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 }