---
title: performance.now()
slug: Web/API/Performance/now
tags:
  - API
  - Performance
  - Prestazioni
  - Reference
  - Riferimento
  - Web Performance API
  - metodo
translation_of: Web/API/Performance/now
---
<div>{{APIRef("High Resolution Timing")}}</div>

<p>Il metodo <code><strong>performance.now()</strong></code> restituisce un {{domxref("DOMHighResTimeStamp")}}, misurato in millisecondi.</p>

<div class="warning">
<p>The timestamp is not actually high-resolution. To mitigate security threats such as <a href="https://spectreattack.com/">Spectre</a>, 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.</p>
</div>

<p>{{AvailableInWorkers}}</p>

<p>Il valore restituito rappresenta il tempo trascorso dal <a href="/en-US/docs/Web/API/DOMHighResTimeStamp#The_time_origin">time origin</a>.</p>

<p>Tieni a mente i seguenti punti:</p>

<ul>
 <li>Negli workers dedicati creati da un contesto {{domxref("Window")}}, il valore nel worker sarà minore di <code>performance.now()</code> nella finestra che ha creato quel worker. Prima era lo stesso di <code>t0</code> nel contesto principale, ma è stato cambiato</li>
 <li>Negli workers condivisi o di servizio, til valore nel worker potrebbe essere più alto di quello del contesto principale, perché quella finestra può essere creata dopo quegli workers.</li>
</ul>

<h2 id="Sintassi">Sintassi</h2>

<pre class="syntaxbox"><em>t</em> = performance.now();</pre>

<h2 id="Esempio">Esempio</h2>

<pre class="brush: js">var t0 = performance.now();
doSomething();
var t1 = performance.now();
console.log("Call to doSomething took " + (t1 - t0) + " milliseconds.");
</pre>

<p>A differenza di altri timing data disponibili con JavaScript (per esempio <a href="/en-US/docs/JavaScript/Reference/Global_Objects/Date/now"><code>Date.now</code></a>), i timestamps restituiti da <code>Performance.now()</code> non sono limitati a una risoluzione di un millisecondo. Invece, rappresentano il tempo come numeri a virgola mobile con una precisione che può arrivare a un microsecondo.</p>

<p>Inolre, a differenza di <code>Date.now()</code>, il valore restituito da <code>Performance.now()</code> aumenta sempre con costanza, indipendentemente dal clock di sistema (che può essere regolato manualmente o distorto da software come NTP). Altrimenti, <code>performance.timing.navigationStart + performance.now()</code> sarà approssimativamente uguale a <code>Date.now()</code>.</p>

<h2 id="Reduced_time_precision">Reduced time precision</h2>

<p>To offer protection against timing attacks and fingerprinting, the precision of <code>performance.now()</code> might get rounded depending on browser settings.<br>
 In Firefox, the <code>privacy.reduceTimerPrecision</code>  preference is enabled by default and defaults to 1ms.</p>

<pre class="brush: js">// reduced time precision (1ms) in Firefox 60
performance.now();
// 8781416
// 8781815
// 8782206
// ...


// reduced time precision with `privacy.resistFingerprinting` enabled
performance.now();
// 8865400
// 8866200
// 8866700
// ...
</pre>

<p>In Firefox, you can also enable <code>privacy.resistFingerprinting</code> — this changes the precision to 100ms or the value of <code>privacy.resistFingerprinting.reduceTimerPrecision.microseconds</code>, whichever is larger.</p>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName('Highres Time Level 2', '#dom-performance-now', 'performance.now()')}}</td>
   <td>{{Spec2('Highres Time Level 2')}}</td>
   <td>Stricter definitions of interfaces and types.</td>
  </tr>
  <tr>
   <td>{{SpecName('Highres Time', '#dom-performance-now', 'performance.now()')}}</td>
   <td>{{Spec2('Highres Time')}}</td>
   <td>Initial definition</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>



<p>{{Compat("api.Performance.now")}}</p>

<h2 id="See_also">See also</h2>

<ul>
 <li><a href="http://updates.html5rocks.com/2012/08/When-milliseconds-are-not-enough-performance-now">When milliseconds are not enough: performance.now() </a>from HTML5 Rocks.</li>
</ul>