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    this

    Summary

    A function's this keyword behaves a little differently in JavaScript compared to other languages. It also has some differences between strict mode and non-strict mode.

    In most cases, the value of this is determined by how a function is called. It can't be set by assignment during execution, and it may be different each time the function is called. ES5 introduced the bind method to set the value of a function's this regardless of how it's called.

    Syntax

     this

     

    Global context

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

    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
    

    Function context

    Inside a function, the value of this depends on how the function is called.

    Simple call

    function f1(){
      return this;
    }
    
    f1() === window; // global object
    

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

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

    In strict mode, the value of this 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 null or 42 or "I am not this".

    Note: In the second example, this should be undefined, because f2 was called without providing any base (e.g. window.f2()). This feature wasn't implemented in some browsers when they first started to support strict mode. As a result, they incorrectly returned the window object.

    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 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 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 thisobject 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
    

    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

    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition. Implemented in JavaScript 1.0
    ECMAScript 5.1 (ECMA-262)
    The definition of 'The this keyword' in that specification.
    Standard  
    ECMAScript 6 (ECMA-262)
    The definition of 'The this keyword' in that specification.
    Draft  

    Browser compatibility

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

    See also

    Document Tags and Contributors

    Last updated by: geoyws,
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