Javascript for loop with iterator In the middle and decrement operator to the left of i?

Question

For a non-recursive example of a factorial function I found this example and am having a little difficulty tracing through the code:

function fact(x) {
   if(x == 0) {
       return 1;
   }
   if(x < 0 ) {
       return undefined;
   }
   for(var i = x; --i; ) {
       x *= i;
   }
   return x;
}

I don't understand the syntax of this for loop-- why is the iteration statement in the middle, shouldn't that be where the test condition normally is? Does that mean there is no test condition or no iteration statement? You can leave one or both out?

Secondly, what is the difference in having ++i or --i in a for loop as opposed toi++ and i--?

If I wanted to find fact(5), in the first iteration of the for loop, is i 4 or 5?

Answer 1

In js, 0 means false, and the other values like 1 are true.

why is the iteration statement in the middle

for(var i = x; --i; /* optional */ )
               ^^ its decrement as well as the condition
                 loop continues until, i is 0

In fact, you can create infinite loop

for(;;);

I wanted to find fact(5), in the first iteration of the for loop, is i 4 or 5?

for(var i = x /* i=5 */; --i/* i=4 */; ) {
       x *= i;/* i=4 */
   }

Answer 2

In JavaScript, 0 evaluates as false. What's done here is to omit the iteration part of the loop, and have the test part perform the iteration itself. --i first decreases the value of i by 1, and then the for loop evaluates it, executing only if it's not 0.

Answer 3

The difference between i-- and --i is that --i first subtracts 1 from i and then evaluates i, while i-- first evaluates i and then subtracts 1 from i. for example:

var i=5;
console.log(i--);

will print 5, while:

var i=5;
console.log(--i);

will print 4.

Answer 4

I guess there is something wrong in the for loop as if you are trying to get the factorial of the number you should stop decreasing the number when it reaches 2. So the for loop should be something like this

for(var i = x; i > 2;--i) {
    x *= i;
}

Also the first if statement should be

if(x == 0 || x == 1) {
    return 1;
}

The difference between i-- and --i is that

i-- decrements i at the end of the loop iteration while --i decrements it before the iteration start.

So in the first iteration i is equals to 4 when you try to get fact(5).