why infinite loop doesn't apply when iterating backwards with for loop

Question

I am learning javascript and I have two questions hope you can help :)
I have two for loops iterating backwards

for (var i = 10; i--;) {
  console.log(i, "backward");
}
// starts at 9 and stops at 0
// without breaking condition

for (var i = 10; i >= 0; i--) {
  console.log(i, "backward");
}
// starts at 10 and stops at 0
// with breaking condition that i >= 0

why in the first example we don't need to set a breaking point for the loop ? like i > 0 ? is it because 0 is false anyway ? so when it reaches 0 it automatically breaks ? and it won't keep iterating like 0, -1, -2, -3, ....etc ?
why in the second example the iterating starts at 10 while in the first example it starts at 9 ?

thanks for your support :D Cheers

developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… shows that all parameters are optional and explains what happens if they're omitted. — WOUNDEDStevenJones
– WOUNDEDStevenJones, Commented Sep 21, 2022 at 21:46

Barmar · Accepted Answer · 2022-09-21 21:46:19Z

2

i-- is the breaking point. A post-decrement expression subtracts 1 from the variable and returns the previous value. So when i is 0, the value of i-- is 0, which is falsey, and the loop stops.
Because in the first example we're decrementing i during the repeat condition, which runs before the body, so the first iteration already has it decremented to 9. In the second example we're decrementing i in the update expression, which runs after the body, so it still has the initial value 10 during the body of the first iteration.

To understand what a for loop is doing, expand it into the analogous while loop.

for(initialize; condition; update) {
    body
}

is roughly equivalent to

initialize;
while (condition) {
    body;
    update;
}

answered Sep 21, 2022 at 21:46

Barmar

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1 Comment

Nicely explained.