👽️ run prettier; ➕ add tests; 📖 update readme

README.md

@@ -128,7 +128,7 @@ const repayment = compoundInterestPerPeriod({
 
 - `amountPerAnnum: number` The amount of contributions per annum (eg 6_000 for 500 per month)
 - `accrualOfPaymentsPerAnnum: boolean` If provided payments accrue interest per annum; Otherwise interest is only accrued on the principal payment.
-- `contributionPerAnnumChange` Changes of annual contribution in percents (to adjust contribution according inflation rates, good for long investments)
+- `contributionPerAnnumChange: number` Changes of annual contribution in percents (to adjust contribution according inflation rates, good for long investments)
 
 ###### Debt Repayment Options
 

calc/compoundInterest.test.ts

@@ -520,4 +520,30 @@ describe("compoundInterestPerPeriod", () => {
       );
     });
   });
+
+  describe("contributionPerAnnumChange", () => {
+    it("when contributionPerAnnumChange is supplied it increases annual additional contribution amountPerAnnum", () => {
+      const options: IOptions = {
+        type: "contribution",
+        principal: 250_000,
+        rate: 7.8,
+        years: 25,
+        paymentsPerAnnum: 1,
+        amountPerAnnum: 12_000,
+        contributionPerAnnumChange: 3,
+        currentPositionInYears: 1,
+        accrualOfPaymentsPerAnnum: true
+      };
+      const result = compoundInterestPerPeriod(options);
+      expect(result).toMatchObject(
+        expect.objectContaining({
+          currentBalance: 282436,
+          totalInterest: 2144895.2679852117,
+          endBalance: 2832406.46,
+          accrualOfPaymentsPerAnnum: true,
+          investmentType: "contribution"
+        })
+      );
+    });
+  });
 });

calc/compoundInterest.ts

@@ -43,12 +43,12 @@ export const calcTotalInvestment = (options: IOptions, investmentType: Investmen
   if (investmentType === "contribution" && "amountPerAnnum" in options) {
     const { amountPerAnnum = 0, contributionPerAnnumChange = 0 } = options;
     // Adjust annual contributions if contributionPerAnnumChange rate provided
-    if ( contributionPerAnnumChange > 0 ) {
+    if (contributionPerAnnumChange > 0) {
       let repaymentWithAnnualChange = amountPerAnnum;
       for (let i = 1; i < years; i++) {
-        repaymentWithAnnualChange += repaymentWithAnnualChange * contributionPerAnnumChange / 100 + amountPerAnnum;
+        repaymentWithAnnualChange += (repaymentWithAnnualChange * contributionPerAnnumChange) / 100 + amountPerAnnum;
       }
-      return principal * years + repaymentWithAnnualChange
+      return principal * years + repaymentWithAnnualChange;
     }
     return principal + amountPerAnnum * years;
   }
@@ -197,7 +197,7 @@ export const compoundInterestPerPeriod = (options: IOptions): CompoundInterestRe
       if (accrualOfPaymentsPerAnnum) {
         // Adjust contributions only from the 2nd year
         if (i >= 1 && contributionPerAnnumChange) {
-          amountPerAnnum = amountPerAnnum * contributionPerAnnumChange / 100 + amountPerAnnum;
+          amountPerAnnum = amountPerAnnum * (1 + contributionPerAnnumChange / 100);
         }
         const newBalanceWithAccrual = prevBalance + amountPerAnnum / paymentsPerAnnum;
         const interest = newBalanceWithAccrual * ratePerPeriod;