feat: add iterative Fibonacci algorithm

Maths/FibonacciIterative.js

@@ -0,0 +1,27 @@
+/**
+ * @function fibonacciIterative
+ * @description Returns the nth Fibonacci number using an iterative approach.
+ *   The Fibonacci sequence starts with 0 and 1, and each subsequent number
+ *   is the sum of the two preceding ones: 0, 1, 1, 2, 3, 5, 8, 13, ...
+ * @param {number} n - The position in the Fibonacci sequence (0-indexed)
+ * @returns {number} - The nth Fibonacci number
+ * @see https://en.wikipedia.org/wiki/Fibonacci_sequence
+ * @example fibonacciIterative(0) => 0
+ * @example fibonacciIterative(6) => 8
+ * @example fibonacciIterative(10) => 55
+ */
+const fibonacciIterative = (n) => {
+  if (!Number.isInteger(n) || n < 0) {
+    throw new TypeError('Input must be a non-negative integer')
+  }
+  if (n === 0) return 0
+  if (n === 1) return 1
+  let a = 0
+  let b = 1
+  for (let i = 2; i <= n; i++) {
+    ;[a, b] = [b, a + b]
+  }
+  return b
+}
+
+export { fibonacciIterative }

Maths/test/FibonacciIterative.test.js

@@ -0,0 +1,39 @@
+import { fibonacciIterative } from '../FibonacciIterative.js'
+
+describe('fibonacciIterative', () => {
+  test('fibonacciIterative(0) returns 0', () => {
+    expect(fibonacciIterative(0)).toBe(0)
+  })
+
+  test('fibonacciIterative(1) returns 1', () => {
+    expect(fibonacciIterative(1)).toBe(1)
+  })
+
+  test('fibonacciIterative(2) returns 1', () => {
+    expect(fibonacciIterative(2)).toBe(1)
+  })
+
+  test('fibonacciIterative(5) returns 5', () => {
+    expect(fibonacciIterative(5)).toBe(5)
+  })
+
+  test('fibonacciIterative(6) returns 8', () => {
+    expect(fibonacciIterative(6)).toBe(8)
+  })
+
+  test('fibonacciIterative(10) returns 55', () => {
+    expect(fibonacciIterative(10)).toBe(55)
+  })
+
+  test('fibonacciIterative(20) returns 6765', () => {
+    expect(fibonacciIterative(20)).toBe(6765)
+  })
+
+  test('throws TypeError for negative input', () => {
+    expect(() => fibonacciIterative(-1)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for float input', () => {
+    expect(() => fibonacciIterative(1.5)).toThrow(TypeError)
+  })
+})