Revision 984197 of handler.has()

<div>{{JSRef}}</div>

<p>The <strong><code>handler.has()</code></strong> method is a trap for the {{jsxref("Operators/in", "in")}} operator.</p>

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

<pre class="brush: js">
var p = new Proxy(target, {
  has: function(target, prop) {
  }
});
</pre>

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

<p>The following parameters are passed to <code>has</code> method. <code>this</code> is bound to the handler.</p>

<dl>
 <dt><code>target</code></dt>
 <dd>The target object.</dd>
 <dt><code>prop</code></dt>
 <dd>The name of the property to check for existence.</dd>
</dl>

<h3 id="Return_value">Return value</h3>

<p>The <code>has</code> method must return a boolean value.</p>

<h2 id="Description">Description</h2>

<p>The <code><strong>handler.has</strong></code> method is a trap for the {{jsxref("Operators/in", "in")}} operator.</p>

<h3 id="Interceptions">Interceptions</h3>

<p>This trap can intercept these operations:</p>

<ul>
 <li>Property query: <code>foo in proxy</code></li>
 <li>Inherited property query: <code>foo in Object.create(proxy)</code></li>
 <li><code>with</code> check<code>: with(proxy) { (foo); }</code></li>
 <li>{{jsxref("Reflect.has()")}}</li>
</ul>

<h3 id="Invariants">Invariants</h3>

<p>If the following invariants are violated, the proxy will throw a {{jsxref("TypeError")}}:</p>

<ul>
 <li>A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.</li>
 <li>A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.</li>
</ul>

<h2 id="Examples">Examples</h2>

<p>The following code traps the {{jsxref("Operators/in", "in")}} operator.</p>

<pre class="brush: js">
var p = new Proxy({}, {
  has: function(target, prop) {
    console.log("called: " + prop);
    return true;
  }
});

console.log("a" in p); // "called: a"
                       // true
</pre>

<p>The following code violates an invariant.</p>

<pre class="brush: js">
var obj = { a: 10 };
Object.preventExtensions(obj);
var p = new Proxy(obj, {
  has: function(target, prop) {
    return false;
  }
});

"a" in p; // TypeError is thrown
</pre>

<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('ES6', '#sec-proxy-object-internal-methods-and-internal-slots-hasproperty-p', '[[HasProperty]]')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-proxy-object-internal-methods-and-internal-slots-hasproperty-p', '[[HasProperty]]')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td>&nbsp;</td>
  </tr>
 </tbody>
</table>

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

<div>{{CompatibilityTable}}</div>

<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>{{CompatUnknown}}</td>
   <td>{{CompatGeckoDesktop("18")}}</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>Chrome for 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>{{CompatUnknown}}</td>
   <td>{{CompatGeckoMobile("18")}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

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

<ul>
 <li>{{jsxref("Proxy")}}</li>
 <li>{{jsxref("Proxy.handler", "handler")}}</li>
 <li>{{jsxref("Operators/in", "in")}} operator</li>
 <li>{{jsxref("Reflect.has()")}}</li>
</ul>