fix tax calculations on next object with single-use coupons

lib/recurly/pricing/checkout/calculations.js

@@ -149,9 +149,7 @@ export default class Calculations {
       this.price.next.discount = discountNext;
     }
 
-    return promise.then(() => {
-      if (coupon.single_use) this.price.next.discount = 0;
-    });
+    return promise;
   }
 
   /**
@@ -329,6 +327,7 @@ export default class Calculations {
    *
    * @return {Object} { discountNow, discountNext }
    */
+
   discountAmounts () {
     const coupon = this.items.coupon;
     let discountNow = 0;
@@ -339,13 +338,18 @@ export default class Calculations {
       } else if (coupon.discount.rate) {
         const { discountableNow, discountableNext } = this.discountableSubtotals(coupon, { setupFees: false });
         discountNow = roundForDiscount(discountableNow * coupon.discount.rate);
-        discountNext = roundForDiscount(discountableNext * coupon.discount.rate);
+        // If coupon is single use, we want discountNext to be zero
+        if (!coupon.single_use) {
+          discountNext = roundForDiscount(discountableNext * coupon.discount.rate);
+        }
       } else if (coupon.discount.amount) {
         const { discountableNow, discountableNext } = this.discountableSubtotals(coupon);
         // Falls back to zero if the coupon does not support the checkout currency
         const discountAmount = coupon.discount.amount[this.items.currency] || 0;
         discountNow = Math.min(discountableNow, discountAmount);
-        discountNext = Math.min(discountableNext, discountAmount);
+        if (!coupon.single_use) {
+          discountNext = Math.min(discountableNext, discountAmount);
+        }
       }
     }
     return { discountNow, discountNext };

test/unit/pricing/checkout/checkout.test.js

@@ -1765,6 +1765,19 @@ describe('CheckoutPricing', function () {
                 done();
               });
           });
+
+          describe('is single-use and applies to subscriptions and adjustments with taxes', () => {
+            beforeEach(applyCoupon('coop-single-use'));
+
+            it('discounts only the subscriptions now, and applies no discounts next cycle', function () {
+              assert.equal(this.price.now.subtotal, 41.99); // 19.99 + 2 (setup fee) + 20 (adj) + 20 (adj) - $20 discount
+              assert.equal(this.price.now.discount, 20); 
+              assert.equal(this.price.next.subscriptions, 19.99);
+              assert.equal(this.price.next.discount, 0);
+              assert.equal(this.price.now.taxes, 3.67);
+              assert.equal(this.price.next.taxes, 1.75);
+            });
+          });
         });
       });