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+---
+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>