feat(cube): Add cube function (#119)

Add a function to find the cube of a number.

Author: akoliyot

src/cube.js

@@ -0,0 +1,12 @@
+export default cube
+
+/**
+ * This method will perform cube of a number.
+ *
+ * @param {Number} num - Input number
+ * @return {Number} - Result of cube of a number
+ */
+
+function cube(num) {
+  return num * num * num
+}

src/index.js

@@ -24,6 +24,7 @@ import subtraction from './subtraction'
 import divide from './divide'
 import multiply from './multiply'
 import square from './square'
+import cube from './cube'
 import sum from './sum'
 import dec2bin from './dec2bin'
 import searchAndReplace from './search-and-replace'
@@ -67,6 +68,7 @@ export {
   divide,
   multiply,
   square,
+  cube,
   sum,
   dec2bin,
   searchAndReplace,

test/cube.test.js

@@ -0,0 +1,9 @@
+import test from 'ava'
+import {cube} from '../src'
+
+test('Cube a number ', t => {
+  const number = 5
+  const expected = 5 * 5 * 5
+  const actual = cube(number)
+  t.deepEqual(actual, expected)
+})