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---
title: Operator instanceof
slug: Web/JavaScript/Reference/Operators/instanceof
tags:
- Dokumentacja_JavaScript
- Dokumentacje
- JavaScript
- Strony_wymagające_dopracowania
- Wszystkie_kategorie
translation_of: Web/JavaScript/Reference/Operators/instanceof
original_slug: Web/JavaScript/Referencje/Operatory/Operator_instanceof
---
<div>{{jsSidebar("Operators")}}</div>
<p><strong>Operator</strong> <strong><code>instanceof</code> </strong>sprawdza czy właściwość konstruktora <code>prototype</code> pojawia się gdziekolwiek w łańcuchu prototypowym obiektu.</p>
<p>{{EmbedInteractiveExample("pages/js/expressions-instanceof.html")}}</p>
<h2 id="Składnia">Składnia</h2>
<pre class="syntaxbox"><em>object</em> instanceof <em>constructor</em></pre>
<dl>
<dt>
<h3 id="Parametry">Parametry</h3>
<code>object</code></dt>
<dd>Obiekt do testowania.</dd>
</dl>
<dl>
<dt><code>constructor</code></dt>
<dd>Funkcja przeciwko której testujemy.</dd>
</dl>
<h2 id="Opis">Opis</h2>
<p>Operator <code>instanceof</code> sprawdza obecność <code>constructor.prototype</code> w łańcuchu prototypowym obiektu <code>object</code></p>
<pre class="brush: js">// definiowanie konstruktorów
function C() {}
function D() {}
var o = new C();
// true, ponieważ: Object.getPrototypeOf(o) === C.prototype
o instanceof C;
// false, ponieważ D.prototype nie występuje w łańcuchu prototypowym o.
o instanceof D;
o instanceof Object; // true, ponieważ:
C.prototype instanceof Object // true
C.prototype = {};
var o2 = new C();
o2 instanceof C; // true
// false, ponieważ C.prototype nie ma już w łańcuchu prototypowym o
o instanceof C;
D.prototype = new C(); // add C to [[Prototype]] linkage of D
var o3 = new D();
o3 instanceof D; // true
o3 instanceof C; // true since C.prototype is now in o3's prototype chain
</pre>
<p>Note that the value of an <code>instanceof</code> test can change based on changes to the <code>prototype</code> property of constructors, and it can also be changed by changing an object prototype using <code>Object.setPrototypeOf</code>. It is also possible using the non-standard <code>__proto__</code> pseudo-property.</p>
<h3 id="instanceof_and_multiple_context_(e.g._frames_or_windows)"><code>instanceof</code> and multiple context (e.g. frames or windows)</h3>
<p>Different scopes have different execution environments. This means that they have different built-ins (different global object, different constructors, etc.). This may result in unexpected results. For instance, <code>[] instanceof window.frames[0].Array</code> will return <code>false</code>, because <code>Array.prototype !== </code><code>window.frames[0].Array</code> and arrays inherit from the former.</p>
<p>This may not make sense at first but when you start dealing with multiple frames or windows in your script and pass objects from one context to another via functions, this will be a valid and strong issue. For instance, you can securely check if a given object is, in fact, an Array using <code>Array.isArray(myObj)</code></p>
<p>For example checking if a <a href="/en-US/docs/Web/API/Node">Nodes</a> is a <a href="/en-US/docs/Web/API/SVGElement">SVGElement</a> in a different context you can use <code>myNode instanceof myNode.ownerDocument.defaultView.SVGElement</code></p>
<div class="note"><strong>Note for Mozilla developers:</strong><br>
In code using XPCOM <code>instanceof</code> has special effect: <code>obj instanceof </code><em><code>xpcomInterface</code></em> (e.g. <code>Components.interfaces.nsIFile</code>) calls <code>obj.QueryInterface(<em>xpcomInterface</em>)</code> and returns <code>true</code> if QueryInterface succeeded. A side effect of such call is that you can use <em><code>xpcomInterface</code></em>'s properties on <code>obj</code> after a successful <code>instanceof</code> test. Unlike standard JavaScript globals, the test <code>obj instanceof xpcomInterface</code> works as expected even if <code>obj</code> is from a different scope.</div>
<h2 id="Examples">Examples</h2>
<h3 id="Demonstrating_that_String_and_Date_are_of_type_Object_and_exceptional_cases">Demonstrating that <code>String</code> and <code>Date</code> are of type <code>Object</code> and exceptional cases</h3>
<p>The following code uses <code>instanceof</code> to demonstrate that <code>String</code> and <code>Date</code> objects are also of type <code>Object</code> (they are derived from <code>Object</code>).</p>
<p>However, objects created with the object literal notation are an exception here: Although the prototype is undefined, <code>instanceof Object</code> returns <code>true</code>.</p>
<pre class="brush: js">var simpleStr = 'This is a simple string';
var myString = new String();
var newStr = new String('String created with constructor');
var myDate = new Date();
var myObj = {};
simpleStr instanceof String; // returns false, checks the prototype chain, finds undefined
myString instanceof String; // returns true
newStr instanceof String; // returns true
myString instanceof Object; // returns true
myObj instanceof Object; // returns true, despite an undefined prototype
({}) instanceof Object; // returns true, same case as above
myString instanceof Date; // returns false
myDate instanceof Date; // returns true
myDate instanceof Object; // returns true
myDate instanceof String; // returns false
</pre>
<h3 id="Demonstrating_that_mycar_is_of_type_Car_and_type_Object">Demonstrating that <code>mycar</code> is of type <code>Car</code> and type <code>Object</code></h3>
<p>The following code creates an object type <code>Car</code> and an instance of that object type, <code>mycar</code>. The <code>instanceof</code> operator demonstrates that the <code>mycar</code> object is of type <code>Car</code> and of type <code>Object</code>.</p>
<pre class="brush: js">function Car(make, model, year) {
this.make = make;
this.model = model;
this.year = year;
}
var mycar = new Car('Honda', 'Accord', 1998);
var a = mycar instanceof Car; // returns true
var b = mycar instanceof Object; // returns true
</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('ESDraft', '#sec-relational-operators', 'Relational Operators')}}</td>
<td>{{Spec2('ESDraft')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES6', '#sec-relational-operators', 'Relational Operators')}}</td>
<td>{{Spec2('ES6')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES5.1', '#sec-11.8.6', 'The instanceof operator')}}</td>
<td>{{Spec2('ES5.1')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES3', '#sec-11.8.6', 'The instanceof operator')}}</td>
<td>{{Spec2('ES3')}}</td>
<td>Initial definition. Implemented in JavaScript 1.4.</td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<p>{{Compat("javascript.operators.instanceof")}}</p>
<h2 id="See_also">See also</h2>
<ul>
<li><code><a href="/en-US/docs/Web/JavaScript/Reference/Operators/typeof" title="/en-US/docs/JavaScript/Reference/Operators/typeof">typeof</a></code></li>
<li>{{jsxref("Symbol.hasInstance")}}</li>
</ul>
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