this

Fonksiyon'un this anahtar kelimesi diğer dillere göre birazcık farklı davranır. Ayrıca strict modu ve non-strict modu arasında farklılıklar bulunur.

Çoğunlukla, this'in değeri fonksiyonun çağrılma biçimine göre belirlenir. Çalışma esnasında değeri değiştirilemez, ve her fonksiyonu çağırışta farklı olabilir. ES5 bu adreste bind method to set the value of a function's this regardless of how it's called.

Sözdizimi

this

Global içerik

Global konumda (fonksiyon dışında), this global nesnesini referans gösterir, strict modunda olmak bu durumu değiştirmez..

console.log(this.document === document); // true

// Web browserlerinde window objesi global objedir.:
console.log(this === window); // true

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

Function içerik

Fonksiyon içerisinde, this'in değeri fonksiyonun nasıl çağrıldığına bağlıdır..

Basit çağrı

function f1(){
  return this;
}

f1() === window; // global obje

Bu durumda, this'in değeri çağrı ile ayarlanmaz. Kod strict modda olmadığı sürece, this'in değeri mutlaka obje olmalıdır bu yüzdende default değer olan global objesi döner.

function f2(){
  "use strict"; // strict modu içerisinde çalıştıralım
  return this;
}

f2() === undefined;

Strict modu içerisinde, this'in değeri çalıştırılma içeriğine nasıl girdiyse o şekilde kalır. Eğer tanımlanmamışsa, undefined olarak kalır. Ayrıca tüm değerlere eşitlenebilir, örneğin null yada 42 yada "I am not this".

Obje methodu cağrısı

Fonksiyon bir objenin methodu olarak çağrıldığında, this çağrıldığı obje olarak atanacaktır.

In the following example, when o.f() is invoked, inside the function this is bound to the o object.

var o = {
  prop: 37,
  f: function() {
    return this.prop;
  }
};

console.log(o.f()); // logs 37

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 f member during the definition of o. However, we could have just as easily defined the function first and later attached it to o.f. Doing so results in the same behavior:

var o = {prop: 37};

function independent() {
  return this.prop;
}

o.f = independent;

console.log(o.f()); // logs 37

This demonstrates that it matters only that the function was invoked from the f member of o.

Similarly, the this 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 g of the object o.b. This time during execution, this inside the function will refer to o.b. The fact that the object is itself a member of o has no consequence; the most immediate reference is all that matters.

o.b = {g: independent, prop: 42};
console.log(o.b.g()); // logs 42

this on the object's prototype chain

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, this refers to the object the method was called on, as if the method was on the object.

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

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

this with a getter or setter

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 this bound to the object from which the property is being set or gotten.

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

As a constructor

When a function is used as a constructor (with the new keyword), its this is bound to the new object being constructed.

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

/*
 * 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

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

call and apply

Where a function uses the this keyword in its body, its value can be bound to a particular object in the call using the call or apply methods that all functions inherit from Function.prototype.

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

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

function bar() {
  console.log(Object.prototype.toString.call(this));
}

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

The bind method

ECMAScript 5 introduced Function.prototype.bind. Calling f.bind(someObject) creates a new function with the same body and scope as f, but where this occurs in the original function, in the new function it is permanently bound to the first argument of bind, regardless of how the function is being used.

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

As a DOM event handler

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

// 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<elements.length ; i++){
  elements[i].addEventListener('click', bluify, false);
}

In an in–line event handler

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

<button onclick="alert(this.tagName.toLowerCase());">
  Show this
</button>

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

<button onclick="alert((function(){return this}()));">
  Show inner this
</button>

In this case, the inner function's this isn't set so it returns the global/window object (i.e. the default object in non–strict mode where this isn't set by the call).

Specifications

Browser compatibility

See also

• Strict mode
• All this, an article about this in different contexts