TheAlgorithms/JavaScript · error · TypeError

The second param should be a boolean

Error message

The second param should be a boolean

What it means

Second-parameter guard in checkWordOccurrence. The optional isCaseSensitive parameter defaults to false, but if a caller passes it explicitly and it is not a boolean, the function throws TypeError. Note: because the default only applies when the argument is omitted, passing undefined explicitly still triggers the default and does NOT throw — but passing 'true' (string), 1, 0, or null does throw. Also note the downstream logic is inverted (isCaseSensitive lowercases the string), but the error itself is purely the type check.

Source

Thrown at String/CheckWordOccurrence.js:14

/**
 * @function checkWordOccurrence
 * @description - this function count all the words in a sentence and return an word occurrence object
 * @param {string} str
 * @param {boolean} isCaseSensitive
 * @returns {Object}
 */
const checkWordOccurrence = (str, isCaseSensitive = false) => {
  if (typeof str !== 'string') {
    throw new TypeError('The first param should be a string')
  }

  if (typeof isCaseSensitive !== 'boolean') {
    throw new TypeError('The second param should be a boolean')
  }

  const modifiedStr = isCaseSensitive ? str.toLowerCase() : str

  return modifiedStr
    .split(/\s+/) // remove all spaces and distribute all word in List
    .reduce((occurrence, word) => {
      occurrence[word] = occurrence[word] + 1 || 1
      return occurrence
    }, {})
}

export { checkWordOccurrence }

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Pass an actual boolean literal or convert first: flag === true or String(flag) === 'true'.
2. Omit the second argument entirely to accept the default (false).
3. Normalize at the boundary: const sens = String(raw).toLowerCase() === 'true'; then call with sens.

Example fix

// before
checkWordOccurrence(text, rawFlag)

// after
checkWordOccurrence(text, rawFlag === true || String(rawFlag).toLowerCase() === 'true')

const isCaseSensitive =
  rawFlag === true || String(rawFlag).toLowerCase() === 'true'
checkWordOccurrence(str, isCaseSensitive)

const isBoolean = (v) => typeof v === 'boolean'

try {
  checkWordOccurrence(str, flag)
} catch (e) {
  if (e instanceof TypeError && /second param/i.test(e.message)) { /* coerce and retry */ } else throw e
}

When it happens

Trigger: Calling checkWordOccurrence('a b a', 'true'), checkWordOccurrence('a b a', 1), checkWordOccurrence('a b a', null), or checkWordOccurrence('a b a', 'yes'). The second arg is present and typeof !== 'boolean'.

Common situations: Reading a flag from a config file or query string where booleans arrive as strings ('true'/'false'); deserialized JSON that used 0/1 for booleans; spreading an object whose flag field is undefined-null.

Related errors

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

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/7ff289b7dd94037a. Report an issue: GitHub.