TheAlgorithms/JavaScript · error · TypeError

Input should be a string

Error message

Input should be a string

What it means

First guard in countLetters. The function counts each letter in a string and rejects non-string input with TypeError before any further processing. This is the outermost of three stacked guards (type, then special characters, then digits).

Source

Thrown at String/CountLetters.js:13

/**
 * @function countLetters
 * @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
}

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Pass a string; if the source is missing, default to '' (note '' returns {}).
2. Validate typeof at the call site.
3. If the source is an array of chars, join first.

Example fix

// before
countLetters(input)

// after
countLetters(typeof input === 'string' ? input : '')

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

const isString = (v) => typeof v === 'string'

try {
  countLetters(input)
} catch (e) {
  if (e instanceof TypeError && /should be a string/i.test(e.message)) { /* handle */ } else throw e
}

When it happens

Trigger: Calling countLetters(undefined), countLetters(null), countLetters(42), countLetters(['a','b']). Any input where typeof !== 'string'.

Common situations: A function that previously received a string now receives a parsed JSON object; user input that was Number()-coerced earlier; an array passed where a flattened string was intended.

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 must not contain special characters

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/cd90e90ee74cdbbb. Report an issue: GitHub.