
            },
            body: make_json_string(request_body_params),
        }).from_json();

        // The responseCode tells us the result.
        switch (response.responseCode) {
            // Success: payment is authorized, transaction created
            case "OK":
                transaction_id = response.transactionId;
                break;

            // More to do: The store must take action
            case "CLIENT_ACTION_REQUIRED":
                // This response parameter tells us what we need to do.
                switch (response.parameters.action) {
                    case "REDIRECT":
                        // Send the user to a particular web page.
                        // request_body_params must include either
                        // notificationUrl or callbackUrl to ensure
                        // the store receives the result asynchronously
                        redirect_device_to({
                            url: response.parameters.url,
                        });
                        break;

                    default:
                        // Unrecognized action.
                        // Check the documentation again.
                        throw "Unrecognized client action";
                }

                break;

            // Failure: unable to proceed with purchase
            case "NOT_AVAILABLE": // None of the requested methods is available
            case "PRICE_NOT_SUPPORTED": // Payment methods don't support supplied prices
            case "USER_EXCEEDED_LIMIT": // User cannot spend more money
            case "DECLINED": // Payment provider declined authorization
            case "SPEED_LIMIT": // Request too soon after previous payment
            case "USER_BARRED": // User not allowed to use payment method
            case "USER_INSUFFICIENT_CREDIT": // User cannot afford purchase
            case "USER_NOT_ENABLED": // Payment provider not enabled user
            case "USER_SPEND_LIMIT": // User reached limit imposed by provider
            case "USER_SUSPENDED": // User temporarily barred from payment method
                throw "Unable to purchase item";

            // Merchant problems
            // These indicate issues in your code, not the Bango Platform.
            case "BAD_REQUEST":   // Invalid request
            case "INVALID_BANGOUSERID":  // Bango ID not recognized
            case "UNAUTHORIZED":  // Check your credentials
                throw "Merchant implementation error";

            // Bango Platform issues
            // These likely indicate transient problems.
            // In production code, you might want to try the request again
            // after a delay, ultimately falling back to an error state.
            case "INTERNAL_ERROR":
            case "CONNECT_ERROR":
            case "FAILURE":
            case "SERVICE_UNAVAILABLE":
            case "CONNECT_TIMEOUT":
                throw "Bango Platform unavailable"

            // This might indicate Bango has added a new response code.
            // Check the documentation again.
            default:
                throw "Unrecognized response code";
        }

    } catch (exception) {
        // Production code might want to notify the merchant in some way,
        // and/or log full details of the exception for later analysis,
        // and degrade gracefully for the user.
        output("An error occurred!");
        output(exception);
    }

    // null if payment not authorized or user redirected.
    // In the redirect case, a transaction ID might be set asynchronously
    // through the notificationUrl or callbackUrl parameter.
    return transaction_id;
}