path: root/files/pl/web/javascript/reference/operators/this/index.html

---
title: this
slug: Web/JavaScript/Reference/Operators/this
translation_of: Web/JavaScript/Reference/Operators/this
original_slug: Web/JavaScript/Referencje/Operatory/this
---
<div>
<div>{{jsSidebar("Operators")}}</div>
</div>

<h2 id="Summary">Summary</h2>

<p>W JavaScript słówko kluczowe <code>this</code> zachowuje się nieco inaczej w porównaniu do innych języków programowania. Istnieje również kilka różnic między trybem <a href="/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode">strict mode</a> oraz non-strict mode.</p>

<p>W większości przypadków wartość <code>this</code> jest ustalana na podstawie tego, jak wywołana została dana funkcja. Wartość ta nie może być przypisana podczas wykonywania się funkcji i może być inna za każdym wywołaniem. ES5 wprowadziło metodę <code><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind">bind</a></code> dzięki której <a href="#The_bind_method">możemy przypisać wartość <code>this</code> w funkcji, niezależnie od tego jak została ona wywołana.</a></p>

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

<pre class="syntaxbox">this</pre>

<h2 id="Global_context">Global context</h2>

<p>In the global execution context (outside of any function), <code>this</code> refers to the global object, whether in strict mode or not.</p>

<pre class="brush:js">console.log(this.document === document); // true

// In web browsers, the window object is also the global object:
console.log(this === window); // true

this.a = 37;
console.log(window.a); // 37
</pre>

<h2 id="Function_context">Function context</h2>

<p>Inside a function, the value of <code>this</code> depends on how the function is called.</p>

<h3 id="Simple_call">Simple call</h3>

<pre class="brush:js">function f1(){
  return this;
}

f1() === window; // global object
</pre>

<p>In this case, the value of <code>this</code> is not set by the call. Since the code is not in strict mode, the value of <code>this</code> must always be an object so it defaults to the global object.</p>

<pre class="brush:js">function f2(){
  "use strict"; // see strict mode
  return this;
}

f2() === undefined;
</pre>

<p>In strict mode, the value of <code>this</code> remains at whatever it's set to when entering the execution context. If it's not defined, it remains undefined. It can also be set to any value, such as <code>null</code> or <code>42</code> or <code>"I am not this"</code>.</p>

<div class="note"><strong>Note:</strong> In the second example, <code>this</code> should be <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined"><code>undefined</code></a>, because <code>f2</code> was called without providing any base (e.g. <code>window.f2()</code>). This feature wasn't implemented in some browsers when they first started to support <a href="/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode" title="Strict mode">strict mode</a>. As a result, they incorrectly returned the <code>window</code> object.</div>

<h3 id="As_an_object_method">As an object method</h3>

<p>When a function is called as a method of an object, its <code>this</code> is set to the object the method is called on.</p>

<p>In the following example, when <code>o.f()</code> is invoked, inside the function <code>this</code> is bound to the <code>o</code> object.</p>

<pre class="brush:js">var o = {
  prop: 37,
  f: function() {
    return this.prop;
  }
};

console.log(o.f()); // logs 37
</pre>

<p>Note that this behavior is not at all affected by how or where the function was defined. In the previous example, we defined the function inline as the <code>f</code> member during the definition of <code>o</code>. However, we could have just as easily defined the function first and later attached it to <code>o.f</code>. Doing so results in the same behavior:</p>

<pre class="brush:js">var o = {prop: 37};

function independent() {
  return this.prop;
}

o.f = independent;

console.log(o.f()); // logs 37
</pre>

<p>This demonstrates that it matters only that the function was invoked from the <code>f</code> member of <code>o</code>.</p>

<p>Similarly, the <code>this</code> binding is only affected by the most immediate member reference. In the following example, when we invoke the function, we call it as a method <code>g</code> of the object <code>o.b</code>. This time during execution, <code>this</code> inside the function will refer to <code>o.b</code>. The fact that the object is itself a member of <code>o</code> has no consequence; the most immediate reference is all that matters.</p>

<pre class="brush:js">o.b = {g: independent, prop: 42};
console.log(o.b.g()); // logs 42
</pre>

<h4 id="this_on_the_objects_prototype_chain"><code>this</code> on the object's prototype chain</h4>

<p>The same notion holds true for methods defined somewhere on the object's prototype chain. If the method is on an object's prototype chain, <code>this</code> refers to the object the method was called on, as if the method was on the object.</p>

<pre class="brush:js">var o = {f:function(){ return this.a + this.b; }};
var p = Object.create(o);
p.a = 1;
p.b = 4;

console.log(p.f()); // 5
</pre>

<p>In this example, the object assigned to the variable <code>p</code> doesn't have its own <code>f</code> property, it inherits it from its prototype. But it doesn't matter that the lookup for <code>f</code> eventually finds a member with that name on <code>o</code>; the lookup began as a reference to <code>p.f</code>, so <code>this</code> inside the function takes the value of the object referred to as <code>p</code>. That is, since <code>f</code> is called as a method of <code>p</code>, its <code>this</code> refers to <code>p</code>. This is an interesting feature of JavaScript's prototype inheritance.</p>

<h4 id="this_with_a_getter_or_setter"><code>this</code> with a getter or setter</h4>

<p>Again, the same notion holds true when a function is invoked from a getter or a setter. A function used as getter or setter has its <code>this</code> bound to the object from which the property is being set or gotten.</p>

<pre class="brush:js">function modulus(){
  return Math.sqrt(this.re * this.re + this.im * this.im);
}

var o = {
  re: 1,
  im: -1,
  get phase(){
    return Math.atan2(this.im, this.re);
  }
};

Object.defineProperty(o, 'modulus', {
    get: modulus, enumerable:true, configurable:true});

console.log(o.phase, o.modulus); // logs -0.78 1.4142
</pre>

<h3 id="As_a_constructor">As a constructor</h3>

<p>When a function is used as a constructor (with the <code><a href="/en-US/docs/Web/JavaScript/Reference/Operators/new">new</a></code> keyword), its <code>this</code> is bound to new object being constructed.</p>

<p>Note: while the default for a constructor is to return the object referenced by <code>this</code>, it can instead return some other object (if the return value isn't an object, then the <code>this</code> object is returned).</p>

<pre class="brush:js">/*
 * Constructors work like this:
 *
 * function MyConstructor(){
 *   // Actual function body code goes here.
 *   // Create properties on |this| as
 *   // desired by assigning to them.  E.g.,
 *   this.fum = "nom";
 *   // et cetera...
 *
 *   // If the function has a return statement that
 *   // returns an object, that object will be the
 *   // result of the |new| expression.  Otherwise,
 *   // the result of the expression is the object
 *   // currently bound to |this|
 *   // (i.e., the common case most usually seen).
 * }
 */

function C(){
  this.a = 37;
}

var o = new C();
console.log(o.a); // logs 37


function C2(){
  this.a = 37;
  return {a:38};
}

o = new C2();
console.log(o.a); // logs 38
</pre>

<p>In the last example (<code>C2</code>), because an object was returned during construction, the new object that <code>this</code> was bound to simply gets discarded. (This essentially makes the statement "<code>this.a = 37;</code>" dead code. It's not exactly dead, because it gets executed, but it can be eliminated with no outside effects.)</p>

<h3 id="call_and_apply"><code>call</code> and <code>apply</code></h3>

<p>Where a function uses the <code>this</code> keyword in its body, its value can be bound to a particular object in the call using the <code><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call">call</a></code> or <code><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply">apply</a></code> methods that all functions inherit from <code>Function.prototype</code>.</p>

<pre class="brush:js">function add(c, d){
  return this.a + this.b + c + d;
}

var o = {a:1, b:3};

// The first parameter is the object to use as
// 'this', subsequent parameters are passed as
// arguments in the function call
add.call(o, 5, 7); // 1 + 3 + 5 + 7 = 16

// The first parameter is the object to use as
// 'this', the second is an array whose
// members are used as the arguments in the function call
add.apply(o, [10, 20]); // 1 + 3 + 10 + 20 = 34
</pre>

<p>Note that with <code>call</code> and <code>apply</code>, if the value passed as <code>this</code> is not an object, an attempt will be made to convert it to an object using the internal <code>ToObject</code> operation. So if the value passed is a primitive like <code>7</code> or <code>'foo'</code>, it will be converted to an Object using the related constructor, so the primitive number <code>7</code> is converted to an object as if by <code>new Number(7)</code> and the string <code>'foo'</code> to an object as if by <code>new String('foo'), e.g.</code></p>

<pre class="brush:js">function bar() {
  console.log(Object.prototype.toString.call(this));
}

bar.call(7); // [object Number]
</pre>

<h3 id="The_bind_method">The <code>bind</code> method</h3>

<p>ECMAScript 5 introduced <code><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind">Function.prototype.bind</a></code>. Calling <code>f.bind(someObject)</code> creates a new function with the same body and scope as <code>f</code>, but where <code>this</code> occurs in the original function, in the new function it is permanently bound to the first argument of <code>bind</code>, regardless of how the function is being used.</p>

<pre class="brush:js">function f(){
  return this.a;
}

var g = f.bind({a:"azerty"});
console.log(g()); // azerty

var o = {a:37, f:f, g:g};
console.log(o.f(), o.g()); // 37, azerty
</pre>

<h3 id="As_a_DOM_event_handler">As a DOM event handler</h3>

<p>When a function is used as an event handler, its <code>this</code> is set to the element the event fired from (some browsers do not follow this convention for listeners added dynamically with methods other than <code>addEventListener</code>).</p>

<pre class="brush:js">// When called as a listener, turns the related element blue
function bluify(e){
  // Always true
  console.log(this === e.currentTarget);
  // true when currentTarget and target are the same object
  console.log(this === e.target);
  this.style.backgroundColor = '#A5D9F3';
}

// Get a list of every element in the document
var elements = document.getElementsByTagName('*');

// Add bluify as a click listener so when the
// element is clicked on, it turns blue
for(var i=0 ; i&lt;elements.length ; i++){
  elements[i].addEventListener('click', bluify, false);
}</pre>

<h3 id="In_an_in–line_event_handler">In an in–line event handler</h3>

<p>When code is called from an in–line handler, its <code>this</code> is set to the DOM element on which the listener is placed:</p>

<pre class="brush:js">&lt;button onclick="alert(this.tagName.toLowerCase());"&gt;
  Show this
&lt;/button&gt;
</pre>

<p>The above alert shows <code>button</code>. Note however that only the outer code has its <code>this</code> set this way:</p>

<pre class="brush:js">&lt;button onclick="alert((function(){return this}()));"&gt;
  Show inner this
&lt;/button&gt;
</pre>

<p>In this case, the inner function's <code>this</code> isn't set so it returns the global/window object (i.e. the default object in non–strict mode where <code>this</code> isn't set by the call).</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>ECMAScript 1st Edition.</td>
   <td>Standard</td>
   <td>Initial definition. Implemented in JavaScript 1.0</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-11.1.1', 'The this keyword')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td></td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-this-keyword', 'The this keyword')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td></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>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</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>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
  </tr>
 </tbody>
</table>
</div>

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

<ul>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode">Strict mode</a></li>
 <li><a href="http://bjorn.tipling.com/all-this">All this</a>, an article about <code>this</code> in different contexts</li>
</ul>