Question

I recently came accross a different way of looping though an array in Javascript.

I used to write loops something like this:

for (var len = 0; len < testData.length; len++) {
  total = total + testData[len];
}

I read some code that did it this way:

for (var len = 0; testData[len]; len++) {
  total = total + testData[len];
}

I was wondering how these would perform so I used jsPerf to find it out. The results are pretty amazing. I expected the second method to be a bit it faster than the first, but it’s actually much, much faster.

Is there a down side I’m missing here? Or is this the best way to loop through the items of a list.

Update:

gray state is coming and Diode pointed me to a simple flaw in the testcase which made is seem to be faster.

After correcting the mistake, this is the fastest:

var datalen = testData.length;
for (var len = 0; len <datalen; len++) {
     total = total + testData[len];
}

Update 2:

After testing in some more browsers this testcase takes a different direction again.
Only in Chrome and Opera the normal for loop is the fastest. In other all other browsers Shmiddty‘s way is just a bit faster.

var i = testData.length, sum=0;
while (i--){
    sum += testData[i];
}

Answer 1

You can also do a loop like so:

var i = testData.length;

while (i--){
    console.log(testData[i]);
}

Note that it traverses the array backwards.

Or, for a summation:

var i = testData.length, sum=0;

while (i--){
    sum += testData[i];
}