TheAlgorithms/JavaScript · error · TypeError

Invalid Input Type

Error message

Invalid Input Type

What it means

Guard in lower. The function lowercases a string by replacing [A-Z] with charCode+32 equivalents and requires the input to be a string, throwing TypeError otherwise. This protects .replace() from non-string values.

Source

Thrown at String/Lower.js:12

/**
 * @function lower
 * @description Will convert the entire string to lowercase letters.
 * @param {String} str - The input string
 * @returns {String} Lowercase string
 * @example lower("HELLO") => hello
 * @example lower("He_llo") => he_llo
 */

const lower = (str) => {
  if (typeof str !== 'string') {
    throw new TypeError('Invalid Input Type')
  }

  return str.replace(/[A-Z]/g, (char) =>
    String.fromCharCode(char.charCodeAt() + 32)
  )
}

export default lower

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Pass a string; default missing values to ''.
2. Validate typeof at the call site.
3. Consider the native str.toLowerCase() for full Unicode coverage.

Example fix

// before
lower(maybeStr)

// after
lower(typeof maybeStr === 'string' ? maybeStr : '')

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

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

try {
  lower(input)
} catch (e) {
  if (e instanceof TypeError && /Invalid Input Type/i.test(e.message)) { /* not a string */ } else throw e
}

When it happens

Trigger: Calling lower(undefined), lower(null), lower(42), lower(['A']). Any input where typeof !== 'string'.

Common situations: Optional field omitted; value coerced to a number earlier; array passed instead of a string. Note: for production use String.prototype.toLowerCase() is more robust (handles Unicode), so calling this custom lower is itself a choice to flag.

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/f94dcf009fd45703. Report an issue: GitHub.