How to factorialize a number in java-script

In Mathematics, the factorial of a number (positif-integer) can be a hard algorithm. In this post, I'm going to explain one approache using for loop.

When you factorialize a number, you are multiplying that number by each consecutive number minus 1.

If your number is 7, you woould have:

7! = 7 * 6 * 5 * 4 * 3 * 2 * 1

The pattern would be:

// Comment
0! = 1
1! = 1
2! = 2 * 1
3! = 3 * 2 * 1
4! = 4 * 3 * 2 * 1
5! = 5 * 4 * 3 * 2 * 1
6! = 6 * 5 * 4 * 3 * 2 * 1
7! = 7 * 6 * 5 * 4 * 3 * 2 * 1

Factorialize number with For loop

function factorialize(num) {
  // If num = 0 OR num = 1, the factorial will return 1
  if (num === 0 || num === 1)
    return 1;
  
  // We start the FOR loop with i = 4
  // We decrement i after each iteration 
  for (var i = num - 1; i >= 1; i--) {
    // We store the value of num at each iteration
    num = num * i; // or num *= i;
    /* 
                    num      var i = num - 1       num *= i         i--       i >= 1?
    1st iteration:   5           4 = 5 - 1         20 = 5 * 4        3          yes   
    2nd iteration:  20           3 = 4 - 1         60 = 20 * 3       2          yes
    3rd iteration:  60           2 = 3 - 1        120 = 60 * 2       1          yes  
    4th iteration: 120           1 = 2 - 1        120 = 120 * 1      0          no             
    5th iteration: 120               0                120
    End of the FOR loop 
    */
  }
  return num; //120
}
factorialize(5);

Without the comments

function factorialize(num) {
  if (num === 0 || num === 1)
    return 1;
  for (var i = num - 1; i >= 1; i--) {
    num *= i;
  }
  return num;
}
factorialize(5);