TheAlgorithms/JavaScript · error · TypeError

The two lists must be of equal length

Error message

The two lists must be of equal length

What it means

The meanSquaredError function requires element-wise comparison, so predicted and expected arrays must have the same length. This guard fires after the array-type check. A length mismatch would cause the loop to silently skip elements or access undefined values.

Source

Thrown at Maths/MeanSquareError.js:9

// Wikipedia: https://en.wikipedia.org/wiki/Mean_squared_error

const meanSquaredError = (predicted, expected) => {
  if (!Array.isArray(predicted) || !Array.isArray(expected)) {
    throw new TypeError('Argument must be an Array')
  }

  if (predicted.length !== expected.length) {
    throw new TypeError('The two lists must be of equal length')
  }

  let err = 0

  for (let i = 0; i < expected.length; i++) {
    err += (expected[i] - predicted[i]) ** 2
  }

  return err / expected.length
}

export { meanSquaredError }

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Ensure both arrays have the same number of elements.
2. Trim or pad arrays to equal length before calling.
3. Check .length equality before invoking.

Example fix

// before
meanSquaredError(pred, actual) // pred.length !== actual.length
// after
if (pred.length !== actual.length) throw new RangeError('arrays must match in length')
meanSquaredError(pred, actual)

if (predicted.length !== expected.length) {
  throw new RangeError('Arrays must have equal length')
}
meanSquaredError(predicted, expected)

const areEqualLengthArrays = (a, b) => Array.isArray(a) && Array.isArray(b) && a.length === b.length

When it happens

Trigger: Calling meanSquaredError([1,2,3], [1,2]) or any pair of arrays with differing lengths.

Common situations: Truncated datasets, filtering that removed elements from one array but not the other, train/test split mismatches, or predictions generated for a different number of samples than ground truth.

Related errors

• Invalid Input
• Argument must be an Array
• Invalid Input
• Number must be greater than zero.
• Input data must be numbers

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/4c8572e2ab68a6ec. Report an issue: GitHub.