Checking available payment methods

function get_payment_methods () {
    // *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 payment_methods = 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"
                        }
                    }
                ]
            }
        ]
    };

    try {
        // send UTF-8 JSON to the Bango Payment API and parse the JSON response
        let response = rest_client_call({
            method: "OPTIONS",
            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: at least one payment method is available.
            case "OK":
                // See below for details of this structure.
                payment_methods = response.availablePaymentMethods;
                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
                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 "SERVICE_UNAVAILABLE":
                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);
    }

    return payment_methods; // null if payment cannot proceed
}