JavaScript For/Loop comparison issue

Question

In my opinion, this JavaScript should work, but it will not work! The loop is never run (it is skipped altogether).

FYI totalPages = 15

        for (var i = 1; i >= totalPages; i++) {
            pagingOptions += '<option value="' + i + '">' + i + '</option> ';
        }

If I change it like this, it works:

        for (var i = 1; i < totalPages; i++) {
            pagingOptions += '<option value="' + i + '">' + i + '</option> ';
        }

What is the problem? The JavaScript will only run the loop if the comparison is "i < totalPages"? Why would this be the case?

Answer 1

The semantics of the for loop:

for (expression1; expression2; expression3) {
  // ...
}
AFTER: // statements

are:

1. evaluate expression1
2. evaluate expression2 and check to see if it's truthy . If not, skip subsequent code (here, to "AFTER")
3. evaluate the statement (usually a block statement) associated with the for statement
4. evaluate expression3
5. go to step 2

Thus, the middle expression should evaluate to true when iteration of the loop body should continue. As soon as it's false — even if it's false on the very first try — the loop finishes.

Another way to think of it is that it's like:

expression1;
while (expression2) {
  // statements
  expression3;
}

Answer 2

I think you have your greater than/less than wrong here:

   for (var i = 1; i <= totalPages; i++) {
        pagingOptions += '<option value="' + i + '">' + i + '</option> ';
    }

In my opinion, this should work (for a given definition of working, as we don't know if you want i to be less than, or less than or equal to totalPages ).