Revision 1044788 of Console.assert()

<div>{{APIRef("Console API")}}{{Non-standard_header}}</div>

<p>Writes an error message to the console if the assertion is false. If the assertion is true, nothing happens.</p>

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

<div class="note">
<p><strong>Note:</strong>&nbsp;<em>The&nbsp;<code>console.assert()</code>&nbsp;method is implemented differently in Node.js than the&nbsp;<code>console.assert()</code>&nbsp;method&nbsp;available in browsers.</em></p>

<p>Specifically, in browsers, calling&nbsp;<code>console.assert()</code>&nbsp;with a falsy assertion will cause the&nbsp;<code>message</code>&nbsp;to be printed to the console without interrupting execution of subsequent code. In Node.js, however, a falsy assertion will cause an&nbsp;<code>AssertionError</code>&nbsp;to be thrown.</p>
</div>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox">
console.assert(<em>assertion</em>, <em>obj1</em> [, <em>obj2</em>, ..., <em>objN</em>]);
console.assert(<em>assertion</em>, <em>msg</em> [, <em>subst1</em>, ..., <em>substN</em>]); // c-like message formatting
</pre>

<h3 id="Parameters">Parameters</h3>

<dl>
 <dt><code>assertion</code></dt>
 <dd>Any boolean expression. If the assertion is false, the message is written to the console.</dd>
 <dt><code>obj1</code> ... <code>objN</code></dt>
 <dd>A list of JavaScript objects to output. The string representations of each of these objects are appended together in the order listed and output.</dd>
 <dt><code>msg</code></dt>
 <dd>A JavaScript string containing zero or more substitution strings.</dd>
 <dt><code>subst1</code> ... <code>substN</code></dt>
 <dd>JavaScript objects with which to replace substitution strings within <code>msg</code>. This parameter gives you additional control over the format of the output.</dd>
</dl>

<p>See <a href="/en-US/docs/Web/API/console#Outputting_text_to_the_console">Outputting text to the console</a> in the documentation of {{domxref("console")}} for details.</p>

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

<table class="standard-table">
 <thead>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>{{SpecName("Console API", "#consoleassertexpression-object", "console.assert()")}}</td>
   <td>{{Spec2("Console API")}}</td>
   <td>Initial definition</td>
  </tr>
 </tbody>
</table>

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

<p>{{CompatibilityTable}}</p>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatGeckoDesktop("28.0")}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
  <tr>
   <td>Available in workers</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatGeckoDesktop("38.0")}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
  <tr>
   <td>c-like message formatting</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatGeckoDesktop("28.0")}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatGeckoMobile("28.0")}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
  </tr>
  <tr>
   <td>Available in workers</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatGeckoMobile("38.0")}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
  </tr>
  <tr>
   <td>c-like message formatting</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatGeckoMobile("28.0")}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

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

<ul>
 <li><a href="https://www.opera.com/dragonfly/documentation/console/">Opera Dragonfly documentation: Console</a></li>
 <li><a href="https://msdn.microsoft.com/library/gg589530">MSDN: Using the F12 Tools Console to View Errors and Status</a></li>
 <li><a href="https://developer.chrome.com/devtools/docs/console#assertions">Chrome Developer Tools: Using the Console</a></li>
</ul>