medianBy

1/**
2 * Calculates the median of an array based on a selector function.
3 *
4 * @param arr - Array of items.
5 * @param fn - Function that returns a numeric value from each item.
6 * @returns The median value, or undefined if array is empty.
7 */
8export function medianBy<T>(arr: T[], fn: (item: T) => number): number | undefined {
9  if (arr.length === 0) return undefined;
10
11  const values = arr.map(fn).sort((a, b) => a - b);
12  const mid = Math.floor(values.length / 2);
13
14  return values.length % 2 === 0
15    ? (values[mid - 1] + values[mid]) / 2
16    : values[mid];
17}

• Custom Selector Support

  Works with complex data structures by allowing a selector function to extract numeric values.

• Type-Safe with Generics

  Leverages TypeScript generics to ensure flexibility without sacrificing type safety.

• Handles Both Even and Odd Lengths

  Correctly computes the median for datasets of any size, using proper averaging when needed.

• Pure and Predictable

  No side effects; consistently returns the same output for the same input, aiding testability.

• Graceful Handling of Empty Input

  Returns undefined instead of throwing an error, enabling safe integration into larger flows.

Tests | Examples

1test('medianBy - odd count', () => {
2  const data = [{ n: 5 }, { n: 1 }, { n: 3 }];
3  expect(medianBy(data, x => x.n)).toBe(3);
4});
5
6test('medianBy - even count', () => {
7  const data = [{ n: 1 }, { n: 2 }, { n: 4 }, { n: 6 }];
8  expect(medianBy(data, x => x.n)).toBe(3);
9});
10
11test('medianBy - all same values', () => {
12  const data = [{ v: 2 }, { v: 2 }, { v: 2 }];
13  expect(medianBy(data, x => x.v)).toBe(2);
14});
15
16test('medianBy - empty array', () => {
17  expect(medianBy([], x => 1)).toBeUndefined();
18});

Common Use Cases

• Statistical Analysis

  Calculate medians from datasets such as survey responses, benchmark results, or sensor readings.

• Financial Tools

  Determine the median value of incomes, prices, or investment returns from structured data.

• Outlier Detection

  Use the median to establish central tendency for further comparison or filtering.

• UI Summaries

  Display median values in dashboards or reports to provide context beyond averages.

• Education & Grading Systems

  Compute the median score in a set of test results or assignments for fairness evaluation.