Display Error Message for Failed Order Discount

[tr-elbdev]

Currently, when a discount is returned, no feedback is provided to the user, when EMPTY_DISCOUNT is returned. I would like to add a functionality to display an error message explaining why the discount was not applied.

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Code Context

Here is the relevant code for the run function:

// @ts-check
import { DiscountApplicationStrategy } from "../generated/api";

/**
 * @typedef {import("../generated/api").RunInput} RunInput
 * @typedef {import("../generated/api").FunctionRunResult} FunctionRunResult
 */

/**
 * @type {FunctionRunResult}
 */
const EMPTY_DISCOUNT = {
  discountApplicationStrategy: DiscountApplicationStrategy.First,
  discounts: [],
};

const isValidNumber = (value) => !isNaN(value) && value >= 0;

/**
 * @param {RunInput} input
 * @returns {FunctionRunResult}
 */
export function run(input) {
  const orderTotal = parseFloat(input.cart.cost.totalAmount.amount);
  const defaultDiscountMessage = 'BLACK FRIDAY';

  const message = input?.discountNode?.message?.value ?? defaultDiscountMessage;

  const discountsStr = input?.discountNode?.discounts?.value;
  if (!discountsStr) {
    return EMPTY_DISCOUNT;
  }

  const discountEntries = discountsStr.split('|')
    .map(entry => entry.split(':').map(value => parseFloat(value)))
    .filter(([threshold, percentage]) =>
      isValidNumber(threshold) && isValidNumber(percentage) && percentage <= 100
    );

  if (discountEntries.length === 0) {
    return EMPTY_DISCOUNT;
  }

  let discountPercentage = discountEntries.reduce((bestPercentage, [threshold, percentage]) => {
    if (orderTotal >= threshold) {
      return Math.max(bestPercentage, percentage);
    }
    return bestPercentage;
  }, 0);

  if (discountPercentage === 0) {
    return EMPTY_DISCOUNT;
  }

  return {
    discountApplicationStrategy: DiscountApplicationStrategy.First,
    discounts: [
      {
        message,
        value: {
          percentage: {
            value: discountPercentage
          }
        },
        targets: [
          {
            orderSubtotal: {
              excludedVariantIds: []
            }
          }
        ]
      }
    ]
  };
}

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Proposed Feature

When the discount calculation fails (e.g., the cart total does not match any thresholds), an error message should be included in the response to inform the user why the discount could not be applied.

Expected Behavior:

• Add a new errorMessage property to the FunctionRunResult when no discount is applied.
• Provide meaningful error messages, such as:
  • "Cart total does not meet any discount thresholds."
  • "Invalid discount format or values."

---

Example Response

Current Response:

{
  discountApplicationStrategy: DiscountApplicationStrategy.First,
  discounts: [],
}

Proposed Response with Error:

{
  discountApplicationStrategy: DiscountApplicationStrategy.First,
  discounts: [],
  errorMessage: "Cart total does not meet any discount thresholds."
}