this

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Das this Schlüsselwort einer Funktion verhält sich verglichen mit anderen Sprachen in JavaScript ein bisschen anders. Es gibt auch einige Unterschiede zwischen strict mode und non-strict mode.

In den meisten Fällen wird der Wert von this aufgrund der Funktion, die aufgerufen wird, gesetzt. Der Wert kann nicht mit einer Zuweisung während der Ausführung gesetzt werden und kann sich jedes Mal, wenn die Funktion aufgerufen wird unterscheiden. ES5 hat die bind Methode eingeführt, um den Wert vom this einer Funktion zu setzen (egal, wie es aufgerufen wird). ECMAScript 2015 hat die arrow functions implementiert. Das this besitzt hier einen lexikalisch eingegrenzten Scope (es ist gesetzt auf den this Wert des einschliessenden Ausführungskontexts).

Syntax

this

Globaler Kontext

Im globalen Ausführungskontext (ausserhalb von Funktionen), referenziert sich this auf das globale Objekt. Dieses Verhalten ist im strict mode und non-strict mode gleich.

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

// Im Webbrowser ist das window Objekt das globale Objekt:
console.log(this === window); // true

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

Funktionskontext

Innerhalb einer Funktion hängt der Wert von this davon ab, wie die Funktion aufgerufen wird.

Einfacher Aufruf

function f1(){
  return this;
}

f1() === window; // globales Objekt

In diesem Fall wird der Wert von this nicht beim Aufruf gesetzt. Weil sich der Code nicht im strict mode befindet, muss der Wert von this immer ein Objekt sein, sodass es im Default auf das global object zeigt.

function f2(){
  "use strict"; // see strict mode
  return this;
}

f2() === undefined;

Im strict mode verbleibt der Wert von this beim gesetzten Wert am Anfang des Ausführungskontext. Wenn this am Anfang nicht gesetzt wurde, bleibt es undefined. Es kann auch jeder Wert gesetzt werden, wie z.B. null, 42 oder "I am not this".

Note: Im zweiten Beispiel sollte this undefined sein, weil f2 direkt und nicht als eine Methode oder Property von einem Objekt (z.B. window.f2()) aufgerufen wird. Diese Funktionalität war noch nicht in allen Browsern implementiert, als der Support für strict mode gestartet wurde. Als Folge, gab es (fälschlicherweise) das window Objekt zurück.

Arrow functions

In arrow functions, wird this lexikalisch gesetzt. Dies bedeutet, dass es auf den Wert des this vom einschliessenden Ausführungskontext gesetzt wird. In globalem Code wird es auf den global object gesezt.

var globalObject = this;
var foo = (() => this);
console.log(foo() === globalObject); // true

Es ist egal, wie foo aufgerufen wird. Sein this wird immer der global object sein. Dieses Verhalten trifft auch dann zu, wenn foo als eine Methode eines Objekts aufgerufen wird (was normalerweise das this auf das Objekt setzen würde) und call, apply oder bind verwendet wird:

// Call as a method of an object
var obj = {foo: foo};
console.log(obj.foo() === globalObject); // true

// Attempt to set this using call
console.log(foo.call(obj) === globalObject); // true

// Attempt to set this using bind
foo = foo.bind(obj);
console.log(foo() === globalObject); // true

No matter what, foo's this is set to what it was when it was created (in the example above, the global object). The same applies for arrow functions created inside other functions: their this is set to that of the outer execution context.

// Create obj with a method bar that returns a function that
// returns its this. The returned function is created as 
// an arrow function, so its this is permanently bound to the
// this of its enclosing function. The value of bar can be set
// in the call, which in turn sets the value of the 
// returned function.
var obj = { bar : function() {
                    var x = (() => this);
                    return x;
                  }
          };

// Call bar as a method of obj, setting its this to obj
// Assign a reference to the returned function to fn
var fn = obj.bar();

// Call fn without setting this, would normally default
// to the global object or undefined in strict mode
console.log(fn() === obj); // true

In the above, the function(call it anonymous function A) assigned to obj.bar returns another function(call it anonymous function B) that is created as an arrow function. As a result, function B's  this is permanently set to the this of obj.bar (function A)when called. When the returned function(function B) is called, its this will always be what it was set to initially. In the above code example, function B's this is set to function A's this which is obj, so it remains set to obj even when called in a manner that would normally set its this to undefined or the global object (or any other method as in the previous example in the global execution context).

As an object method

When a function is called as a method of an object, its this is set to the object the method is called on.

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 sum(){
  return this.a + this.b + this.c;
}

var o = {
  a: 1,
  b: 2,
  c: 3,
  get average(){
    return (this.a + this.b + this.c) / 3;
  }
};

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

console.log(o.average, o.sum); // logs 2, 6

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 on-event 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).

Spezifikationen

Spezifikation Status Kommentar
ECMAScript 2017 Draft (ECMA-262)
Die Definition von 'The this keyword' in dieser Spezifikation.
Entwurf  
ECMAScript 2015 (6th Edition, ECMA-262)
Die Definition von 'The this keyword' in dieser Spezifikation.
Standard  
ECMAScript 5.1 (ECMA-262)
Die Definition von 'The this keyword' in dieser Spezifikation.
Standard  
ECMAScript 3rd Edition (ECMA-262)
Die Definition von 'The this keyword' in dieser Spezifikation.
Standard  
ECMAScript 1st Edition (ECMA-262)
Die Definition von 'The this keyword' in dieser Spezifikation.
Standard Erste Definition. Implementiert in JavaScript 1.0.

Browser Kompatibilität

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Ja) (Ja) (Ja) (Ja) (Ja)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Ja) (Ja) (Ja) (Ja) (Ja) (Ja)

Siehe auch

Schlagwörter des Dokuments und Mitwirkende

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