TheAlgorithms/JavaScript · error · TypeError

Input must not contain numbers

Error message

Input must not contain numbers

What it means

Third guard in countLetters, fired by /\d/. After the type check and the special-character check, any decimal digit (0-9) throws TypeError. Combined with the previous guard this means countLetters only accepts pure ASCII letters [A-Za-z] (and underscores, which survive \W but produce an underscore key).

Source

Thrown at String/CountLetters.js:21

 * @description Given a string, count the number of each letter.
 * @param {String} str - The input string
 * @return {Object} - Object with letters and number of times
 * @example countLetters("hello") => {h: 1, e: 1, l: 2, o: 1}
 */

const countLetters = (str) => {
  const specialChars = /\W/g

  if (typeof str !== 'string') {
    throw new TypeError('Input should be a string')
  }

  if (specialChars.test(str)) {
    throw new TypeError('Input must not contain special characters')
  }

  if (/\d/.test(str)) {
    throw new TypeError('Input must not contain numbers')
  }

  const obj = {}
  for (let i = 0; i < str.toLowerCase().length; i++) {
    const char = str.toLowerCase().charAt(i)
    obj[char] = (obj[char] || 0) + 1
  }

  return obj
}

export { countLetters }

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Strip digits first: str.replace(/\d/g, '').
2. Use a more permissive counter if you need digits included.
3. Pre-validate the input is letters-only: /^[A-Za-z]+$/.test(str).

Example fix

// before
countLetters('abc123')

// after
countLetters('abc123'.replace(/\d/g, ''))

const clean = str.replace(/\d/g, '')
countLetters(clean)

const hasNoDigits = (s) => typeof s === 'string' && !/\d/.test(s)

try {
  countLetters(str)
} catch (e) {
  if (e instanceof TypeError && /numbers/i.test(e.message)) {
    countLetters(str.replace(/\d/g, ''))
  } else throw e
}

When it happens

Trigger: Calling countLetters('abc123'), countLetters('a1b2'), countLetters('2024'). Any string containing a digit that already passed the special-char check.

Common situations: Alphanumeric identifiers (usernames, codes) passed where letters-only was assumed; alphanumeric reference numbers; forgetting the function is letters-only despite its name.

Related errors

• Input must not contain special characters
• Input should be a string
• Argument is not a valid HEX code!
• Invalid hex string.
• The given value is not a string

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/7a2d91d664ca0880. Report an issue: GitHub.