JavaScript/Maths/MeanSquareError.js

// 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
}

// testing
(() => {
  console.log(meanSquaredError([1, 2, 3, 4], [1, 2, 3, 4]) === 0)
  console.log(meanSquaredError([4, 3, 2, 1], [1, 2, 3, 4]) === 5)
  console.log(meanSquaredError([2, 0, 2, 0], [0, 0, 0, 0]) === 3)
})()