TheAlgorithms/JavaScript · error · TypeError

The given value is not a string

Error message

The given value is not a string

What it means

Thrown by reverseWords() in String/ReverseWords.js when typeof str !== 'string'. A TypeError raised before the .split(/\s+/) pipeline runs, because split only exists meaningfully on strings and the reduceRight builds a new string. Only the type is checked, so an empty string '' is accepted and returns '' after trim.

Source

Thrown at String/ReverseWords.js:8

/**
 * @function reverseWords
 * @param {string} str
 * @returns {string} - reverse string
 */
const reverseWords = (str) => {
  if (typeof str !== 'string') {
    throw new TypeError('The given value is not a string')
  }

  return str
    .split(/\s+/) // create an array with each word in string
    .reduceRight((reverseStr, word) => `${reverseStr} ${word}`, '') // traverse the array from last & create an string
    .trim() // remove the first useless space
}

export default reverseWords

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Pass a string primitive: reverseWords('hello world').
2. Coerce at the boundary: reverseWords(String(value)).
3. Verify the upstream data shape — if value is sometimes non-string, fix the producer (e.g. schema-validate request bodies).
4. If you intentionally store text in an object, read the right property before passing it.

Example fix

// before
const r = reverseWords(req.body.title) // title is sometimes a number in malformed payloads

// after
const r = reverseWords(typeof req.body.title === 'string' ? req.body.title : String(req.body.title ?? ''))

function reverseWordsSafe(v) {
  if (typeof v !== 'string') v = String(v ?? '')
  return reverseWords(v)
}

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

try { reverseWords(s) } catch (e) { if (e instanceof TypeError && /not a string/.test(e.message)) s = String(s); else throw e }

When it happens

Trigger: Calling reverseWords(42), reverseWords(null), reverseWords(undefined), reverseWords(['hello world']), reverseWords({ text: 'hi' }). Empty string reverseWords('') does NOT throw — it returns ''.

Common situations: Reading req.body fields that arrived as a number from JSON; passing a Node Buffer; forgetting to .toString() a value read from a stream; mixing up argument order so an options object lands in str.

Related errors

• The arg must be a valid, non empty string
• The given value is not a string
• The given value is not a string
• Email Address String Null or Empty.
• Invalid Input

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/0da2bd8620e7c4cb. Report an issue: GitHub.