TheAlgorithms/JavaScript · error · Error

Index out of range

Error message

Index out of range

What it means

Thrown by findMaxRecursion(arr, left, right) (FindMaxRecursion.js:24) as a plain Error when left or right falls outside the valid index range of the array. The function accepts negative indices (down to -len) as well as positive indices, mirroring JS array indexing, and rejects anything beyond those bounds. Note: an empty array does NOT throw here — it returns undefined at line 20, so the bounds check only matters for non-empty arrays.

Source

Thrown at Maths/FindMaxRecursion.js:25

 * @param {Integer} right Index of the last element
 *
 * @return {Integer} Maximum value of the array
 *
 * @see [Maximum value](https://en.wikipedia.org/wiki/Maximum_value)
 *
 * @example findMaxRecursion([1, 2, 4, 5]) = 5
 * @example findMaxRecursion([10, 40, 100, 20]) = 100
 * @example findMaxRecursion([-1, -2, -4, -5]) = -1
 */
function findMaxRecursion(arr, left, right) {
  const len = arr.length

  if (len === 0 || !arr) {
    return undefined
  }

  if (left >= len || left < -len || right >= len || right < -len) {
    throw new Error('Index out of range')
  }

  if (left === right) {
    return arr[left]
  }

  // n >> m is equivalent to floor(n / pow(2, m)), floor(n / 2) in this case, which is the mid index
  const mid = (left + right) >> 1

  const leftMax = findMaxRecursion(arr, left, mid)
  const rightMax = findMaxRecursion(arr, mid + 1, right)

  // Return the maximum
  return Math.max(leftMax, rightMax)
}

export { findMaxRecursion }

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Use the conventional call pattern: findMaxRecursion(arr, 0, arr.length - 1).
2. Clamp indices before calling: left = Math.max(0, left); right = Math.min(arr.length - 1, right).
3. Always pass both left and right explicitly — the function has no defaults.
4. Recompute indices from the current array length rather than caching them.

Example fix

// before
const m = findMaxRecursion(arr, 0, arr.length) // off-by-one: length not length-1

// after
const m = findMaxRecursion(arr, 0, arr.length - 1)

if (!Array.isArray(arr) || arr.length === 0) return undefined
const left = Math.max(0, Math.min(start, arr.length - 1))
const right = Math.max(0, Math.min(end, arr.length - 1))
const m = findMaxRecursion(arr, left, right)

const areValidIndices = (arr, l, r) =>
  Array.isArray(arr) && arr.length > 0 &&
  l >= -arr.length && l < arr.length &&
  r >= -arr.length && r < arr.length

try {
  m = findMaxRecursion(arr, left, right)
} catch (e) {
  if (e instanceof Error && e.message === 'Index out of range') {
    // clamp indices to bounds and retry
    m = findMaxRecursion(arr, 0, arr.length - 1)
  } else throw e
}

When it happens

Trigger: Call findMaxRecursion([1,2,3], 0, 5) where right=5 exceeds length 3; findMaxRecursion([1,2,3], -4, 2) where left=-4 is below -len (-3); passing left/right that were computed from a different (longer or shorter) array than the one supplied; omitting right so it becomes undefined and fails the comparison.

Common situations: Off-by-one errors computing the last index (using arr.length instead of arr.length - 1); stale indices cached before the array was mutated/truncated; omitting the right argument expecting a default (there is no default); passing indices from a 0-based vs 1-based source mismatch.

Related errors

• Index Out of Bound
• Input is not a valid 2D matrix.
• Array is empty
• Invalid Month Number.
• Provided input is not an array

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/ad44d0bdbfcf9fbb. Report an issue: GitHub.