TheAlgorithms/JavaScript · error · TypeError

Invalid phone number!

Error message

Invalid phone number!

What it means

Guard in formatPhoneNumber: it throws when phoneNumber.length !== 10 OR isNaN(phoneNumber). The function expects a STRING of exactly 10 numeric digits and rebuilds the '(XXX) XXX-XXXX' pattern by indexing into the string. Because it reads .length, a Number has no length (undefined !== 10) and throws; because it uses the coercion-based isNaN, a string of 10 spaces incorrectly passes the numeric check (latent bug). Empty or wrong-length strings throw.

Source

Thrown at String/FormatPhoneNumber.js:9

/**
 * @description - function that takes 10 digits and returns a string of the formatted phone number e.g.: 1234567890 -> (123) 456-7890
 * @param {string} phoneNumber
 * @returns {string} - Format to (XXX) XXX-XXXX pattern
 */
const formatPhoneNumber = (phoneNumber) => {
  if (phoneNumber.length !== 10 || isNaN(phoneNumber)) {
    // return "Invalid phone number."
    throw new TypeError('Invalid phone number!')
  }

  let index = 0
  return '(XXX) XXX-XXXX'.replace(/X/g, () => phoneNumber[index++])
}

export default formatPhoneNumber

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Pass a string of exactly 10 digits: strip non-digits first, e.g. String(n).replace(/\D/g, '').
2. Validate length === 10 and /^[0-9]{10}$/.test(value) before calling.
3. Prefer Number.isFinite over the library's isNaN if you fork/patch, to close the whitespace hole.

Example fix

// before
formatPhoneNumber(rawPhone)

// after
const digits = String(rawPhone).replace(/\D/g, '')
if (/^[0-9]{10}$/.test(digits)) formatPhoneNumber(digits)

const digits = String(phoneNumber).replace(/\D/g, '')
if (!/^[0-9]{10}$/.test(digits)) {
  throw new TypeError('phoneNumber must be exactly 10 digits')
}
formatPhoneNumber(digits)

const isTenDigitString = (v) => typeof v === 'string' && /^[0-9]{10}$/.test(v)

try {
  formatPhoneNumber(raw)
} catch (e) {
  if (e instanceof TypeError && /Invalid phone number/i.test(e.message)) {
    // sanitize and retry, or surface to user
  } else throw e
}

When it happens

Trigger: Calling formatPhoneNumber(1234567890) (number, no .length), formatPhoneNumber('123') (too short), formatPhoneNumber('12345678901') (too long), formatPhoneNumber('12345678ab') (non-numeric), formatPhoneNumber('') (length 0), formatPhoneNumber(null) (throws on .length access, possibly a different error).

Common situations: Receiving a phone number as a Number from a form/database instead of a string; formatted input with dashes/parens already applied (length > 10); stripped input that retained a country code; whitespace-only input slipping through due to the isNaN coercion bug.

Related errors

• Argument is not a string.
• Argument is not a string.
• The first param should be a string
• The second param should be a boolean
• Input should be a string

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/5f1f01585511d89f. Report an issue: GitHub.