Implement array functions

- sum: sum of all elements in the array
- product: product of all elements in the array
- average: average of all elements in the array

packages/mymath/src/array/index.spec.ts

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+import { describe, expect, it, jest } from '@jest/globals'
+
+import { average, product, sum } from './index'
+
+describe('sum', () => {
+  it('should return sum of all values', () => {
+    expect(sum([1, 2, 3, 4])).toBe(10)
+  })
+
+  it('should return 0 for []', () => {
+    expect(sum([])).toBe(0)
+  })
+})
+
+describe('product', () => {
+  it('should return product of all values', () => {
+    expect(product([1, 2, 3, 4])).toBe(24)
+  })
+
+  it('should return 1 for []', () => {
+    expect(product([])).toBe(1)
+  })
+})
+
+describe('average', () => {
+  it('should return average of all values', () => {
+    expect(average([1, 2, 3, 4])).toBe(2.5)
+  })
+
+  it('should return 0 for []', () => {
+    expect(average([])).toBe(0)
+  })
+
+  it('should return 5 for [] and 5', () => {
+    expect(average([], 5)).toBe(5)
+  })
+})

packages/mymath/src/array/index.ts

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+/**
+ * Sum of all values in the array
+ * Example: sum([1, 2, 3, 4]) => 10
+ * If the array is empty, it returns 0 (0 is identity element for addition)
+ * @param values
+ * @returns sum of all values
+ */
+export const sum = (values: number[]): number => {
+  let sum = 0
+  for (const value of values) {
+    sum += value
+  }
+  return sum
+}
+
+/**
+ * Product of all values in the array
+ * Example: product([1, 2, 3, 4]) => 24
+ * If the array is empty, it returns 1 (1 is identity element for multiplication)
+ * @param values
+ * @returns product of all values
+ */
+export const product = (values: number[]): number => {
+  let product = 1
+  for (const value of values) {
+    product *= value
+  }
+  return product
+}
+
+/**
+ * Average of all values in the array
+ * Example: average([1, 2, 3, 4]) => 2.5
+ * If the array is empty, it returns 0 or the defaultValue
+ * @param values
+ * @param defaultValue
+ * @returns average of all values
+ */
+export const average = (values: number[], defaultValue?: number): number => {
+  if (values.length === 0) {
+    return defaultValue || 0
+  }
+  return sum(values) / values.length
+}