Factorial of a Number in JavaScript

1// Factorial: n! = n × (n-1) × (n-2) × ... × 2 × 1
2// Example: 5! = 5 × 4 × 3 × 2 × 1 = 120
3
4// Method 1: Using for loop
5function factorialLoop(n) {
6    if (n < 0) return "Invalid input";
7    if (n === 0 || n === 1) return 1;
8    
9    let result = 1;
10    for (let i = 2; i <= n; i++) {
11        result *= i;
12    }
13    return result;
14}
15
16console.log("5! =", factorialLoop(5));
17console.log("0! =", factorialLoop(0));
18console.log("10! =", factorialLoop(10));
19
20// Method 2: Using recursion
21function factorialRecursive(n) {
22    if (n < 0) return "Invalid input";
23    if (n === 0 || n === 1) return 1;
24    return n * factorialRecursive(n - 1);
25}
26
27console.log("\nUsing recursion:");
28console.log("5! =", factorialRecursive(5));
29console.log("7! =", factorialRecursive(7));
30
31// Method 3: Using while loop
32function factorialWhile(n) {
33    if (n < 0) return "Invalid input";
34    if (n === 0 || n === 1) return 1;
35    
36    let result = 1;
37    let i = n;
38    while (i > 1) {
39        result *= i;
40        i--;
41    }
42    return result;
43}
44
45console.log("\nUsing while:");
46console.log("6! =", factorialWhile(6));
47
48// Method 4: Using reduce
49function factorialReduce(n) {
50    if (n < 0) return "Invalid input";
51    if (n === 0 || n === 1) return 1;
52    
53    return Array.from({length: n}, (_, i) => i + 1)
54                .reduce((product, num) => product * num, 1);
55}
56
57console.log("\nUsing reduce:");
58console.log("4! =", factorialReduce(4));