TheAlgorithms/JavaScript · error · TypeError

Argument is not a string.

Error message

Argument is not a string.

What it means

Thrown as a TypeError by checkFlatCase() when varname is not a string. The function tests varname against /^[a-z]*$/ via regex; the explicit guard rejects non-strings before coercion could mask bugs. Flatcase means all-lowercase with no separators (e.g. 'thisvariable').

Source

Thrown at String/CheckFlatCase.js:15

// checkFlatCase method checks if the given string is in flatcase or not. Flatcase is a convention
// where all letters are in lowercase, and there are no spaces between words.
// thisvariable is an example of flatcase. In camelCase it would be thisVariable, snake_case this_variable and so on.

// Problem Source & Explanation: https://en.wikipedia.org/wiki/Naming_convention_(programming)

/**
 * checkFlatCase method returns true if the string in flatcase, else return the false.
 * @param {string} varname the name of the variable to check.
 * @returns {boolean} return true if the string is in flatcase, else return false.
 */
const checkFlatCase = (varname) => {
  // firstly, check that input is a string or not.
  if (typeof varname !== 'string') {
    throw new TypeError('Argument is not a string.')
  }

  const pat = /^[a-z]*$/
  return pat.test(varname)
}

export { checkFlatCase }

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Coerce: checkFlatCase(String(name)).
2. Type-guard: if (typeof name === 'string').
3. Default missing values to empty string.

Example fix

// before
checkFlatCase(identifier)

// after
checkFlatCase(String(identifier ?? ''))

function safeCheckFlatCase(name) {
  if (typeof name !== 'string') {
    throw new TypeError('Expected a string')
  }
  return checkFlatCase(name)
}

function isString(v) {
  return typeof v === 'string'
}

try {
  checkFlatCase(name)
} catch (e) {
  if (e instanceof TypeError && e.message === 'Argument is not a string.') {
    return false
  }
  throw e
}

When it happens

Trigger: Calling checkFlatCase(null), checkFlatCase(undefined), checkFlatCase(123), or checkFlatCase(['name']).

Common situations: Identifier validation on data that may be missing; numeric or symbol identifiers from generated code.

Related errors

• Argument is not a string.
• Argument is not a string.
• Invalid Input
• Argument should be string
• Both arguments should be strings.

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/ccf5312e303ce797. Report an issue: GitHub.