TheAlgorithms/JavaScript · error · TypeError

Argument should be string

Error message

Argument should be string

What it means

Guard in firstUniqChar. The function finds the index of the first non-repeating character and requires the input to be a string, throwing TypeError otherwise. (Note: the body misuses a Map with bracket access, but the guard itself is a clean type check.)

Source

Thrown at String/FirstUniqueCharacter.js:13

/**
 * @function firstUniqChar
 * @description Given a string str, find the first non-repeating character in it and return its index. If it does not exist, return -1.
 * @param {String} str - The input string
 * @return {Number} - The index of first unique character.
 * @example firstUniqChar("javascript") => 0
 * @example firstUniqChar("sesquipedalian") => 3
 * @example firstUniqChar("aabb") => -1
 */

const firstUniqChar = (str) => {
  if (typeof str !== 'string') {
    throw new TypeError('Argument should be string')
  }
  const count = new Map()

  for (const char of str) {
    if (!count[char]) {
      count[char] = 1
    } else {
      count[char]++
    }
  }
  for (let i = 0; i < str.length; i++) {
    if (count[str[i]] === 1) return i
  }
  return -1
}

export { firstUniqChar }

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Pass a string; coerce numbers with String(n) or n.toString() first.
2. Default missing values to '' (returns -1).
3. Validate typeof at the boundary.

Example fix

// before
firstUniqChar(maybeStr)

// after
firstUniqChar(typeof maybeStr === 'string' ? maybeStr : String(maybeStr ?? ''))

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

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

try {
  firstUniqChar(input)
} catch (e) {
  if (e instanceof TypeError) { /* not a string */ } else throw e
}

When it happens

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

Common situations: A LeetCode-style input that arrives as a number when a string was expected; a payload field that is null on miss; a destructured value that does not exist.

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/a9b1fce3a2ddfc53. Report an issue: GitHub.