feat: convers a number to string representation (#227)

Converts a positive integer up to 2^53-1 to its English string representation.

src/index.js

@@ -85,6 +85,7 @@ import first from './first'
 import mode from './mode-array'
 import rollDice from './rollDice'
 import inchesToMetric from './inches-to-metric'
+import numberToString from './numberToString'
 
 export {
   reverseArrayInPlace,
@@ -174,4 +175,5 @@ export {
   mode,
   rollDice,
   inchesToMetric,
+  numberToString,
 }

src/numberToString.js

@@ -0,0 +1,69 @@
+/*eslint complexity: ["error", 10]*/
+export default numberToString
+
+/**
+ * Original source: https://stackoverflow.com/questions/5529934/javascript-numbers-to-words
+ *
+ * Converts an int to its string representation
+ *
+ * @param {Number} number - input integer (MAX_SAFE_INTEGER)
+ * @return {String} - String representation of the integer
+ */
+
+const stringLookup = [
+  {key: 1000000000000000, value: 'quadrillion'},
+  {key: 1000000000000, value: 'trillion'},
+  {key: 1000000000, value: 'billion'},
+  {key: 1000000, value: 'million'},
+  {key: 1000, value: 'thousand'},
+  {key: 100, value: 'hundred'},
+  {key: 90, value: 'ninety'},
+  {key: 80, value: 'eighty'},
+  {key: 70, value: 'seventy'},
+  {key: 60, value: 'sixty'},
+  {key: 50, value: 'fifty'},
+  {key: 40, value: 'forty'},
+  {key: 30, value: 'thirty'},
+  {key: 20, value: 'twenty'},
+  {key: 19, value: 'nineteen'},
+  {key: 18, value: 'eighteen'},
+  {key: 17, value: 'seventeen'},
+  {key: 16, value: 'sixteen'},
+  {key: 15, value: 'fifteen'},
+  {key: 14, value: 'fourteen'},
+  {key: 13, value: 'thirteen'},
+  {key: 12, value: 'twelve'},
+  {key: 11, value: 'eleven'},
+  {key: 10, value: 'ten'},
+  {key: 9, value: 'nine'},
+  {key: 8, value: 'eight'},
+  {key: 7, value: 'seven'},
+  {key: 6, value: 'six'},
+  {key: 5, value: 'five'},
+  {key: 4, value: 'four'},
+  {key: 3, value: 'three'},
+  {key: 2, value: 'two'},
+  {key: 1, value: 'one'},
+]
+
+function numberToString(number) {
+  if (number <= 0 || number > Number.MAX_SAFE_INTEGER) {
+    return 'Number needs to be grater than 0 or less than 2^53-1.'
+  }
+  let result = ''
+
+  for (const n of stringLookup) {
+    if (number >= n.key) {
+      if (number < 100) {
+        result += n.value
+        number -= n.key
+        number > 0 ? result += ' ' : result
+      } else {
+        const t = Math.floor(number / n.key)
+        const d = number % n.key
+        return d > 0 ? `${numberToString(t)} ${n.value} ${numberToString(d)}` : `${numberToString(t)} ${n.value}`
+      }
+    }
+  }
+  return result
+}

test/numberToString.test.js

@@ -0,0 +1,35 @@
+import test from 'ava'
+import {numberToString} from '../src'
+
+test('Convert lowest allowed number to string', t => {
+  const number = 1
+  const stringNum = 'one'
+  const actual = numberToString(number)
+  t.deepEqual(actual, stringNum)
+})
+
+test('Convert high number like 214124124 to string', t => {
+  const number = 214124124
+  const stringNum = `two hundred fourteen million one hundred twenty four thousand one hundred twenty four`
+  const actual = numberToString(number)
+  t.deepEqual(actual, stringNum)
+})
+
+test('Convert 1000 to string', t => {
+  const number = 1000
+  const stringNum = `one thousand`
+  const actual = numberToString(number)
+  t.deepEqual(actual, stringNum)
+})
+
+test('Zero and negative numbers should return error message', t => {
+  const actual = numberToString(0)
+  const errorMessage = 'Number needs to be grater than 0 or less than 2^53-1.'
+  t.deepEqual(actual, errorMessage)
+})
+
+test('Numbers greater than Number.MAX_SAFE_INTEGER should return error message', t => {
+  const actual = numberToString(Number.MAX_SAFE_INTEGER + 1)
+  const errorMessage = 'Number needs to be grater than 0 or less than 2^53-1.'
+  t.deepEqual(actual, errorMessage)
+})