test: add more tests for xStandardDeviation (#298)

Author: lpatiny

src/x/__tests__/xStandardDeviation.test.ts

@@ -2,24 +2,30 @@ import { expect, test } from 'vitest';
 
 import { xStandardDeviation } from '../xStandardDeviation';
 
-const data = [15, 13, 17, 7];
-
-const typedArray = new Uint16Array(4);
-typedArray[0] = 15;
-typedArray[1] = 13;
-typedArray[2] = 17;
-typedArray[3] = 7;
-
 test('standard deviation', () => {
+  const data = [15, 13, 17, 7];
   const s = xStandardDeviation(data);
   expect(s).toBeCloseTo(Math.sqrt(18.667), 3);
   expect(xStandardDeviation(data, { unbiased: true })).toBe(s);
   expect(xStandardDeviation(data, { unbiased: false })).toBe(Math.sqrt(14));
 });
 
 test('standard deviation of typed array', () => {
+  const data = [15, 13, 17, 7];
+  const typedArray = Uint16Array.from(data);
   const s = xStandardDeviation(typedArray);
   expect(s).toBeCloseTo(Math.sqrt(18.667), 3);
   expect(xStandardDeviation(data, { unbiased: true })).toBe(s);
   expect(xStandardDeviation(data, { unbiased: false })).toBe(Math.sqrt(14));
 });
+
+test('one element', () => {
+  const data = [15];
+  expect(xStandardDeviation(data)).toBe(Number.NaN);
+  expect(xStandardDeviation(data, { unbiased: false })).toBe(0);
+});
+
+test('empty array', () => {
+  const data: number[] = [];
+  expect(() => xStandardDeviation(data)).toThrow('input must not be empty');
+});

src/x/xVariance.ts

@@ -11,7 +11,7 @@ export interface XVarianceOptions {
   unbiased?: boolean;
 
   /**
-   * Mean of the data
+   * Precalculated mean of the data. If undefined it will be recalculated internally
    * @default mean calculated
    */
   mean?: number;