JavaScript For Loop - execution termination condition

Question

I was looking at source code of Ext object in ExtJs docs here and I noticed this for loop:

for (j = enumerables.length; j--;) {
    k = enumerables[j];
    if (config.hasOwnProperty(k)) {
        object[k] = config[k];
    }
}

So, normally in a for loop, we have do initialization, specify condition and then increment/decrement the counter. I do see an initial condition and j-- which decrements the counter after each iteration. However, what I can't figure out is how is the loop going to be terminated? I neither see any condition nor a break keyword that will terminate the loop.

What am I missing here?

Andras · Accepted Answer · 2015-02-21 02:40:45Z

2

the j-- is the loop condition, that's how it will terminate. the trailing ;) is the missing increment/decrement.

The j-- construct both decrements j and tests the old un-decremented value for non-zero to decide whether to continue looping. The loop stops when j is zero; below the loop, the value of j will be -1.

The key parts of the for loop to look for are the semicolons -- for ( ; ; ) Any valid expression (or comma-separated expression list) can go in any of the three slots, before, between, and after the semicolons.

The loop initializers go after the ( and before the first ;. The loop condition is the value of the expression between the two ; ;. The post-loop update goes after the last ; and before the ).

answered Feb 21, 2015 at 2:40

Andras

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5 Comments

Ricardo Sanchez Over a year ago

That is correct. This approach is often used to improve the performance of a loop, to decrement the iterator toward 0 instead of incrementing it.

Andras Over a year ago

eg the classic C string copy loop: for ( ; *d++ = *s++ ; ) ; (with apologies to the original, which used while)

Tachyon Over a year ago

Why would the loop stop when j is 0?

tourniquet_grab Over a year ago

Because 0 is implicitly converted to the boolean value false.

Andras Over a year ago

because the conditional tests (if, while, for, do) test for truthy, and 0, false, null, undefined, "" (empty string) (and in php, "0" string containing a single zero - go figure) are all "falsy"

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