feat: add array count occurrences algorithm

Data-Structures/Array/CountOccurrences.js

@@ -0,0 +1,19 @@
+/**
+ * @function countOccurrences
+ * @description Counts the number of times a value appears in an array.
+ * @param {Array} arr - The input array
+ * @param {*} value - The value to count
+ * @returns {number} - The number of times value appears in arr
+ * @see https://en.wikipedia.org/wiki/Frequency_(statistics)
+ * @example countOccurrences([1, 2, 2, 3, 2], 2) => 3
+ * @example countOccurrences(['a', 'b', 'a'], 'a') => 2
+ * @example countOccurrences([1, 2, 3], 4) => 0
+ */
+const countOccurrences = (arr, value) => {
+  if (!Array.isArray(arr)) {
+    throw new TypeError('First argument must be an array')
+  }
+  return arr.reduce((count, item) => (item === value ? count + 1 : count), 0)
+}
+
+export { countOccurrences }

Data-Structures/Array/test/CountOccurrences.test.js

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+import { countOccurrences } from '../CountOccurrences.js'
+
+describe('countOccurrences', () => {
+  test('countOccurrences([1,2,2,3,2], 2) returns 3', () => {
+    expect(countOccurrences([1, 2, 2, 3, 2], 2)).toBe(3)
+  })
+
+  test("countOccurrences(['a','b','a'], 'a') returns 2", () => {
+    expect(countOccurrences(['a', 'b', 'a'], 'a')).toBe(2)
+  })
+
+  test('countOccurrences([1,2,3], 4) returns 0', () => {
+    expect(countOccurrences([1, 2, 3], 4)).toBe(0)
+  })
+
+  test('countOccurrences empty array returns 0', () => {
+    expect(countOccurrences([], 1)).toBe(0)
+  })
+
+  test('countOccurrences all same elements', () => {
+    expect(countOccurrences([2, 2, 2, 2], 2)).toBe(4)
+  })
+
+  test('countOccurrences with boolean values', () => {
+    expect(countOccurrences([true, false, true, true], true)).toBe(3)
+  })
+
+  test('countOccurrences with null values', () => {
+    expect(countOccurrences([null, 1, null, 2], null)).toBe(2)
+  })
+
+  test('countOccurrences does not mutate original array', () => {
+    const arr = [1, 2, 2, 3]
+    countOccurrences(arr, 2)
+    expect(arr).toEqual([1, 2, 2, 3])
+  })
+
+  test('throws TypeError for non-array input', () => {
+    expect(() => countOccurrences('123', 1)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for null input', () => {
+    expect(() => countOccurrences(null, 1)).toThrow(TypeError)
+  })
+})