refactor: extracted rule-applying code to a separate method (closes #10)

src/shared/rules.ts

@@ -0,0 +1,11 @@
+import _ from 'lodash';
+
+type RuleResult = number | false | undefined;
+
+export const getMinimumRuleOutput = async <T>(
+  rules: ((input: T) => RuleResult | Promise<RuleResult>)[],
+  input: T,
+) => {
+  const results = await Promise.all(rules.map((rule) => rule(input)));
+  return Math.min(..._.compact(results));
+};

src/transaction/transaction.controller.spec.ts

@@ -71,91 +71,55 @@ describe('TransactionController Unit Tests', () => {
 
   it('calling applyRules method minimum result of applied rules should be got', async () => {
     {
-      const result = await transactionController.applyRules(
-        [
-          transactionController.discountRule,
-          transactionController.turnoverRule,
-        ],
-        {
-          base_amount: 1000,
-          client_id: 1,
-        },
-      );
+      const result = await transactionController.applyRules({
+        base_amount: 1000,
+        client_id: 1,
+      });
 
       expect(result).to.eql(5);
     }
 
     {
-      const result = await transactionController.applyRules(
-        [
-          transactionController.discountRule,
-          transactionController.turnoverRule,
-        ],
-        {
-          base_amount: 1,
-          client_id: 1,
-        },
-      );
+      const result = await transactionController.applyRules({
+        base_amount: 1,
+        client_id: 1,
+      });
 
       expect(result).to.eql(0.05);
     }
 
     {
-      const result = await transactionController.applyRules(
-        [
-          transactionController.discountRule,
-          transactionController.turnoverRule,
-        ],
-        {
-          base_amount: 1000,
-          client_id: 42,
-        },
-      );
+      const result = await transactionController.applyRules({
+        base_amount: 1000,
+        client_id: 42,
+      });
 
       expect(result).to.eql(0.03);
     }
 
     {
-      const result = await transactionController.applyRules(
-        [
-          transactionController.discountRule,
-          transactionController.turnoverRule,
-        ],
-        {
-          base_amount: 1,
-          client_id: 42,
-        },
-      );
+      const result = await transactionController.applyRules({
+        base_amount: 1,
+        client_id: 42,
+      });
 
       expect(result).to.eql(0.03);
     }
 
     {
-      const result = await transactionController.applyRules(
-        [
-          transactionController.discountRule,
-          transactionController.turnoverRule,
-        ],
-        {
-          base_amount: 1000,
-          client_id: 99,
-        },
-      );
+      const result = await transactionController.applyRules({
+        base_amount: 1000,
+        client_id: 99,
+      });
 
       expect(result).to.eql(0.03);
     }
 
     {
-      const result = await transactionController.applyRules(
-        [
-          transactionController.discountRule,
-          transactionController.turnoverRule,
-        ],
-        {
-          base_amount: 1,
-          client_id: 99,
-        },
-      );
+      const result = await transactionController.applyRules({
+        base_amount: 1,
+        client_id: 99,
+      });
 
       expect(result).to.eql(0.03);
     }

src/transaction/transaction.controller.ts

@@ -1,5 +1,4 @@
 import { Controller, Post, UsePipes, Body } from '@nestjs/common';
-import _ from 'lodash';
 import {
   ExchangeRateInput,
   ExchangeRateResponse,
@@ -17,6 +16,7 @@ import {
   HighTurnoverDiscount,
 } from './transaction.dto';
 import { Transaction } from './transaction.entity';
+import { getMinimumRuleOutput } from 'src/shared/rules';
 
 type TransactionRuleData = Pick<Transaction, 'base_amount' | 'client_id'>;
 
@@ -90,16 +90,11 @@ export class TransactionController {
       : commissionAmount;
   }
 
-  async applyRules(
-    rules: ((
-      transactionInput: TransactionRuleData,
-    ) => number | false | undefined | Promise<number | false | undefined>)[],
-    transactionInput: TransactionRuleData,
-  ) {
-    const commissions = await Promise.all(
-      [...rules, this.defaultRule].map((rule) => rule(transactionInput)),
+  async applyRules(transactionInput: TransactionRuleData) {
+    return getMinimumRuleOutput(
+      [this.turnoverRule, this.discountRule, this.defaultRule],
+      transactionInput,
     );
-    return Math.min(..._.compact(commissions));
   }
 
   getExchangeRate(exchangeRateInput: ExchangeRateInput) {
@@ -131,10 +126,7 @@ export class TransactionController {
         transactionAmount / exchangeRateResponse[transactionInput.currency],
     };
 
-    const commissionAmount = this.applyRules(
-      [this.turnoverRule, this.discountRule],
-      transactionData,
-    );
+    const commissionAmount = this.applyRules(transactionData);
 
     commissionAmount
       .then((commission) =>
@@ -158,10 +150,7 @@ export class TransactionController {
       base_amount: parseInt(transactionInput.amount),
     };
 
-    const commissionAmount = await this.applyRules(
-      [this.turnoverRule, this.discountRule],
-      transactionData,
-    );
+    const commissionAmount = await this.applyRules(transactionData);
     try {
       this.transactionService
         .insertOne({