Javascript is a powerful but sometimes strange programming language, especially when it comes to performance. Some effects are the result of implementation details and performance may vary between browsers and even between different versions of the same browser. So, the only way to find out is to try. But even that can be a strange journey, as you may see.

Generally, Javascript performance is very high, so we should not care too much about it. There are many things going on while a website is rendered that take much longer than Javascript execution, so it is unlikely to make a slow page fast just by optimizing Javascript. But if you are dealing with large arrays, text parsers or other kind of operations, that include manipulation of thousands or even millions of elements, performance may become critical.

There are several ways to measure time in Javascript (See here), usually I´m using the performance API. It is just not as precise as it could be, as for some reason it is intentionally limited to 100 ms to avoid browser fingerprinting (see here for details), so you need to repeat the code multiple times to get better results. But there is one effect that is important to know and that I cannot really explain.

Can somebody explain?

See the example below. This is a simple loop that changes a static array of 10.000 elements. To make it measurable, the operation is repeated 1000 times. Finally, it finishes in about 100 ms, which means, the total array of 10.000 elements was changed in 0,1 ms - amazing. Each operation took 0,01 us or 10 ns only!

    const count = 1000 // Number of repetitions

    function run() {
      ar1 = Array(10000).fill(" ")
      for (k = 0; k < count; k++) {
        for (i = 0; i < ar1.length; i++) {
          ar1[i] = "+"
        }
      }
    }

    t0 = performance.now()
    run()
    t1 = performance.now()
    run()
    t2 = performance.now()
    run()
    t3 = performance.now()
    run()
    t4 = performance.now()

console.log(`Time 1 = ${(t1 - t0).toFixed()}ms`)
console.log(`Time 2 = ${(t2 - t1).toFixed()}ms`)
....

Console ------>

Time 1 = 167ms  <--
Time 2 = 124ms
Time 3 = 117ms
Time 4 = 124ms

But the strange thing: The first loop took about 40% longer than the next three. I tried this several times, but always the first loop took longer than the rest.

Thigs get even stranger if I rerun the page:

Time 1 = 106ms
Time 2 = 84ms
Time 3 = 89ms
Time 4 = 80ms

The first loop is still slower, but now all loops run about 30% faster. After the second rerun, results get fairly stable, but still vary by about 5%.

I repeated this multiple times with different codes, but always the first loop is significantly slower than the following.

A second test

Here is an extension to the example above. I added a second routine. This is the same code, just instead of for ( ; ; ) I used for ( ..in.. ), which is known to be slower:

    function run2() {
      ar2 = Array(10000).fill(" ")
      for (k = 0; k < count; k++) {
        for (i in ar2) {
          ar2[i] = "+"
        }
      }

    t0 = performance.now()
    run()
    t1 = performance.now()
    run()
    t2 = performance.now()
    run2()
    t3 = performance.now()
    run2()
    t4 = performance.now()

Console ------>

Time 1 = 141ms
Time 2 = 115ms
Time 3 = 1621ms
Time 4 = 1663ms

Amazing enough, for ( ..in.. ) is about 10 time slower than the conventional loop. But the first routine still is also much slower on the first run. Imagine, I would have only compared two loops: My results would have been significantly wrong.

Let´s test in reverse order:

Time 1 = 1737ms
Time 2 = 1533ms
Time 3 = 103ms
Time 4 = 101ms

Differences are - relatively - smaller, but still, the first loop takes about 200ms longer than the second.

I cannot really explain, why the first loop is always so much slower. But it is in any way important to know, as otherwise your comparison could be misleading. Currently, I´m repeating all my tests multiple times to get more reliable results.

If you have any different experience or know a more reliable way to benchmark code, please let me know.