使用 javascript performance.now() 计时
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Timing with javascript performance.now()
提问by slrom
I am trying to time the execution of my function in milliseconds. I use performance.now()in order to do that. I am able to get the time on the first run, but on the second, third, and so on runs I get 0 milliseconds. Here is an example:
我试图以毫秒为单位计时我的函数的执行时间。我使用performance.now()来做到这一点。我能够在第一次运行时获得时间，但是在第二次、第三次等运行中我得到 0 毫秒。下面是一个例子：
function someFunction (){
var t0 = performance.now();
//Function calculations
var t1 = performance.now();
Console.log(t1 - t0);
}
I launch the function onclick. It works when I first launch the page. It stops working on the second click. t0 and t1 get the same values and when I subtract them I get 0 for the time. Is there anyway around it? I don't necessarily need to use performance.now(). I just want to measure time in milliseconds.
我启动了 onclick 功能。当我第一次启动页面时它起作用。它在第二次点击时停止工作。t0 和 t1 得到相同的值，当我减去它们时，我得到了 0。反正周围有吗？我不一定需要使用 performance.now()。我只想以毫秒为单位测量时间。
Thank you.
谢谢你。
UpdateI think it has everything to do with the speed. For example:
更新我认为这与速度有关。例如：
<html>
<script type="text/javascript">
function someFunction (){
var t0 = performance.now();
console.log(t0);
//Function calculations
//Some loop
var counter = 0;
for (i = 0; i < 1000000; i++) {
counter ++;
}
var t1 = performance.now();
console.log(t1);
console.log(t1 - t0);
}
</script>
<button type="button" onclick="someFunction()">Click me</button>
</hmtl>Works as I would expect, but with the loop for (i = 0; i < 1000; i++)it doesn't.
像我期望的那样工作，但在循环中for (i = 0; i < 1000; i++)却没有。
Thank you for the pointers in the right direction.
感谢您提供正确方向的指示。
回答by jdphenix
The actual code you use will change the results here, and why the test comes to 0 as the result is a matter of speculation without that.
您使用的实际代码将改变这里的结果，以及为什么测试结果为 0 是一个推测问题，没有那个。
That said, micro benchmarks in JavaScript these days are subject to optimizations. For example:
也就是说，如今 JavaScript 中的微基准测试需要优化。例如：
function spiffy() {
/* long bit of code that
loops and loops and runs in
O(n!) time then finally */
return result;
}
Spiffy!
漂亮！
Let's say spiffy()deterministically always outputs the same result. The optimizer is allowed to effectively run this as:
假设spiffy()确定性地总是输出相同的结果。优化器可以有效地运行它：
function spiffy() {
return 42;
}
Which turns
哪个转弯
function someFunction (){
var t0 = performance.now();
var result = spiffy();
var t1 = performance.now();
Console.log(t1 - t0);
}
into a useless test result.
变成无用的测试结果。
If you've got a bona-fide performance problem in your JavaScript app, I would profile it when it's running slower than molassesand analyze the busiest portions of your code. And I don't mean micro benchmarks, but examining run-time, look at the algorithmyou're using in that section and see if there might be a better one for your circumstances, and finally, ask someone else about the actual codein question, in the same contextit's running in.
如果您的 JavaScript 应用程序存在真正的性能问题，我会在它的运行速度比 molasses 慢时对其进行分析，并分析代码中最繁忙的部分。而且我不是指微基准测试，但检查的运行时间，看看算法您使用的是一节，看看是否有可能是一个更好的为您的情况，最后，问别人对实际的代码中问题，它在相同的上下文中运行。
回答by Naor Tedgi
performance.now() upgraded and the question should be closed and not being bumpedanymore
performance.now（）升级，这个问题应该被关闭，没有被撞到了
https://developer.mozilla.org/en-US/docs/Web/API/Performance/now
https://developer.mozilla.org/en-US/docs/Web/API/Performance/now
the timestamps returned by Performance.now() are not limited to one-millisecond resolution. Instead, they represent times as floating-point numbers with up to microsecond precision.
Performance.now() 返回的时间戳不限于一毫秒的分辨率。相反，它们将时间表示为精度高达微秒的浮点数。
<html>
<script type="text/javascript">
function someFunction (){
var t0 = performance.now();
console.log(t0);
//Function calculations
//Some loop
var counter = 0;
for (i = 0; i < 1000; i++) {
counter ++;
}
var t1 = performance.now();
console.log(t1);
console.log(t1 - t0);
}
</script>
<button type="button" onclick="someFunction()">Click me</button>
</hmtl>回答by alphakevin
According to MDN doc:
根据 MDN 文档：
https://developer.mozilla.org/en-US/docs/Web/API/Performance/now
https://developer.mozilla.org/en-US/docs/Web/API/Performance/now
The timestamp is not actually high-resolution. To mitigate security threats such as Spectre, browsers currently round the results to varying degrees. (Firefox started rounding to 1 millisecond in Firefox 60.) Some browsers may also slightly randomize the timestamp. The precision may improve again in future releases; browser developers are still investigating these timing attacks and how best to mitigate them.
时间戳实际上不是高分辨率的。为了减轻 Spectre 等安全威胁，浏览器目前会在不同程度上对结果进行四舍五入。（Firefox 在 Firefox 60 中开始四舍五入到 1 毫秒。）一些浏览器也可能会稍微随机化时间戳。在未来的版本中，精度可能会再次提高；浏览器开发人员仍在研究这些定时攻击以及如何最好地缓解它们。
In such case you should not rely on performance.now()in browsers, or only rely it in millisecond resolution (like Date.now()does).
在这种情况下，你不应该依赖performance.now()浏览器，或者只依赖毫秒级的分辨率（就像Date.now()做的那样）。
One workaround, wrapping your code with another for{}loop with 1000 times, so the time spend on the wrapped code is roughly 1000 times of the original code.
一种解决方法是，用另一个for{}循环将代码包装1000 次，因此在包装代码上花费的时间大约是原始代码的 1000 倍。
function benchmark(func) {
var start = Date.now()
for (var i=0;i<1000;i++) {
func();
}
var end = Date.now();
var diff = (end - start) / 1000;
console.log('running 1000 times, average time is '+ diff + 'ms');
}
benchmark(someFunction);
Or you can test your code in NodeJS, if your code has no DOM operation:
或者你可以在 NodeJS 中测试你的代码，如果你的代码没有 DOM 操作：
