Weird inner loop not executing certain iterations

Question

I was trying to answer this question where I encountered a weird loop condition.

for (index1 = 1; index1 < 8; index1++) {
  var op = '#';
  for (index2 = index1; index2 - 1; index2--) { //this loop is weird to me
    op = op + '#';
  }
  console.log(op);
}

Upon checking how many iterations the inner loop is making for each outer loop iteration I get this:

var x = 0;

for (index1 = 1; index1 < 8; index1++) {
  //var op = '#';
  for (index2 = index1; index2 - 1; index2--) {
    var log = {};
    log.a = x; //check value before increment
    x++;
    log.b = x; //check value after increment
    console.log(`outer: ${index1}, inner: ${index2}`, log);
  }
  console.log(x);
  x = 0;
  //console.log(op);
}

As you can see, it logs 0, 1, 2, 3, 4, 5, 6.

My questions is:

Is the inner loop not iterating on the first outer loop iteration because index2 - 1 is equal to zero, which is falsy?

@Someprogrammerdude, already did that. It seems it's not stepping inside the inner loop if `inde2 - 1 == 0` — xGeo, Oct 07 '17 at 15:30
In C-like languages (like JavaScript) "true" is anything non-zero, and "false" is zero. JavaScript adds a little more to that, but that's basically it, if you have zero, then it's false. Your condition could be rewritten as `index2 - 1 != 0`. — Some programmer dude, Oct 07 '17 at 15:31
That's what I thought @Someprogrammerdude. Anyone want to post an answer? #2 question too please. — xGeo, Oct 07 '17 at 15:38
yeah.. sorry. i got confused with the additional 0 when i tried to run the code in the chrome debugger XD — xGeo, Oct 07 '17 at 15:46
Right, on the first loop iteration `index1` == 1, `index1` is then assigned to `index2`, therefore `index2 - 1` == 0, which equates to false and your inner loop does not execute. — bloodyKnuckles, Oct 07 '17 at 18:07

score 1 · Accepted Answer · answered Oct 07 '17 at 16:00

1

Is the inner loop not iterating on the first outer loop iteration because index2 - 1 is equal to zero, which is falsy?

That's correct. Any falsey value provided to the condition to the loop immediately halts the loop.

Falsey values are 0, NaN, null, undefined, "" and false.

answered Oct 07 '17 at 16:00

llama

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There's your vote since you're the first to post an answer though @Someprogrammerdude should also be given the credit since he clarified my guess. As a comment though. – xGeo Oct 07 '17 at 16:05

Weird inner loop not executing certain iterations

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