Auto-Populate Fields, Calculate, and Update Totals On Set/Change of Item

"recurring-services" - subrecord internalId
463908 - Items recordId
34148 - Xero Tax Rate recodId (default)

async function handler(C) {
    const index = C.getEventPayload().index;
    const value = C.getSubValueBasedOnIndex("recurring-services", index);
    const itemId = value.item[0];
    const quantity = value.quantity;
    const rate = value.rate;
    function getParentUpdates({ net, tax, total }) {
        const lineValues = C.state.subValues["recurring-services"];
        const newNet =
            lineValues
                .map((v, i) => (i === index ? 0 : +v.net))
                .reduce((agg, v) => v + agg, 0) + net;
        const newTaxTotal =
            lineValues
                .map((v, i) => (i === index ? 0 : +v.tax))
                .reduce((agg, v) => v + agg, 0) + tax;
        const newTotal =
            lineValues
                .map((v, i) => (i === index ? 0 : +v.total))
                .reduce((agg, v) => v + agg, 0) + total;
        return C.mergeAll(
            C.setValue("net-total", newNet),
            C.setValue("tax-total", newTaxTotal),
            C.setValue("total", newTotal)
        );
    }
    if (itemId !== undefined && itemId !== null) {
        const entry = await C.api.getEntry({
            id: [itemId],
            responseType: "iov",
            recordId: 463908,
        });
        // console.log("items entry: ", entry);
        let effectiveTax = 0;
        let taxRateSelectValue = [];
        try {
            taxRateSelectValue = JSON.parse(entry["tax-rate"]);
            const taxRateEntryId = taxRateSelectValue[0];
            if (taxRateEntryId) {
                const taxRateEntry = await C.api.getEntry({
                    id: [taxRateEntryId],
                    responseType: "iov",
                    recordId: 34148,
                });
                // console.log("tax rate entry id:", taxRateEntry);
                effectiveTax = +taxRateEntry["effective-tax-rate"];
            }
        } catch {}

        const net = (quantity || 0) * +(entry.rate || 0);
        const tax = (effectiveTax / 100) * net;
        const total = net + tax;
        const updatedLineValues = C.updateSubValueBasedOnIndex(
            "recurring-services",
            index,
            {
                description: entry.description,
                rate: +entry.rate,
                "tax-rate": taxRateSelectValue,
                net,
                tax,
                total,
            }
        );

        // merged return value
        const returnValue = C.mergeAll(
            updatedLineValues,
            getParentUpdates({ net, tax, total })
        );

        // console.log("return value:", returnValue);
        return returnValue;
    }
    const updatedValues = C.updateSubValueBasedOnIndex(
        "recurring-services",
        index,
        {
            description: "",
            rate: null,
            frequency: [],
            "tax-rate": [],
            quantity: 1,
            net: null,
            tax: null,
            total: null,
        }
    );

    const returnValue = C.mergeAll(
        updatedValues,
        getParentUpdates({ net: 0, tax: 0, total: 0 })
    );
    return returnValue;
}