TheAlgorithms/JavaScript · error · TypeError

Argument should be a string

Error message

Argument should be a string

What it means

First guard in maxCharacter. The function returns the most-frequent character (optionally ignoring a RegExp pattern) and requires str to be a string, throwing TypeError otherwise. This is the type branch; the emptiness branch is a separate error ([156]). Note the empty-string check (!str) is unreachable from a non-string because typeof short-circuits first.

Source

Thrown at String/MaxCharacter.js:11

/**
 * @function maxCharacter
 * @example - Given a string of characters, return the character that appears the most often. Example: input = "Hello World!" return "l"
 * @param {string} str
 * @param {RegExp} ignorePattern - ignore the char in str that is not required
 * @returns {string} - char
 */
const maxCharacter = (str, ignorePattern) => {
  // initially it's count only alphabets
  if (typeof str !== 'string') {
    throw new TypeError('Argument should be a string')
  } else if (!str) {
    throw new Error('The param should be a nonempty string')
  }

  // store all char in occurrence map
  const occurrenceMap = new Map()

  for (const char of str) {
    if (!ignorePattern?.test(char)) {
      occurrenceMap.set(char, occurrenceMap.get(char) + 1 || 1)
    }
  }

  // find the max char from the occurrence map
  let max = { char: '', occur: -Infinity }

  for (const [char, occur] of occurrenceMap) {
    if (occur > max.occur) {

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Pass a string; default missing values to '' (note '' triggers the empty-string error, so pick a real default).
2. Validate typeof at the call site.
3. If the source is an array, join first.

Example fix

// before
maxCharacter(maybeStr)

// after
if (typeof maybeStr === 'string' && maybeStr.length) maxCharacter(maybeStr)

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

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

try {
  maxCharacter(input)
} catch (e) {
  if (e instanceof TypeError && /should be a string/i.test(e.message)) { /* not a string */ } else throw e
}

When it happens

Trigger: Calling maxCharacter(undefined), maxCharacter(null), maxCharacter(0), maxCharacter({}), maxCharacter([]). Any input where typeof !== 'string'.

Common situations: A field that is null on miss; a value coerced to a number; an array passed where a string was expected; refactoring that dropped the argument.

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/582aa0eaa93069d6. Report an issue: GitHub.