TheAlgorithms/JavaScript · error · TypeError

Duplicate characters in character set are not allowed

Error message

Duplicate characters in character set are not allowed

What it means

Thrown by convertArbitraryBase after the type guard passes, when either baseOneCharacterString or baseTwoCharacterString contains repeated characters. Uniqueness is required because each character maps bijectively to a digit value; a duplicate would make the value of a digit ambiguous and break both encoding and decoding. The check compares array length against a Set of the same characters.

Source

Thrown at Conversions/ArbitraryBase.js:39

const convertArbitraryBase = (
  stringInBaseOne,
  baseOneCharacterString,
  baseTwoCharacterString
) => {
  if (
    [stringInBaseOne, baseOneCharacterString, baseTwoCharacterString]
      .map((arg) => typeof arg)
      .some((type) => type !== 'string')
  ) {
    throw new TypeError('Only string arguments are allowed')
  }

  const baseOneCharacters = [...baseOneCharacterString]
  const baseTwoCharacters = [...baseTwoCharacterString]

  for (const charactersInBase of [baseOneCharacters, baseTwoCharacters]) {
    if (charactersInBase.length !== new Set(charactersInBase).size) {
      throw new TypeError(
        'Duplicate characters in character set are not allowed'
      )
    }
  }
  const reversedStringOneChars = [...stringInBaseOne].reverse()
  const stringOneBase = baseOneCharacters.length
  let value = 0
  let placeValue = 1
  for (const digit of reversedStringOneChars) {
    const digitNumber = baseOneCharacters.indexOf(digit)
    if (digitNumber === -1) {
      throw new TypeError(`Not a valid character: ${digit}`)
    }
    value += digitNumber * placeValue
    placeValue *= stringOneBase
  }
  const outputChars = []
  const stringTwoBase = baseTwoCharacters.length

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Audit each character-set string and remove duplicates (e.g. [...set].join('')).
2. Generate alphabets programmatically from a known-unique source, e.g. '0123456789abcdef...', rather than typing them.
3. Add a pre-flight assertion in your caller: new Set(alpha).size === alpha.length.

Example fix

// before
convertArbitraryBase('10', '01', '0011')
// after
const dedupe = (s) => [...new Set([...s])].join('')
convertArbitraryBase('10', '01', dedupe('0011')) // -> '01'

const hasUniqueChars = (s) => [...s].length === new Set([...s]).size
if (!hasUniqueChars(srcAlpha) || !hasUniqueChars(dstAlpha)) {
  throw new Error('Character set must have unique glyphs')
}
convertArbitraryBase(input, srcAlpha, dstAlpha)

const isUniqueCharset = (s) =>
  typeof s === 'string' && [...s].length === new Set([...s]).size

try {
  convertArbitraryBase(input, srcAlpha, dstAlpha)
} catch (e) {
  if (/Duplicate characters/.test(e.message)) {
    srcAlpha = [...new Set([...srcAlpha])].join('')
    dstAlpha = [...new Set([...dstAlpha])].join('')
    return convertArbitraryBase(input, srcAlpha, dstAlpha)
  }
  throw e
}

When it happens

Trigger: Calling convertArbitraryBase('10', '01', '0011') (output set '0011' has duplicate 0 and 1), or a source alphabet like '01234567890' with a repeated 0. Either character set triggering the duplicate is sufficient since the loop covers both arrays.

Common situations: Hand-typing a custom alphabet and accidentally repeating a glyph; reusing a charset constant that was concatenated incorrectly; using multibyte/emoji strings where a grapheme got duplicated; building a base alphabet from a range with an off-by-one overlap.

Related errors

• Not a valid character: ${digit}
• Only string arguments are allowed
• Argument is not a valid HEX code!
• Invalid hex string.
• Invalid units

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/63fc198315a34fa9. Report an issue: GitHub.