Fix On Balance Volume (OBV) calculation. (#428)

# Describe Request

Fix On Balance Volume (OBV)  calculation.

Fixed #426 

# Change Type

Bug Fix

src/indicator/volume/onBalanceVolume.test.ts

@@ -0,0 +1,16 @@
+// Copyright (c) 2022 Onur Cinar. All Rights Reserved.
+// https://github.com/cinar/indicatorts
+
+import { roundDigitsAll } from '../../index';
+import { onBalanceVolume } from './onBalanceVolume';
+
+describe('On Balance Volume (OBV)', () => {
+  it('should be able to compute OBV', () => {
+    const closings = [9, 11, 7, 10, 8];
+    const volumes = [100, 110, 80, 120, 90];
+    const expected = [0, 110, 30, 150, 60];
+
+    const actual = onBalanceVolume(closings, volumes);
+    expect(roundDigitsAll(2, actual)).toStrictEqual(expected);
+  });
+});

src/indicator/volume/onBalanceVolume.ts

@@ -23,17 +23,15 @@ export function onBalanceVolume(
 
   const result = new Array<number>(closings.length);
 
+  result[0] = 0;
+
   for (let i = 1; i < result.length; i++) {
-    if (i === 0) {
-      result[i] = 0;
-    } else {
-      result[i] = result[i - 1];
+    result[i] = result[i - 1];
 
-      if (closings[i] > closings[i - 1]) {
-        result[i] += volumes[i];
-      } else if (closings[i] < closings[i - 1]) {
-        result[i] -= volumes[i];
-      }
+    if (closings[i] > closings[i - 1]) {
+      result[i] += volumes[i];
+    } else if (closings[i] < closings[i - 1]) {
+      result[i] -= volumes[i];
     }
   }