From 68a477cb7b177b48d8091a9375e93def8816915a Mon Sep 17 00:00:00 2001
From: Song Chuansheng <songchuansheng@91jinrong.com>
Date: Tue, 4 Dec 2018 15:13:31 +0800
Subject: [PATCH 1/4] add rate / calculate rate of interest per period

---
 finance.js | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/finance.js b/finance.js
index f31c8fd..0ad0bd2 100755
--- a/finance.js
+++ b/finance.js
@@ -201,6 +201,37 @@ Finance.prototype.IAR = function(investmentReturn, inflationRate){
   return 100 * (((1 + investmentReturn) / (1 + inflationRate)) - 1);
 }
 
+/**
+ * Caculate the rate of interest per period.
+ * @param numOfPayments
+ * @param pmt Loan Payment
+ * @param pv Present Value
+ * @param fv Future Value
+ * @param when begin for 1, end for 0
+ */
+Finance.prototype.RATE = function(numOfPayments, pmt, pv, fv, when) {
+
+  var guess = 0.1;
+  var limit = 100; //loop limit
+  var guess_last;
+
+  do {
+    guess_last = guess;
+    guess = guess_last - guess_diff(guess_last, numOfPayments, pmt, pv, fv, when);
+    limit--;
+  }while(guess_last.toFixed(5) != guess.toFixed(5) && limit > 0);
+
+  return guess;
+
+  function guess_diff(guess, nper, pmt, pv, fv, when) {
+    t1 = (guess+1)**nper;
+    t2 = (guess+1)**(nper-1);
+    return ((fv + t1*pv + pmt*(t1 - 1)*(guess*when + 1)/guess) /
+                (nper*t2*pv - pmt*(t1 - 1)*(guess*when + 1)/(guess**2) + nper*pmt*t2*(guess*when + 1)/guess +
+                 pmt*(t1 - 1)*when/guess));
+  }
+}
+
 // XIRR - IRR for irregular intervals
 Finance.prototype.XIRR = function(cfs, dts, guess) {
   if (cfs.length != dts.length) throw new Error('Number of cash flows and dates should match');

From f9f654a5f76946261b0a6a0adf32819971588a5d Mon Sep 17 00:00:00 2001
From: Song Chuansheng <songchuansheng@91jinrong.com>
Date: Tue, 4 Dec 2018 15:30:52 +0800
Subject: [PATCH 2/4] RATE definition

---
 finance.d.ts | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/finance.d.ts b/finance.d.ts
index 52c6a7f..4fe859b 100644
--- a/finance.d.ts
+++ b/finance.d.ts
@@ -138,6 +138,16 @@ export class Finance {
      */
     public PMT(fractionalRate: number, numOfPayments: number, principal: number): number;
 
+    /**
+     * Caculate the rate of interest per period.
+     * @param numOfPayments
+     * @param pmt Loan Payment
+     * @param pv Present Value
+     * @param fv Future Value
+     * @param when begin for 1, end for 0
+     */
+    public RATE(numOfPayments: number, pmt: number, pv: number, fv: number, when: number): number;
+
     /**
      * Inflation-adjusted Return (IAR)
      * Measure the return taking into account the time period's inflation rate

From 93fe8c7576426484bc087f789dc985223e45a917 Mon Sep 17 00:00:00 2001
From: Song Chuansheng <songchuansheng@91jinrong.com>
Date: Tue, 4 Dec 2018 15:36:55 +0800
Subject: [PATCH 3/4] add RATE test

---
 test/index.js | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/test/index.js b/test/index.js
index 67cd382..6e5c33c 100755
--- a/test/index.js
+++ b/test/index.js
@@ -120,6 +120,10 @@ describe('FinanceJS', function() {
     cal.IAR(0.08, 0.03).should.equal(4.854368932038833);
   });
 
+  it('should compute RATE', function() {
+    Number(cal.RATE(12, -933.33, 10000, 0, 0).toFixed(5)).should.equal(0.01788);
+  });
+
   it('should compute XIRR', function() {
     cal.XIRR([-1000, -100, 1200],[new Date(2015, 11, 1 ), new Date(2016, 7, 1 ), new Date(2016, 7, 19 )],0 ).should.equal(14.11);
   });

From 6c5ee9cee87d6e1e9017750569ea6977ebb9cf53 Mon Sep 17 00:00:00 2001
From: Song Chuansheng <songchuansheng@91jinrong.com>
Date: Tue, 4 Dec 2018 16:24:34 +0800
Subject: [PATCH 4/4] use Math.pow instead of **

---
 finance.js | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/finance.js b/finance.js
index 0ad0bd2..c7fc8a7 100755
--- a/finance.js
+++ b/finance.js
@@ -224,10 +224,10 @@ Finance.prototype.RATE = function(numOfPayments, pmt, pv, fv, when) {
   return guess;
 
   function guess_diff(guess, nper, pmt, pv, fv, when) {
-    t1 = (guess+1)**nper;
-    t2 = (guess+1)**(nper-1);
+    t1 = Math.pow((guess+1), nper);
+    t2 = Math.pow((guess+1), nper-1);
     return ((fv + t1*pv + pmt*(t1 - 1)*(guess*when + 1)/guess) /
-                (nper*t2*pv - pmt*(t1 - 1)*(guess*when + 1)/(guess**2) + nper*pmt*t2*(guess*when + 1)/guess +
+                (nper*t2*pv - pmt*(t1 - 1)*(guess*when + 1)/Math.pow(guess,2) + nper*pmt*t2*(guess*when + 1)/guess +
                  pmt*(t1 - 1)*when/guess));
   }
 }