feat(promoCode): enhance discount logic and add tests for plan applicability

Author: Dobrunia

src/utils/promoCodeService.ts

@@ -182,6 +182,15 @@ function isPlanApplicable(benefit: PromoCodeBenefit, plan: PlanModel): boolean {
   return benefit.applicablePlanIds.some((planId): boolean => planId.toString() === plan._id.toString());
 }
 
+/**
+ * Returns whether discount promo can affect plan price.
+ *
+ * @param plan - tariff plan
+ */
+function isDiscountablePlan(plan: PlanModel): boolean {
+  return plan.monthlyCharge > 0 && isPlanAvailable(plan);
+}
+
 /**
  * Calculates discounted price for one plan.
  *
@@ -190,7 +199,9 @@ function isPlanApplicable(benefit: PromoCodeBenefit, plan: PlanModel): boolean {
  */
 export function calculatePromoCodePlanPrice(benefit: PromoCodeBenefit, plan: PlanModel): PromoCodePlanPrice {
   const originalAmount = plan.monthlyCharge;
-  const isApplicable = benefit.type !== 'grant_plan' && isPlanAvailable(plan) && isPlanApplicable(benefit, plan);
+  const isApplicable = benefit.type !== 'grant_plan' &&
+    isDiscountablePlan(plan) &&
+    isPlanApplicable(benefit, plan);
 
   if (!isApplicable) {
     return {
@@ -208,6 +219,16 @@ export function calculatePromoCodePlanPrice(benefit: PromoCodeBenefit, plan: Pla
       const discountAmount = Math.floor(originalAmount * benefit.percent / 100);
       const finalAmount = Math.max(originalAmount - discountAmount, minFinalPrice);
 
+      if (finalAmount >= originalAmount) {
+        return {
+          planId: plan._id.toString(),
+          isApplicable: false,
+          originalAmount,
+          finalAmount: originalAmount,
+          discountAmount: 0,
+        };
+      }
+
       return {
         planId: plan._id.toString(),
         isApplicable: true,
@@ -221,6 +242,16 @@ export function calculatePromoCodePlanPrice(benefit: PromoCodeBenefit, plan: Pla
       const minFinalPrice = benefit.minFinalPrice ?? DEFAULT_MIN_FINAL_PRICE;
       const finalAmount = Math.max(originalAmount - benefit.amount, minFinalPrice);
 
+      if (finalAmount >= originalAmount) {
+        return {
+          planId: plan._id.toString(),
+          isApplicable: false,
+          originalAmount,
+          finalAmount: originalAmount,
+          discountAmount: 0,
+        };
+      }
+
       return {
         planId: plan._id.toString(),
         isApplicable: true,
@@ -231,12 +262,22 @@ export function calculatePromoCodePlanPrice(benefit: PromoCodeBenefit, plan: Pla
     }
 
     case 'fixed_price':
+      if (benefit.amount >= originalAmount) {
+        return {
+          planId: plan._id.toString(),
+          isApplicable: false,
+          originalAmount,
+          finalAmount: originalAmount,
+          discountAmount: 0,
+        };
+      }
+
       return {
         planId: plan._id.toString(),
         isApplicable: true,
         originalAmount,
         finalAmount: benefit.amount,
-        discountAmount: Math.max(originalAmount - benefit.amount, 0),
+        discountAmount: originalAmount - benefit.amount,
       };
 
     default:

test/utils/promoCodeService.test.ts

@@ -122,6 +122,35 @@ describe('PromoCodeService', () => {
     });
   });
 
+  it('does not apply discount promos to free plan', () => {
+    const plan = createPlan({ monthlyCharge: 0 });
+    const price = calculatePromoCodePlanPrice({
+      type: 'percent_discount',
+      percent: 20,
+    } as any, plan);
+
+    expect(price).toMatchObject({
+      isApplicable: false,
+      originalAmount: 0,
+      finalAmount: 0,
+      discountAmount: 0,
+    });
+  });
+
+  it('does not apply fixed price promo when it is not cheaper than plan price', () => {
+    const plan = createPlan({ monthlyCharge: 100 });
+    const price = calculatePromoCodePlanPrice({
+      type: 'fixed_price',
+      amount: 100,
+    } as any, plan);
+
+    expect(price).toMatchObject({
+      isApplicable: false,
+      finalAmount: 100,
+      discountAmount: 0,
+    });
+  });
+
   it('returns preview for percent discount promo', async () => {
     const plan = createPlan({ monthlyCharge: 1000 });
     const promoCode = createPromoCode({