feat(round): function for simple decimal rounding (#94)

Author: mjclyde

src/index.js

@@ -30,6 +30,7 @@ import mod from './mod'
 import shallowEqual from './shallow-equal'
 import swapCase from './swap-case'
 import endsWith from './endsWith'
+import round from './round'
 
 export {
   initArray,
@@ -64,4 +65,5 @@ export {
   shallowEqual,
   swapCase,
   endsWith,
+  round,
 }

src/round.js

@@ -0,0 +1,19 @@
+export default round
+
+/**
+ * Original Source: https://stackoverflow.com/questions/11832914/round-to-at-most-2-decimal-places-only-if-necessary
+ *
+ * This method will round a number to the specified decimal places.
+ *
+ * @param {number} num - Number to round
+ * @param {number} decimalPlaces - Decimal places to be rounded to
+ * @return {number} - Rounded number
+ */
+
+function round(num, decimalPlaces) {
+  if (decimalPlaces < 0) {
+    throw new Error('decimalPlaces cannot be negative')
+  }
+
+  return +(`${Math.round(+`${num}e+${decimalPlaces}`)}e-${decimalPlaces}`)
+}

test/round.test.js

@@ -0,0 +1,34 @@
+import test from 'ava'
+import {round} from '../src'
+
+test('Round to 0 places', t => {
+  t.is(round(1.5, 0), 2)
+})
+
+test('Round to 1 places down', t => {
+  t.is(round(1.54, 1), 1.5)
+})
+
+test('Round to 1 places up', t => {
+  t.is(round(1.55, 1), 1.6)
+})
+
+test('Round to 2 places down', t => {
+  t.is(round(100.334, 2), 100.33)
+})
+
+test('Round to 2 places up', t => {
+  t.is(round(100.335, 2), 100.34)
+})
+
+test('Round negative number', t => {
+  t.is(round(-123.4532, 1), -123.5)
+})
+
+test('Throws error if decimal places is negative', t => {
+  const error = t.throws(() => {
+    round(5.876, -1)
+  })
+  t.is(error.message, 'decimalPlaces cannot be negative')
+})
+