Add Phone Number Formatting Function

String/FormatPhoneNumber.js

@@ -0,0 +1,17 @@
+// function that takes 10 digits and returns a string of the formatted phone number
+// e.g.: 1234567890 -> (123) 456-7890
+
+const formatPhoneNumber = (numbers) => {
+  const numbersString = numbers.toString()
+  if ((numbersString.length !== 10) || isNaN(numbersString)) {
+    // return "Invalid phone number."
+    throw new TypeError('Invalid phone number.')
+  }
+  const arr = '(XXX) XXX-XXXX'.split('')
+  Array.from(numbersString).forEach(n => {
+    arr[arr.indexOf('X')] = n
+  })
+  return arr.join('')
+}
+
+export { formatPhoneNumber }

String/FormatPhoneNumber.test.js

@@ -0,0 +1,23 @@
+import { formatPhoneNumber } from './FormatPhoneNumber'
+
+describe('PhoneNumberFormatting', () => {
+  it('expects to return the formatted phone number', () => {
+    expect(formatPhoneNumber('1234567890')).toEqual('(123) 456-7890')
+  })
+
+  it('expects to return the formatted phone number', () => {
+    expect(formatPhoneNumber(1234567890)).toEqual('(123) 456-7890')
+  })
+
+  it('expects to throw a type error', () => {
+    expect(() => { formatPhoneNumber('1234567') }).toThrow('Invalid phone number.')
+  })
+
+  it('expects to throw a type error', () => {
+    expect(() => { formatPhoneNumber('123456text') }).toThrow('Invalid phone number.')
+  })
+
+  it('expects to throw a type error', () => {
+    expect(() => { formatPhoneNumber(12345) }).toThrow('Invalid phone number.')
+  })
+})