Computing the mean has two steps: add every value, then divide by the number of items. A common trap is to average too early or special-case empty arrays incorrectly. Every implementation keeps the same condition: after index i, total is the sum of all values from 0 through i.

Solution

Approach 1: Sum with a for loop

Track a running total, then divide that total by array.length once the loop is done. The for loop version is the recommended one to internalize first because it makes both steps explicit.

The loop state has one job: preserve the exact sum of the values seen so far. The denominator does not change during the loop, so mixing division into the accumulation only adds rounding opportunities without making the algorithm simpler.

There is no "valid value" filter in this version. Every entry contributes to the total, and the denominator is always the original array length, including 0 and negative numbers.

For mean([4, 2, 8, 6]):

The final answer is 20 / 4 = 5.

/**
 * @param {Array<number>} array
 * @returns {number}
 */
export default function mean(array) {
  let total = 0;

  // Calculate the sum of all numbers in the array.
  for (let i = 0; i < array.length; i++) {
    total += array[i];
  }

  // Calculate the mean from the sum.
  return total / array.length;
}

Approach 2: Sum with Array.prototype.reduce()

This version is shorter, but it is doing the same work: accumulate a total first, then divide by the array length.

export default function mean(array: Array<number>): number {
  return array.reduce((a, b) => a + b, 0) / array.length;
}

Common pitfalls

• Do not divide on every iteration. Accumulate the total first, then divide once by the final length.

Notes

• An empty array does not need special handling here. 0 / 0 evaluates to NaN, which matches the expected behavior.
• Very large totals can overflow to Infinity. That is beyond the normal scope of this interview question, but it is worth knowing that JavaScript numbers do not keep arbitrary precision for huge sums.
• The function assumes every array entry is a number. Coercing strings or filtering non-numeric values would change the prompt's scope.

Resources

• Lodash _.mean