Array's reverse vs simple arithmethic

Question

I was asked to print numbers from 100 to 1 in a for loop with the index starting from 1. I came up with this solution:

var a = [];
for(var i = 1; i <= 100; i++) {
 a.push(i);
}
console.log(a.reverse());

I was thinking of a better way of doing this after the interview and came up with this

for(var i = 1; i <= 100; i++) {
 console.log((100-i)+1);
}

Now what blows my mind is, jsperf.com's test indicates the former is faster. How is this even possible? Creating an array, pushing something into it and reversing it before printing definitely seems slower than simple arithmetic. Could anyone help me understand this?

• Dair: I honestly think this should probably be on stackoverflow instead of codereview. Nevertheless, really interesting question.
• greybeard: Figure out the number of calls to console.log(). Try timing building the output as a single string, and logging that.

Answer 1

It turns out accessing console take's more time than pushing a value in an array.

Console.log is indeed implemented as a synchronous function (next print starts only after last print finishes).

Meaning, there is no buffering system. The program goes to blocking state until console.log() finish printing, everytime.

Performance difference in above two cases is due to the difference in the number of console.log() call.

• Blindman67: The test results are meaningless, the error on 4 samples ranges 2 orders of magnitude. That is statistically called noise. You need to test 1000+ samples, 1000+ times, include the error bars. Then you can make a deduction about performance, use that to make a prediction, and only when you confirm your prediction via experiment are you able to make a meaningful statement about performance.
• snehm: Thank you, sir. Though I am not predicting anything, It was only for demonstration.

Answer 2

You can also push the reversed order of values in the array so you can avoid reverse() and can call console.log() outside the loop.

var a = [];
for(var i = 1; i <= 100; i++) {
 a.push(101-i);
}
console.log(a)

Answer 3

Both variants presented look somewhat indirect. What about

for (var i = 100 ; 1 <= i ; --i)
 console.log(i);

(I grant that print numbers from 100 to 1 in a for loop with the index starting from 1 reads use an array - not sure about starting from 1.)

for (var index = 1 ; 1 <= (99+index) ; --index)
 console.log(99+index);

• greybeard: Personally preferring for (var i = 101 ; 0 < --i ; ).