TheAlgorithms/JavaScript · error · TypeError

Argument is not a string.

Error message

Argument is not a string.

What it means

Pre-condition guard at the top of CheckPascalCase. The function tests a variable name against the regex /^[A-Z][A-Za-z]*$/ (an uppercase letter followed by zero or more letters). Before it can call regex.test(), it rejects any input whose typeof is not 'string' with TypeError. This is the library's standard pattern for protecting .test() from non-string arguments.

Source

Thrown at String/CheckPascalCase.js:13

// CheckPascalCase method checks the given string is in PascalCase or not.

// Problem Source & Explanation: https://www.theserverside.com/definition/Pascal-case

/**
 * CheckPascalCase method returns true if the string in PascalCase, else return the false.
 * @param {String} VarName the name of the variable to check.
 * @returns `Boolean` return true if the string is in PascalCase, else return false.
 */
const CheckPascalCase = (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][A-Za-z]*$/
  return pat.test(VarName)
}

export { CheckPascalCase }

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Pass only string values: coerce or validate the source before calling, e.g. ensure the value is a string via typeof.
2. Default the variable when it may be missing: const name = raw?.name ?? ''.
3. Wrap the call in try/catch when the input crosses a trust boundary (user input, external API payload).

Example fix

// before
CheckPascalCase(maybeName)

// after
if (typeof maybeName === 'string') CheckPascalCase(maybeName)
// or
CheckPascalCase(typeof maybeName === 'string' ? maybeName : '')

if (typeof VarName !== 'string') {
  throw new TypeError('VarName must be a string, got ' + typeof VarName)
}
CheckPascalCase(VarName)

const isNonEmptyString = (v) => typeof v === 'string' && v.length > 0

try {
  CheckPascalCase(name)
} catch (e) {
  if (e instanceof TypeError && /not a string/i.test(e.message)) {
    // handle bad input type
  } else throw e
}

When it happens

Trigger: Calling CheckPascalCase(undefined), CheckPascalCase(null), CheckPascalCase(42), CheckPascalCase({name:'Foo'}), or CheckPascalCase(['Foo']). Any value where typeof !== 'string'.

Common situations: Reading an optional field from JSON.parse where the key is missing (yields undefined); form/CLI input parsed as a number; a destructured object property that was renamed or absent; spreading an array element that is undefined.

Related errors

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

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/ca476222f41b5c2e. Report an issue: GitHub.