「this」キーワードはどのように機能しますか?

1346
Maxim Gershkovich 2010-06-28 03:12.

thisStack OverflowサイトのJavaScriptでキーワードが何であり、どのように正しく(そして誤って)使用されているかについて明確な説明がないように思われることに気づきました。

私はそれでいくつかの非常に奇妙な振る舞いを目撃しました、そしてそれがなぜ起こったのか理解できませんでした。

どのように機能しthis、いつ使用する必要がありますか?

21 answers

1378
Daniel Trebbien 2010-06-28 03:15.

私は読んでお勧めのマイク・西の記事JavaScriptで適用範囲(ミラー)最初に。これは、thisJavaScriptの概念とスコープチェーンの優れたフレンドリーな紹介です。

慣れ始めるthisと、ルールは実際には非常に単純です。ECMAScriptの5.1標準の定義this

§11.1.1thisキーワード

this現在の実行コンテキストのThisBindingの値に評価さキーワード

ThisBindingは、オブジェクトへの参照を保持する特別なCPUレジスタのように、JavaScriptインタープリターがJavaScriptコードを評価するときに維持するものです。インタプリタは、次の3つの異なるケースのいずれかで実行コンテキストを確立するたびに、ThisBindingを更新します。

1.初期グローバル実行コンテキスト

これは、トップレベルで評価されるJavaScriptコードの場合です。たとえば、<script>:の内部に直接ある場合などです。

<script>
  alert("I'm evaluated in the initial global execution context!");

  setTimeout(function () {
      alert("I'm NOT evaluated in the initial global execution context.");
  }, 1);
</script>

初期のグローバル実行コンテキストでコードを評価する場合、ThisBindingはグローバルオブジェクトに設定されますwindow(§10.4.1.1)。

2.評価コードの入力

  • eval()ThisBindingへの直接呼び出しによるものは変更されません。これは、呼び出し元の実行コンテキストのThisBindingと同じ値です(§10.4.2(2)(a))。

  • eval()
    ThisBindingを直接呼び出さない場合は、最初のグローバル実行コンテキストで実行されているかのようにグローバルオブジェクトに設定されます(§10.4.2(1))。

§15.1.2.1.1は、直接の呼び出しが何であるかを定義しますeval()。基本的に、eval(...)は直接呼び出しですが、(0, eval)(...)またはのようなものvar indirectEval = eval; indirectEval(...);はへの間接呼び出しeval()です。JavaScriptの(1、eval)( 'this')vs eval( 'this')に対するchuckjの答えを参照してください。とドミトリーソシュニコフのECMA-262-5の詳細。第2章厳密モード。間接eval()呼び出しを使用する可能性がある場合。

3.機能コードの入力

これは、関数を呼び出すときに発生します。inobj.myMethod()または同等のオブジェクトなどで関数が呼び出されるとobj["myMethod"]()、ThisBindingがオブジェクトに設定されます(obj例では、§13.2.1)。他のほとんどの場合、ThisBindingはグローバルオブジェクトに設定されます(§10.4.3)。

「他のほとんどの場合」と書く理由は、ThisBindingを引数リストで指定できるようにする8つのECMAScript5組み込み関数があるためです。これらの特別な関数は、関数をthisArg呼び出すときにThisBindingになるいわゆるを取ります(§10.4.3)。

これらの特別な組み込み関数は次のとおりです。

  • Function.prototype.apply( thisArg, argArray )
  • Function.prototype.call( thisArg [ , arg1 [ , arg2, ... ] ] )
  • Function.prototype.bind( thisArg [ , arg1 [ , arg2, ... ] ] )
  • Array.prototype.every( callbackfn [ , thisArg ] )
  • Array.prototype.some( callbackfn [ , thisArg ] )
  • Array.prototype.forEach( callbackfn [ , thisArg ] )
  • Array.prototype.map( callbackfn [ , thisArg ] )
  • Array.prototype.filter( callbackfn [ , thisArg ] )

Function.prototype関数の場合、関数オブジェクトで呼び出されますが、ThisBindingを関数オブジェクトに設定するのではなく、ThisBindingをに設定しますthisArg

Array.prototype関数の場合、指定されたものcallbackfnは、ThisBindingが指定されているthisArg場合に設定されている実行コンテキストで呼び出されます。それ以外の場合は、グローバルオブジェクトに。

これらはプレーンJavaScriptのルールです。JavaScriptライブラリ(jQueryなど)の使用を開始すると、特定のライブラリ関数がの値を操作する場合がありthisます。これらのJavaScriptライブラリの開発者は、最も一般的なユースケースをサポートする傾向があるため、これを行います。ライブラリのユーザーは通常、この動作がより便利であると感じています。thisライブラリ関数を参照するコールバック関数を渡すthisときは、関数が呼び出されたときの値が何であるかについての保証について、ドキュメントを参照する必要があります。

JavaScriptライブラリがの値をどのように操作するのか疑問に思っている場合this、ライブラリは、を受け入れる組み込みのJavaScript関数の1つを使用しているだけthisArgです。あなたも、コールバック関数を使って独自の関数を書くことができますthisArg

function doWork(callbackfn, thisArg) {
    //...
    if (callbackfn != null) callbackfn.call(thisArg);
}

私がまだ言及しなかった特別なケースがあります。new演算子を使用して新しいオブジェクトを作成する場合、JavaScriptインタープリターは新しい空のオブジェクトを作成し、いくつかの内部プロパティを設定してから、新しいオブジェクトに対してコンストラクター関数を呼び出します。したがって、コンストラクターコンテキストで関数が呼び出されると、の値はthisインタープリターが作成した新しいオブジェクトになります。

function MyType() {
    this.someData = "a string";
}

var instance = new MyType();
// Kind of like the following, but there are more steps involved:
// var instance = {};
// MyType.call(instance);

矢印関数

矢印関数(ECMA6で導入)は、のスコープを変更しますthis。既存の標準的な質問、矢印関数と関数宣言/式を参照してください:それらは同等/交換可能ですか?詳細については。しかし、要するに:

矢印関数には独自のthis....バインディングがありません。代わりに、これらの識別子は、他の変数と同様に字句スコープで解決されます。つまり、矢印関数内で、this...this矢印関数が定義されている環境内の値を参照します。

楽しみのために、いくつかの例であなたの理解をテストしてください

答えを表示するには、薄い灰色のボックスにマウスを合わせます。

  1. thisマークされた行の値は何ですか?どうして?

window —マークされた行は、最初のグローバル実行コンテキストで評価されます。

    if (true) {
        // What is `this` here?
    }
  1. が実行されたthisときのマークされた行の値は何obj.staticFunction()ですか?どうして?

obj —オブジェクトで関数を呼び出すと、ThisBindingがオブジェクトに設定されます。

var obj = {
    someData: "a string"
};

function myFun() {
    return this // What is `this` here?
}

obj.staticFunction = myFun;

console.log("this is window:", obj.staticFunction() == window);
console.log("this is obj:", obj.staticFunction() == obj);
  

  1. thisマークされた行の値は何ですか?どうして?

window

この例では、JavaScriptインタープリターが関数コードを入力しますが、myFun/obj.myMethodはオブジェクトで呼び出されないため、ThisBindingはに設定されwindowます。

これは、メソッド(obj.myMethod)にアクセスするとバインドされたメソッドオブジェクトが作成されるPythonとは異なります。

var obj = {
    myMethod: function () {
        return this; // What is `this` here?
    }
};
var myFun = obj.myMethod;
console.log("this is window:", myFun() == window);
console.log("this is obj:", myFun() == obj);
  

  1. thisマークされた行の値は何ですか?どうして?

window

これはトリッキーでした。評価コードを評価する場合、thisobjです。ただし、evalコードでmyFunは、はオブジェクトで呼び出されないため、ThisBindingはwindow呼び出しに対してに設定されます。

 <!-- no snippet because, seemingly, eval doesn’t work in snippets -->

    function myFun() {
        return this; // What is `this` here?
    }
    var obj = {
        myMethod: function () {
            eval("myFun()");
        }
    };
  1. thisマークされた行の値は何ですか?どうして?

obj

この行myFun.call(obj);は、最初の引数としてFunction.prototype.call()受け入れる特別な組み込み関数を呼び出していthisArgます。

function myFun() {
    return this; // What is `this` here?
}
var obj = {
    someData: "a string"
};
console.log("this is window:", myFun.call(obj) == window);
console.log("this is obj:", myFun.call(obj) == obj);
  

168
Mahesha999 2013-07-08 07:32.

this異なっJavaScriptでキーワード振る舞いは他の言語と比較します。オブジェクト指向言語では、thisキーワードはクラスの現在のインスタンスを参照します。JavaScriptでは、の値はthis関数(context.function())の呼び出しコンテキストとそれが呼び出される場所によって決定されます。

1.グローバルコンテキストで使用する場合

thisグローバルコンテキストで使用する場合、グローバルオブジェクト(windowブラウザ内)にバインドされます

document.write(this);  //[object Window]

thisグローバルコンテキストで定義された関数内で使用する場合this、関数は実際にはグローバルコンテキストのメソッドになっているため、は引き続きグローバルオブジェクトにバインドされます。

function f1()
{
   return this;
}
document.write(f1());  //[object Window]

上記f1はグローバルオブジェクトのメソッドになっています。したがってwindow、次のようにオブジェクトで呼び出すこともできます。

function f()
{
    return this;
}

document.write(window.f()); //[object Window]

2.オブジェクトメソッド内で使用する場合

thisオブジェクトメソッド内でキーワードを使用するthisと、は「即時」の囲んでいるオブジェクトにバインドされます。

var obj = {
    name: "obj",
    f: function () {
        return this + ":" + this.name;
    }
};
document.write(obj.f());  //[object Object]:obj

上記では、即時という単語を二重引用符で囲んでいます。オブジェクトを別のオブジェクト内にネストするthisと、直接の親にバインドされることを強調する必要があります。

var obj = {
    name: "obj1",
    nestedobj: {
        name:"nestedobj",
        f: function () {
            return this + ":" + this.name;
        }
    }            
}

document.write(obj.nestedobj.f()); //[object Object]:nestedobj

メソッドとしてオブジェクトに関数を明示的に追加した場合でも、上記のルールに従いthisます。つまり、直接の親オブジェクトを指します。

var obj1 = {
    name: "obj1",
}

function returnName() {
    return this + ":" + this.name;
}

obj1.f = returnName; //add method to object
document.write(obj1.f()); //[object Object]:obj1

3.コンテキストレス関数を呼び出す場合

thisコンテキストなしで(つまり、オブジェクト上で)呼び出されない内部関数を使用すると、(window関数がオブジェクト内で定義されている場合でも)グローバルオブジェクトにバインドされます(ブラウザー内)。

var context = "global";

var obj = {  
    context: "object",
    method: function () {                  
        function f() {
            var context = "function";
            return this + ":" +this.context; 
        };
        return f(); //invoked without context
    }
};

document.write(obj.method()); //[object Window]:global 

関数ですべて試してみる

上記の点を関数で試すこともできます。ただし、いくつかの違いがあります。

  • 上記では、オブジェクトリテラル表記を使用してオブジェクトにメンバーを追加しました。を使用して、関数にメンバーを追加できthisます。それらを指定します。
  • オブジェクトリテラル表記は、すぐに使用できるオブジェクトのインスタンスを作成します。関数では、最初にnew演算子を使用してインスタンスを作成する必要がある場合があります。
  • また、オブジェクトリテラルのアプローチでは、ドット演算子を使用して、定義済みのオブジェクトにメンバーを明示的に追加できます。これは特定のインスタンスにのみ追加されます。ただし、関数のすべてのインスタンスに反映されるように、関数プロトタイプに変数を追加しました。

以下では、Objectthis以上で行ったすべてのことを試しましたが、オブジェクトを直接書き込むのではなく、最初に関数を作成しました。

/********************************************************************* 
  1. When you add variable to the function using this keyword, it 
     gets added to the function prototype, thus allowing all function 
     instances to have their own copy of the variables added.
*********************************************************************/
function functionDef()
{
    this.name = "ObjDefinition";
    this.getName = function(){                
        return this+":"+this.name;
    }
}        

obj1 = new functionDef();
document.write(obj1.getName() + "<br />"); //[object Object]:ObjDefinition   

/********************************************************************* 
   2. Members explicitly added to the function protorype also behave 
      as above: all function instances have their own copy of the 
      variable added.
*********************************************************************/
functionDef.prototype.version = 1;
functionDef.prototype.getVersion = function(){
    return "v"+this.version; //see how this.version refers to the
                             //version variable added through 
                             //prototype
}
document.write(obj1.getVersion() + "<br />"); //v1

/********************************************************************* 
   3. Illustrating that the function variables added by both above 
      ways have their own copies across function instances
*********************************************************************/
functionDef.prototype.incrementVersion = function(){
    this.version = this.version + 1;
}
var obj2 = new functionDef();
document.write(obj2.getVersion() + "<br />"); //v1

obj2.incrementVersion();      //incrementing version in obj2
                              //does not affect obj1 version

document.write(obj2.getVersion() + "<br />"); //v2
document.write(obj1.getVersion() + "<br />"); //v1

/********************************************************************* 
   4. `this` keyword refers to the immediate parent object. If you 
       nest the object through function prototype, then `this` inside 
       object refers to the nested object not the function instance
*********************************************************************/
functionDef.prototype.nestedObj = { name: 'nestedObj', 
                                    getName1 : function(){
                                        return this+":"+this.name;
                                    }                            
                                  };

document.write(obj2.nestedObj.getName1() + "<br />"); //[object Object]:nestedObj

/********************************************************************* 
   5. 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 ProtoObj = { fun: function () { return this.a } };
var obj3 = Object.create(ProtoObj); //creating an object setting ProtoObj
                                    //as its prototype
obj3.a = 999;                       //adding instance member to obj3
document.write(obj3.fun()+"<br />");//999
                                    //calling obj3.fun() makes 
                                    //ProtoObj.fun() to access obj3.a as 
                                    //if fun() is defined on obj3

4.コンストラクター関数内で使用する場合

関数がコンストラクターとして使用される場合(つまり、newキーワードを使用して呼び出される場合)、this関数本体の内部は、構築される新しいオブジェクトを指します。

var myname = "global context";
function SimpleFun()
{
    this.myname = "simple function";
}

var obj1 = new SimpleFun(); //adds myname to obj1
//1. `new` causes `this` inside the SimpleFun() to point to the
//   object being constructed thus adding any member
//   created inside SimipleFun() using this.membername to the
//   object being constructed
//2. And by default `new` makes function to return newly 
//   constructed object if no explicit return value is specified

document.write(obj1.myname); //simple function

5.プロトタイプチェーンで定義された関数内で使用する場合

メソッドがオブジェクトのプロトタイプチェーン上にある場合、thisそのようなメソッド内では、メソッドがオブジェクトで定義されているかのように、メソッドが呼び出されたオブジェクトを参照します。

var ProtoObj = {
    fun: function () {
        return this.a;
    }
};
//Object.create() creates object with ProtoObj as its
//prototype and assigns it to obj3, thus making fun() 
//to be the method on its prototype chain

var obj3 = Object.create(ProtoObj);
obj3.a = 999;
document.write(obj3.fun()); //999

//Notice that fun() is defined on obj3's prototype but 
//`this.a` inside fun() retrieves obj3.a   

6. call()内で、apply()およびbind()関数

  • これらのメソッドはすべて、で定義されていFunction.prototypeます。
  • これらのメソッドを使用すると、関数を1回記述して、別のコンテキストで呼び出すことができます。つまり、this関数の実行中に使用される値を指定できます。また、呼び出されたときに元の関数に渡されるパラメーターも受け取ります。
  • fun.apply(obj1 [, argsArray])insideobj1の値として設定し、引数としての受け渡し要素を呼び出します。thisfun()fun()argsArray
  • fun.call(obj1 [, arg1 [, arg2 [,arg3 [, ...]]]])-設定obj1の値として、this内部fun()および通話fun()渡すarg1, arg2, arg3, ...引数として。
  • fun.bind(obj1 [, arg1 [, arg2 [,arg3 [, ...]]]])-指定されたパラメーターにバインドされたinsidefunとバインドされたパラメーターをfun持つ関数への参照を返します。thisobj1funarg1, arg2, arg3,...
  • ここまででapplycallとのbind違いが明らかになったに違いありません。apply配列のようなオブジェクト、つまり数値lengthプロパティと対応する非負の整数プロパティを持つオブジェクトとして機能する引数を指定できます。一方call、関数への引数を直接指定することができます。とは両方とも、指定されたコンテキストで指定された引数を使用して関数applycallすぐに呼び出します。一方、bind指定されたthis値と引数にバインドされた関数を返すだけです。この返された関数への参照を変数に割り当てることでキャプチャでき、後でいつでも呼び出すことができます。
function add(inc1, inc2)
{
    return this.a + inc1 + inc2;
}

var o = { a : 4 };
document.write(add.call(o, 5, 6)+"<br />"); //15
      //above add.call(o,5,6) sets `this` inside
      //add() to `o` and calls add() resulting:
      // this.a + inc1 + inc2 = 
      // `o.a` i.e. 4 + 5 + 6 = 15
document.write(add.apply(o, [5, 6]) + "<br />"); //15
      // `o.a` i.e. 4 + 5 + 6 = 15

var g = add.bind(o, 5, 6);       //g: `o.a` i.e. 4 + 5 + 6
document.write(g()+"<br />");    //15

var h = add.bind(o, 5);          //h: `o.a` i.e. 4 + 5 + ?
document.write(h(6) + "<br />"); //15
      // 4 + 5 + 6 = 15
document.write(h() + "<br />");  //NaN
      //no parameter is passed to h()
      //thus inc2 inside add() is `undefined`
      //4 + 5 + undefined = NaN</code>

7.thisイベントハンドラー内

  • 要素のイベントハンドラーに関数を直接割り当てる場合、thisイベント処理関数内で直接使用すると、対応する要素が参照されます。このような直接関数の割り当てはaddeventListener、メソッドを使用して、またはのような従来のイベント登録メソッドを介して実行できますonclick
  • 同様に、要素のthisイベントプロパティ(など<button onclick="...this..." >)内で直接使用すると、要素を参照します。
  • ただしthis、イベント処理関数またはイベントプロパティ内で呼び出される他の関数を介して間接的に使用すると、グローバルオブジェクトに解決されますwindow
  • Microsoftのイベント登録モデルメソッドを使用して関数をイベントハンドラーにアタッチすると、上記と同じ動作が実現されattachEventます。関数をイベントハンドラーに割り当てる(したがって、要素の関数メソッドを作成する)代わりに、イベントで関数を呼び出します(事実上グローバルコンテキストで呼び出します)。

JSFiddleでこれを試してみることをお勧めします。

<script> 
    function clickedMe() {
       alert(this + " : " + this.tagName + " : " + this.id);
    } 
    document.getElementById("button1").addEventListener("click", clickedMe, false);
    document.getElementById("button2").onclick = clickedMe;
    document.getElementById("button5").attachEvent('onclick', clickedMe);   
</script>

<h3>Using `this` "directly" inside event handler or event property</h3>
<button id="button1">click() "assigned" using addEventListner() </button><br />
<button id="button2">click() "assigned" using click() </button><br />
<button id="button3" onclick="alert(this+ ' : ' + this.tagName + ' : ' + this.id);">used `this` directly in click event property</button>

<h3>Using `this` "indirectly" inside event handler or event property</h3>
<button onclick="alert((function(){return this + ' : ' + this.tagName + ' : ' + this.id;})());">`this` used indirectly, inside function <br /> defined & called inside event property</button><br />

<button id="button4" onclick="clickedMe()">`this` used indirectly, inside function <br /> called inside event property</button> <br />

IE only: <button id="button5">click() "attached" using attachEvent() </button>

8. thisES6矢印機能で

矢印関数でthisは、は一般的な変数のように動作します。字句スコープから継承されます。関数のthis矢印関数が定義され、矢印機能のだろうthis

つまり、これは次と同じ動作です。

(function(){}).bind(this)

次のコードを参照してください。

const globalArrowFunction = () => {
  return this;
};

console.log(globalArrowFunction()); //window

const contextObject = {
  method1: () => {return this},
  method2: function(){
    return () => {return this};
  }
};

console.log(contextObject.method1()); //window

const contextLessFunction = contextObject.method1;

console.log(contextLessFunction()); //window

console.log(contextObject.method2()()) //contextObject

const innerArrowFunction = contextObject.method2();

console.log(innerArrowFunction()); //contextObject 
66
user3459110 2014-10-27 05:07.

Javascriptの this

単純な関数呼び出し

次の関数について考えてみます。

function foo() {
    console.log("bar");
    console.log(this);
}
foo(); // calling the function

Note that we are running this in the normal mode, i.e. strict mode is not used.

When running in a browser, the value of this would be logged as window. This is because window is the global variable in a web browser's scope.

If you run this same piece of code in an environment like node.js, this would refer to the global variable in your app.

Now if we run this in strict mode by adding the statement "use strict"; to the beginning of the function declaration, this would no longer refer to the global variable in either of the environments. This is done to avoid confusions in strict mode. this would, in this case just log undefined, because that is what it is, it is not defined.

In the following cases, we would see how to manipulate the value of this.

Calling a function on an object

There are different ways to do this. If you have called native methods in Javascript like forEach and slice, you should already know that the this variable in that case refers to the Object on which you called that function (Note that in javascript, just about everything is an Object, including Arrays and Functions). Take the following code for example.

var myObj = {key: "Obj"};
myObj.logThis = function () {
    // I am a method
    console.log(this);
}
myObj.logThis(); // myObj is logged

If an Object contains a property which holds a Function, the property is called a method. This method, when called, will always have it's this variable set to the Object it is associated with. This is true for both strict and non-strict modes.

Note that if a method is stored (or rather, copied) in another variable, the reference to this is no longer preserved in the new variable. For example:

// continuing with the previous code snippet

var myVar = myObj.logThis;
myVar();
// logs either of window/global/undefined based on mode of operation

Considering a more commonly practical scenario:

var el = document.getElementById('idOfEl');
el.addEventListener('click', function() { console.log(this) });
// the function called by addEventListener contains this as the reference to the element
// so clicking on our element would log that element itself

The new keyword

Consider a constructor function in Javascript:

function Person (name) {
    this.name = name;
    this.sayHello = function () {
        console.log ("Hello", this);
    }
}

var awal = new Person("Awal");
awal.sayHello();
// In `awal.sayHello`, `this` contains the reference to the variable `awal`

How does this work? Well, let's see what happens when we use the new keyword.

  1. Calling the function with the new keyword would immediately initialize an Object of type Person.
  2. The constructor of this Object has its constructor set to Person. Also, note that typeof awal would return Object only.
  3. This new Object would be assigned the prototype of Person.prototype. This means that any method or property in the Person prototype would be available to all instances of Person, including awal.
  4. The function Person itself is now invoked; this being a reference to the newly constructed object awal.

Pretty straightforward, eh?

Note that the official ECMAScript spec nowhere states that such types of functions are actual constructor functions. They are just normal functions, and new can be used on any function. It's just that we use them as such, and so we call them as such only.

Calling functions on Functions: call and apply

So yeah, since functions are also Objects (and in-fact first class variables in Javascript), even functions have methods which are... well, functions themselves.

All functions inherit from the global Function, and two of its many methods are call and apply, and both can be used to manipulate the value of this in the function on which they are called.

function foo () { console.log (this, arguments); }
var thisArg = {myObj: "is cool"};
foo.call(thisArg, 1, 2, 3);

This is a typical example of using call. It basically takes the first parameter and sets this in the function foo as a reference to thisArg. All other parameters passed to call is passed to the function foo as arguments.
So the above code will log {myObj: "is cool"}, [1, 2, 3] in the console. Pretty nice way to change the value of this in any function.

apply is almost the same as call accept that it takes only two parameters: thisArg and an array which contains the arguments to be passed to the function. So the above call call can be translated to apply like this:

foo.apply(thisArg, [1,2,3])

Note that call and apply can override the value of this set by dot method invocation we discussed in the second bullet. Simple enough :)

Presenting.... bind!

bind is a brother of call and apply. It is also a method inherited by all functions from the global Function constructor in Javascript. The difference between bind and call/apply is that both call and apply will actually invoke the function. bind, on the other hand, returns a new function with the thisArg and arguments pre-set. Let's take an example to better understand this:

function foo (a, b) {
    console.log (this, arguments);
}
var thisArg = {myObj: "even more cool now"};
var bound = foo.bind(thisArg, 1, 2);
console.log (typeof bound); // logs `function`
console.log (bound);
/* logs `function () { native code }` */

bound(); // calling the function returned by `.bind`
// logs `{myObj: "even more cool now"}, [1, 2]`

See the difference between the three? It is subtle, but they are used differently. Like call and apply, bind will also over-ride the value of this set by dot-method invocation.

Also note that neither of these three functions do any change to the original function. call and apply would return the value from freshly constructed functions while bind will return the freshly constructed function itself, ready to be called.

Extra stuff, copy this

Sometimes, you don't like the fact that this changes with scope, especially nested scope. Take a look at the following example.

var myObj = {
    hello: function () {
        return "world"
        },
    myMethod: function () {
        // copy this, variable names are case-sensitive
        var that = this;
        // callbacks ftw \o/
        foo.bar("args", function () {
            // I want to call `hello` here
            this.hello(); // error
            // but `this` references to `foo` damn!
            // oh wait we have a backup \o/
            that.hello(); // "world"
        });
    }
  };

In the above code, we see that the value of this changed with the nested scope, but we wanted the value of this from the original scope. So we 'copied' this to that and used the copy instead of this. Clever, eh?

Index:

  1. What is held in this by default?
  2. What if we call the function as a method with Object-dot notation?
  3. What if we use the new keyword?
  4. How do we manipulate this with call and apply?
  5. Using bind.
  6. Copying this to solve nested-scope issues.
49
arunjitsingh 2010-06-28 04:10.

"this" is all about scope. Every function has its own scope, and since everything in JS is an object, even a function can store some values into itself using "this". OOP 101 teaches that "this" is only applicable to instances of an object. Therefore, every-time a function executes, a new "instance" of that function has a new meaning of "this".

Most people get confused when they try to use "this" inside of anonymous closure functions like:

(function(value) {
    this.value = value;
    $('.some-elements').each(function(elt){
        elt.innerHTML = this.value;        // uh oh!! possibly undefined
    });
})(2);

So here, inside each(), "this" doesn't hold the "value" that you expect it to (from

this.value = value;
above it). So, to get over this (no pun intended) problem, a developer could:

(function(value) {
    var self = this;            // small change
    self.value = value;
    $('.some-elements').each(function(elt){
        elt.innerHTML = self.value;        // phew!! == 2 
    });
})(2);

Try it out; you'll begin to like this pattern of programming

18
carlodurso 2014-10-30 17:58.

Since this thread has bumped up, I have compiled few points for readers new to this topic.

How is the value of this determined?

We use this similar to the way we use pronouns in natural languages like English: “John is running fast because he is trying to catch the train.” Instead we could have written “… John is trying to catch the train”.

var person = {    
    firstName: "Penelope",
    lastName: "Barrymore",
    fullName: function () {

    // We use "this" just as in the sentence above:
       console.log(this.firstName + " " + this.lastName);

    // We could have also written:
       console.log(person.firstName + " " + person.lastName);
    }
}

this is not assigned a value until an object invokes the function where it is defined. In the global scope, all global variables and functions are defined on the window object. Therefore, this in a global function refers to (and has the value of) the global window object.

When use strict, this in global and in anonymous functions that are not bound to any object holds a value of undefined.

The this keyword is most misunderstood when: 1) we borrow a method that uses this, 2) we assign a method that uses this to a variable, 3) a function that uses this is passed as a callback function, and 4) this is used inside a closure — an inner function. (2)

What holds the future

Defined in ECMA Script 6, arrow-functions adopt the this binding from the enclosing (function or global) scope.

function foo() {
     // return an arrow function
     return (a) => {
     // `this` here is lexically inherited from `foo()`
     console.log(this.a);
  };
}
var obj1 = { a: 2 };
var obj2 = { a: 3 };

var bar = foo.call(obj1);
bar.call( obj2 ); // 2, not 3!

While arrow-functions provide an alternative to using bind(), it’s important to note that they essentially are disabling the traditional this mechanism in favor of more widely understood lexical scoping. (1)


References:

  1. this & Object Prototypes, by Kyle Simpson. © 2014 Getify Solutions.
  2. javascriptissexy.com - http://goo.gl/pvl0GX
  3. Angus Croll - http://goo.gl/Z2RacU
18
Seph 2013-10-07 09:46.

this in JavaScript always refers to the 'owner' of the function that is being executed.

If no explicit owner is defined, then the top most owner, the window object, is referenced.

So if I did

function someKindOfFunction() {
   this.style = 'foo';
}

element.onclick = someKindOfFunction;

this would refer to the element object. But be careful, a lot of people make this mistake.

<element onclick="someKindOfFunction()">

In the latter case, you merely reference the function, not hand it over to the element. Therefore, this will refer to the window object.

15
blockhead 2010-06-28 03:15.

Every execution context in javascript has a this parameter that is set by:

  1. How the function is called (including as an object method, use of call and apply, use of new)
  2. Use of bind
  3. Lexically for arrow functions (they adopt the this of their outer execution context)
  4. Whether the code is in strict or non-strict mode
  5. Whether the code was invoked using eval

You can set the value of this using func.call, func.apply or func.bind.

By default, and what confuses most beginners, when a listener is called after an event is raised on a DOM element, the this value of the function is the DOM element.

jQuery makes this trivial to change with jQuery.proxy.

11
Arman McHitarian 2013-04-24 02:57.

Daniel, awesome explanation! A couple of words on this and good list of this execution context pointer in case of event handlers.

In two words, this in JavaScript points the object from whom (or from whose execution context) the current function was run and it's always read-only, you can't set it anyway (such an attempt will end up with 'Invalid left-hand side in assignment' message.

For event handlers: inline event handlers, such as <element onclick="foo">, override any other handlers attached earlier and before, so be careful and it's better to stay off of inline event delegation at all. And thanks to Zara Alaverdyan who inspired me to this list of examples through a dissenting debate :)

  • el.onclick = foo; // in the foo - obj
  • el.onclick = function () {this.style.color = '#fff';} // obj
  • el.onclick = function() {doSomething();} // In the doSomething - Window
  • el.addEventListener('click',foo,false) // in the foo - obj
  • el.attachEvent('onclick, function () { // this }') // window, all the compliance to IE :)
  • <button onclick="this.style.color = '#fff';"> // obj
  • <button onclick="foo"> // In the foo - window, but you can <button onclick="foo(this)">
11
zangw 2015-11-29 20:01.

Here is one good source of this in JavaScript.

Here is the summary:

  • global this

    In a browser, at the global scope, this is the windowobject

    <script type="text/javascript">
      console.log(this === window); // true
      var foo = "bar";
      console.log(this.foo); // "bar"
      console.log(window.foo); // "bar"
    

    In node using the repl, this is the top namespace. You can refer to it as global.

    >this
      { ArrayBuffer: [Function: ArrayBuffer],
        Int8Array: { [Function: Int8Array] BYTES_PER_ELEMENT: 1 },
        Uint8Array: { [Function: Uint8Array] BYTES_PER_ELEMENT: 1 },
        ...
    >global === this
     true
    

    In node executing from a script, this at the global scope starts as an empty object. It is not the same as global

    \\test.js
    console.log(this);  \\ {}
    console.log(this === global); \\ fasle
    
  • function this

Except in the case of DOM event handlers or when a thisArg is provided (see further down), both in node and in a browser using this in a function that is not called with new references the global scope…

<script type="text/javascript">
    foo = "bar";

    function testThis() {
      this.foo = "foo";
    }

    console.log(this.foo); //logs "bar"
    testThis();
    console.log(this.foo); //logs "foo"
</script>

If you use use strict;, in which case this will be undefined

<script type="text/javascript">
    foo = "bar";

    function testThis() {
      "use strict";
      this.foo = "foo";
    }

    console.log(this.foo); //logs "bar"
    testThis();  //Uncaught TypeError: Cannot set property 'foo' of undefined 
</script>

If you call a function with new the this will be a new context, it will not reference the global this.

<script type="text/javascript">
    foo = "bar";

    function testThis() {
      this.foo = "foo";
    }

    console.log(this.foo); //logs "bar"
    new testThis();
    console.log(this.foo); //logs "bar"

    console.log(new testThis().foo); //logs "foo"
</script>
  • prototype this

Functions you create become function objects. They automatically get a special prototype property, which is something you can assign values to. When you create an instance by calling your function with new you get access to the values you assigned to the prototype property. You access those values using this.

function Thing() {
  console.log(this.foo);
}

Thing.prototype.foo = "bar";

var thing = new Thing(); //logs "bar"
console.log(thing.foo);  //logs "bar"

It is usually a mistake to assign arrays or objects on the prototype. If you want instances to each have their own arrays, create them in the function, not the prototype.

function Thing() {
    this.things = [];
}

var thing1 = new Thing();
var thing2 = new Thing();
thing1.things.push("foo");
console.log(thing1.things); //logs ["foo"]
console.log(thing2.things); //logs []
  • object this

You can use this in any function on an object to refer to other properties on that object. This is not the same as an instance created with new.

var obj = {
    foo: "bar",
    logFoo: function () {
        console.log(this.foo);
    }
};

obj.logFoo(); //logs "bar"
  • DOM event this

In an HTML DOM event handler, this is always a reference to the DOM element the event was attached to

function Listener() {
    document.getElementById("foo").addEventListener("click",
       this.handleClick);
}
Listener.prototype.handleClick = function (event) {
    console.log(this); //logs "<div id="foo"></div>"
}

var listener = new Listener();
document.getElementById("foo").click();

Unless you bind the context

function Listener() {
    document.getElementById("foo").addEventListener("click", 
        this.handleClick.bind(this));
}
Listener.prototype.handleClick = function (event) {
    console.log(this); //logs Listener {handleClick: function}
}

var listener = new Listener();
document.getElementById("foo").click();
  • HTML this

Inside HTML attributes in which you can put JavaScript, this is a reference to the element.

<div id="foo" onclick="console.log(this);"></div>
<script type="text/javascript">
document.getElementById("foo").click(); //logs <div id="foo"...
</script>
  • eval this

You can use eval to access this.

function Thing () {
}
Thing.prototype.foo = "bar";
Thing.prototype.logFoo = function () {
    eval("console.log(this.foo)"); //logs "bar"
}

var thing = new Thing();
thing.logFoo();
  • with this

You can use with to add this to the current scope to read and write to values on this without referring to this explicitly.

function Thing () {
}
Thing.prototype.foo = "bar";
Thing.prototype.logFoo = function () {
    with (this) {
        console.log(foo);
        foo = "foo";
    }
}

var thing = new Thing();
thing.logFoo(); // logs "bar"
console.log(thing.foo); // logs "foo"
  • jQuery this

the jQuery will in many places have this refer to a DOM element.

<div class="foo bar1"></div>
<div class="foo bar2"></div>
<script type="text/javascript">
$(".foo").each(function () { console.log(this); //logs <div class="foo... }); $(".foo").on("click", function () {
    console.log(this); //logs <div class="foo...
});
$(".foo").each(function () {
    this.click();
});
</script>
9
Arup Hore 2017-01-31 22:29.

There is a lot of confusion regarding how "this" keyword is interpreted in JavaScript. Hopefully this article will lay all those to rest once and for all. And a lot more. Please read the entire article carefully. Be forewarned that this article is long.

Irrespective of the context in which it is used, "this" always references the "current object" in Javascript. However, what the "current object" is differs according to context. The context may be exactly 1 of the 6 following:

  1. Global (i.e. Outside all functions)
  2. Inside Direct "Non Bound Function" Call (i.e. a function that has not been bound by calling functionName.bind)
  3. Inside Indirect "Non Bound Function" Call through functionName.call and functionName.apply
  4. Inside "Bound Function" Call (i.e. a function that has been bound by calling functionName.bind)
  5. While Object Creation through "new"
  6. Inside Inline DOM event handler

The following describes each of this contexts one by one:

  1. Global Context (i.e. Outside all functions):

    Outside all functions (i.e. in global context) the "current object" (and hence the value of "this") is always the "window" object for browsers.

  2. Inside Direct "Non Bound Function" Call:

    Inside a Direct "Non Bound Function" Call, the object that invoked the function call becomes the "current object" (and hence the value of "this"). If a function is called without a explicit current object, the current object is either the "window" object (For Non Strict Mode) or undefined (For Strict Mode) . Any function (or variable) defined in Global Context automatically becomes a property of the "window" object.For e.g Suppose function is defined in Global Context as

    function UserDefinedFunction(){
        alert(this)
        }
    

    it becomes the property of the window object, as if you have defined it as

    window.UserDefinedFunction=function(){
      alert(this)
    }  
    

    In "Non Strict Mode", Calling/Invoking this function directly through "UserDefinedFunction()" will automatically call/invoke it as "window.UserDefinedFunction()" making "window" as the "current object" (and hence the value of "this") within "UserDefinedFunction".Invoking this function in "Non Strict Mode" will result in the following

    UserDefinedFunction() // displays [object Window]  as it automatically gets invoked as window.UserDefinedFunction()
    

    In "Strict Mode", Calling/Invoking the function directly through "UserDefinedFunction()" will "NOT" automatically call/invoke it as "window.UserDefinedFunction()".Hence the "current object" (and the value of "this") within "UserDefinedFunction" shall be undefined. Invoking this function in "Strict Mode" will result in the following

    UserDefinedFunction() // displays undefined
    

    However, invoking it explicitly using window object shall result in the following

    window.UserDefinedFunction() // "always displays [object Window]   irrespective of mode."
    

    Let us look at another example. Please look at the following code

     function UserDefinedFunction()
        {
            alert(this.a + ","  + this.b + ","  + this.c  + ","  + this.d)
        }
    
    var o1={
                a:1,
                b:2,
                f:UserDefinedFunction
          }
    var o2={
                c:3,
                d:4,
                f:UserDefinedFunction
           }
    
    o1.f() // Shall display 1,2,undefined,undefined
    o2.f() // Shall display undefined,undefined,3,4
    

    In the above example we see that when "UserDefinedFunction" was invoked through o1, "this" takes value of o1 and the value of its properties "a" and "b" get displayed. The value of "c" and "d" were shown as undefined as o1 does not define these properties

    Similarly when "UserDefinedFunction" was invoked through o2, "this" takes value of o2 and the value of its properties "c" and "d" get displayed.The value of "a" and "b" were shown as undefined as o2 does not define these properties.

  3. Inside Indirect "Non Bound Function" Call through functionName.call and functionName.apply:

    When a "Non Bound Function" is called through functionName.call or functionName.apply, the "current object" (and hence the value of "this") is set to the value of "this" parameter (first parameter) passed to call/apply. The following code demonstrates the same.

    function UserDefinedFunction()
    {
        alert(this.a + ","  + this.b + ","  + this.c  + ","  + this.d)
    }
    var o1={
                a:1,
                b:2,
                f:UserDefinedFunction
           }
    var o2={
                c:3,
                d:4,
                f:UserDefinedFunction
           }
    
    UserDefinedFunction.call(o1) // Shall display 1,2,undefined,undefined
    UserDefinedFunction.apply(o1) // Shall display 1,2,undefined,undefined
    
    UserDefinedFunction.call(o2) // Shall display undefined,undefined,3,4
    UserDefinedFunction.apply(o2) // Shall display undefined,undefined,3,4
    
    o1.f.call(o2) // Shall display undefined,undefined,3,4
    o1.f.apply(o2) // Shall display undefined,undefined,3,4
    
    o2.f.call(o1) // Shall display 1,2,undefined,undefined
    o2.f.apply(o1) // Shall display 1,2,undefined,undefined
    

    The above code clearly shows that the "this" value for any "NON Bound Function" can be altered through call/apply. Also,if the "this" parameter is not explicitly passed to call/apply, "current object" (and hence the value of "this") is set to "window" in Non strict mode and "undefined" in strict mode.

  4. Inside "Bound Function" Call (i.e. a function that has been bound by calling functionName.bind):

    A bound function is a function whose "this" value has been fixed. The following code demonstrated how "this" works in case of bound function

    function UserDefinedFunction()
    {
        alert(this.a + ","  + this.b + ","  + this.c  + ","  + this.d)
    }
    var o1={
              a:1,
              b:2,
              f:UserDefinedFunction,
              bf:null
           }
    var o2={
               c:3,
               d:4,
               f:UserDefinedFunction,
               bf:null
            }
    
    var bound1=UserDefinedFunction.bind(o1); // permanantly fixes "this" value of function "bound1" to Object o1
    bound1() // Shall display 1,2,undefined,undefined
    
    var bound2=UserDefinedFunction.bind(o2); // permanantly fixes "this" value of function "bound2" to Object o2
    bound2() // Shall display undefined,undefined,3,4
    
    var bound3=o1.f.bind(o2); // permanantly fixes "this" value of function "bound3" to Object o2
    bound3() // Shall display undefined,undefined,3,4
    
    var bound4=o2.f.bind(o1); // permanantly fixes "this" value of function "bound4" to Object o1
    bound4() // Shall display 1,2,undefined,undefined
    
    o1.bf=UserDefinedFunction.bind(o2) // permanantly fixes "this" value of function "o1.bf" to Object o2
    o1.bf() // Shall display undefined,undefined,3,4
    
    o2.bf=UserDefinedFunction.bind(o1) // permanantly fixes "this" value of function "o2.bf" to Object o1
    o2.bf() // Shall display 1,2,undefined,undefined
    
    bound1.call(o2) // Shall still display 1,2,undefined,undefined. "call" cannot alter the value of "this" for bound function
    
    bound1.apply(o2) // Shall still display 1,2,undefined,undefined. "apply" cannot alter the value of "this" for bound function
    
    o2.bf.call(o2) // Shall still display 1,2,undefined,undefined. "call" cannot alter the value of "this" for bound function
    o2.bf.apply(o2) // Shall still display 1,2,undefined,undefined."apply" cannot alter the value of "this" for bound function
    

    As given in the code above, "this" value for any "Bound Function" CANNOT be altered through call/apply. Also, if the "this" parameter is not explicitly passed to bind, "current object" (and hence the value of "this" ) is set to "window" in Non strict mode and "undefined" in strict mode. One more thing. Binding an already bound function does not change the value of "this". It remains set as the value set by first bind function.

  5. While Object Creation through "new":

    Inside a constructor function, the "current object" (and hence the value of "this") references the object that is currently being created through "new" irrespective of the bind status of the function. However if the constructor is a bound function it shall get called with predefined set of arguments as set for the bound function.

  6. Inside Inline DOM event handler:

    Please look at the following HTML Snippet

    <button onclick='this.style.color=white'>Hello World</button>
    <div style='width:100px;height:100px;' onclick='OnDivClick(event,this)'>Hello World</div>
    

    The "this" in above examples refer to "button" element and the "div" element respectively.

    In the first example, the font color of the button shall be set to white when it is clicked.

    In the second example when the "div" element is clicked it shall call the OnDivClick function with its second parameter referencing the clicked div element. However the value of "this" within OnDivClick SHALL NOT reference the clicked div element. It shall be set as the "window object" or "undefined" in Non strict and Strict Modes respectively (if OnDivClick is an unbound function) or set to a predefined Bound value (if OnDivClick is a bound function)

The following summarizes the entire article

  1. In Global Context "this" always refers to the "window" object

  2. Whenever a function is invoked, it is invoked in context of an object ("current object"). If the current object is not explicitly provided, the current object is the "window object" in NON Strict Mode and "undefined" in Strict Mode by default.

  3. The value of "this" within a Non Bound function is the reference to object in context of which the function is invoked ("current object")

  4. The value of "this" within a Non Bound function can be overriden by call and apply methods of the function.

  5. The value of "this" is fixed for a Bound function and cannot be overriden by call and apply methods of the function.

  6. Binding and already bound function does not change the value of "this". It remains set as the value set by first bind function.

  7. The value of "this" within a constructor is the object that is being created and initialized

  8. The value of "this" within an inline DOM event handler is reference to the element for which the event handler is given.

9
Dmitri Pavlutin 2016-11-09 02:17.

Probably the most detailed and comprehensive article on this is the following:

Gentle explanation of 'this' keyword in JavaScript

The idea behind this is to understand that the function invocation types have the significant importance on setting this value.


When having troubles identifying this, do not ask yourself:

Where is this taken from?

but do ask yourself:

How is the function invoked?

For an arrow function (special case of context transparency) ask yourself:

What value has this where the arrow function is defined?

This mindset is correct when dealing with this and will save you from headache.

7
James Drinkard 2016-07-13 09:03.

This is the best explanation I've seen: Understand JavaScripts this with Clarity

The this reference ALWAYS refers to (and holds the value of) an object—a singular object—and it is usually used inside a function or a method, although it can be used outside a function in the global scope. Note that when we use strict mode, this holds the value of undefined in global functions and in anonymous functions that are not bound to any object.

There are Four Scenarios where this can be confusing:

  1. When we pass a method (that uses this) as an argument to be used as a callback function.
  2. When we use an inner function (a closure). It is important to take note that closures cannot access the outer function’s this variable by using the this keyword because the this variable is accessible only by the function itself, not by inner functions.
  3. When a method which relies on this is assigned to a variable across contexts, in which case this references another object than originally intended.
  4. When using this along with the bind, apply, and call methods.

He gives code examples, explanations, and solutions, which I thought was very helpful.

6
Nipuna 2014-08-09 23:22.

this is one of the misunderstood concept in JavaScript because it behaves little differently from place to place. Simply, this refers to the "owner" of the function we are currently executing.

this helps to get the current object (a.k.a. execution context) we work with. If you understand in which object the current function is getting executed, you can understand easily what current this is

var val = "window.val"

var obj = {
    val: "obj.val",
    innerMethod: function () {
        var val = "obj.val.inner",
            func = function () {
                var self = this;
                return self.val;
            };

        return func;
    },
    outerMethod: function(){
        return this.val;
    }
};

//This actually gets executed inside window object 
console.log(obj.innerMethod()()); //returns window.val

//Breakdown in to 2 lines explains this in detail
var _inn = obj.innerMethod();
console.log(_inn()); //returns window.val

console.log(obj.outerMethod()); //returns obj.val

Above we create 3 variables with same name 'val'. One in global context, one inside obj and the other inside innerMethod of obj. JavaScript resolves identifiers within a particular context by going up the scope chain from local go global.


Few places where this can be differentiated

Calling a method of a object

var status = 1;
var helper = {
    status : 2,
    getStatus: function () {
        return this.status;
    }
};

var theStatus1 = helper.getStatus(); //line1
console.log(theStatus1); //2

var theStatus2 = helper.getStatus;
console.log(theStatus2()); //1

When line1 is executed, JavaScript establishes an execution context (EC) for the function call, setting this to the object referenced by whatever came before the last ".". so in the last line you can understand that a() was executed in the global context which is the window.

With Constructor

this can be used to refer to the object being created

function Person(name){
    this.personName = name;
    this.sayHello = function(){
        return "Hello " + this.personName;
    }
}

var person1 = new Person('Scott');
console.log(person1.sayHello()); //Hello Scott

var person2 = new Person('Hugh');
var sayHelloP2 = person2.sayHello;
console.log(sayHelloP2()); //Hello undefined

When new Person() is executed, a completely new object is created. Person is called and its this is set to reference that new object.

Function call

function testFunc() {
    this.name = "Name";
    this.myCustomAttribute = "Custom Attribute";
    return this;
}

var whatIsThis = testFunc();
console.log(whatIsThis); //window

var whatIsThis2 = new testFunc();
console.log(whatIsThis2);  //testFunc() / object

console.log(window.myCustomAttribute); //Custom Attribute 

If we miss new keyword, whatIsThis referes to the most global context it can find(window)

With event handlers

If the event handler is inline, this refers to global object

<script type="application/javascript">
    function click_handler() {
        alert(this); // alerts the window object
    }
</script>

<button id='thebutton' onclick='click_handler()'>Click me!</button>

When adding event handler through JavaScript, this refers to DOM element that generated the event.


  • You can also manipulate the context using .apply() .call() and .bind()
  • JQuery proxy is another way you can use to make sure this in a function will be the value you desire. (Check Understanding $.proxy(), jQuery.proxy() usage)
  • What does var that = this means in JavaScript
6
mrmaclean89 2017-09-17 12:11.

In pseudoclassical terms, the way many lectures teach the 'this' keyword is as an object instantiated by a class or object constructor. Each time a new object is constructed from a class, imagine that under the hood a local instance of a 'this' object is created and returned. I remember it taught like this:

function Car(make, model, year) {
var this = {}; // under the hood, so to speak
this.make = make;
this.model = model;
this.year = year;
return this; // under the hood
}

var mycar = new Car('Eagle', 'Talon TSi', 1993);
// ========= under the hood
var this = {};
this.make = 'Eagle';
this.model = 'Talon TSi';
this.year = 1993;
return this;
5
Trombe 2017-07-04 00:07.

The value of "this" depends on the "context" in which the function is executed. The context can be any object or the global object, i.e., window.

So the Semantic of "this" is different from the traditional OOP languages. And it causes problems: 1. when a function is passed to another variable (most likely, a callback); and 2. when a closure is invoked from a member method of a class.

In both cases, this is set to window.

4
Simon Groenewolt 2010-06-28 03:15.

Whould this help? (Most confusion of 'this' in javascript is coming from the fact that it generally is not linked to your object, but to the current executing scope -- that might not be exactly how it works but is always feels like that to me -- see the article for a complete explanation)

3
unclexo 2018-07-25 21:04.

A little bit info about this keyword

Let's log this keyword to the console in global scope without any more code but

console.log(this)

In Client/Browser this keyword is a global object which is window

console.log(this === window) // true

and

In Server/Node/Javascript runtime this keyword is also a global object which is module.exports

console.log(this === module.exports) // true
console.log(this === exports) // true

Keep in mind exports is just a reference to module.exports

1
PRADEEP SINGH Chundawat 2016-04-06 21:15.

this use for Scope just like this

  <script type="text/javascript" language="javascript">
$('#tbleName tbody tr').each(function{ var txt=''; txt += $(this).find("td").eq(0).text();
\\same as above but synatx different
var txt1='';
 txt1+=$('#tbleName tbody tr').eq(0).text();
alert(txt1)
});
</script>

value of txt1 and txt is same in Above example $(this)=$('#tbleName tbody tr') is Same

1
gman 2019-01-29 08:37.

I have a different take on this from the other answers that I hope is helpful.

One way to look at JavaScript is to see that there are only 1 way to call a function1. It is

functionObject.call(objectForThis, arg0, arg1, arg2, ...);

There is always some value supplied for objectForThis.

Everything else is syntactic sugar for functionObject.call

So, everything else can be described by how it translates into functionObject.call.

If you just call a function then this is the "global object" which in the browser is the window

function foo() {
  console.log(this);
}

foo();  // this is the window object

In other words,

foo();

was effectively translated into

foo.call(window);

Note that if you use strict mode then this will be undefined

'use strict';

function foo() {
  console.log(this);
}

foo();  // this is the window object

which means

In other words,

foo();

was effectively translated into

foo.call(undefined);

In JavaScript there are operators like + and - and *. There is also the dot operator which is .

The . operator when used with a function on the right and an object on the left effectively means "pass object as this to function.

Example

const bar = {
  name: 'bar',
  foo() { 
    console.log(this); 
  },
};

bar.foo();  // this is bar

In other words bar.foo() translates into const temp = bar.foo; temp.call(bar);

Note that it doesn't matter how the function was created (mostly...). All of these will produce the same results

const bar = {
  name: 'bar',
  fn1() { console.log(this); },
  fn2: function() { console.log(this); },
  fn3: otherFunction,
};

function otherFunction() { console.log(this) };

bar.fn1();  // this is bar
bar.fn2();  // this is bar
bar.fn3();  // this is bar

Again these all are just syntactic sugar for

{ const temp = bar.fn1; temp.call(bar); }
{ const temp = bar.fn2; temp.call(bar); }
{ const temp = bar.fn3; temp.call(bar); }

One other wrinkle is the prototype chain. When you use a.b JavaScript first looks on the object referenced directly by a for the property b. If b is not found on the object then JavaScript will look in the object's prototype to find b.

There are various ways to define an object's prototype, the most common in 2019 is the class keyword. For the purposes of this though it doesn't matter. What matters is that as it looks in object a for property b if it finds property b on the object or in it's prototype chain if b ends up being a function then the same rules as above apply. The function b references will be called using the call method and passing a as objectForThis as shown a the top of this answer.

Now. Let's imagine we make a function that explicitly sets this before calling another function and then call it with the . (dot) operator

function foo() {
  console.log(this);
}

function bar() {
  const objectForThis = {name: 'moo'}
  foo.call(objectForThis);  // explicitly passing objectForThis
}

const obj = {
  bar,
};

obj.bar();  

Following the translation to use call, obj.bar() becomes const temp = obj.bar; temp.call(obj);. When we enter the bar function we call foo but we explicitly passed in another object for objectForThis so when we arrive at foo this is that inner object.

This is what both bind and => functions effectively do. They are more syntactic sugar. They effectively build a new invisible function exactly like bar above that explicitly sets this before it calls whatever function is specified. In the case of bind this is set to whatever you pass to bind.

function foo() {
  console.log(this);
}

const bar = foo.bind({name: 'moo'});

// bind created a new invisible function that calls foo with the bound object.

bar();  

// the objectForThis we are passing to bar here is ignored because
// the invisible function that bind created will call foo with with
// the object we bound above

bar.call({name: 'other'});

Note that if functionObject.bind did not exist we could make our own like this

function bind(fn, objectForThis) {
  return function(...args) {
    return fn.call(objectForthis, ...args);
  };
}

and then we could call it like this

function foo() {
  console.log(this);
}

const bar = bind(foo, {name:'abc'});

Arrow functions, the => operator are syntactic sugar for bind

const a = () => {console.log(this)};

is the same as

const tempFn = function() {console.log(this)}; 
const a = tempFn.bind(this);

Just like bind, a new invisible function is created that calls the given function with a bound value for objectForThis but unlike bind the object to be bound is implicit. It's whatever this happens to be when the => operator is used.

So, just like the rules above

const a = () => { console.log(this); }  // this is the global object
'use strict';
const a = () => { console.log(this); }  // this is undefined
function foo() {
  return () => { console.log(this); }
}

const obj = {
  foo,
};
const b = obj.foo();
b();

obj.foo() translates to const temp = obj.foo; temp.call(obj); which means the arrow operator inside foo will bind obj to a new invisible function and return that new invisible function which is assigned to b. b() will work as it always has as b.call(window) or b.call(undefined) calling the new invisible function that foo created. That invisible function ignores the this passed into it and passes obj as objectForThis` to the arrow function.

The code above translates to

function foo() {
  function tempFn() {
    console.log(this);
  }
  return tempFn.bind(this);
}

const obj = {
  foo,
};
const b = obj.foo();
b.call(window or undefined if strict mode);

1apply is another function similar to call

functionName.apply(objectForThis, arrayOfArgs);

But as of ES6 conceptually you can even translate that into

functionName.call(objectForThis, ...arrayOfArgs);
0
Willem van der Veen 2018-08-22 07:12.

Summary this Javascript:

  • The value of this is determined by how the function is invoked not, where it was created!
  • Usually the value of this is determined by the Object which is left of the dot. (window in global space)
  • In event listeners the value of this refers to the DOM element on which the event was called.
  • When in function is called with the new keyword the value of this refers to the newly created object
  • You can manipulate the value of this with the functions: call, apply, bind

Example:

let object = {
  prop1: function () {console.log(this);}
}

object.prop1();   // object is left of the dot, thus this is object

const myFunction = object.prop1 // We store the function in the variable myFunction

myFunction(); // Here we are in the global space
              // myFunction is a property on the global object
              // Therefore it logs the window object
              
             

Example event listeners:

document.querySelector('.foo').addEventListener('click', function () {
  console.log(this);   // This refers to the DOM element the eventListener was invoked from
})


document.querySelector('.foo').addEventListener('click', () => {
  console.log(this);  // Tip, es6 arrow function don't have their own binding to the this v
})                    // Therefore this will log the global object
.foo:hover {
  color: red;
  cursor: pointer;
}
<div class="foo">click me</div>

Example constructor:

function Person (name) {
  this.name = name;
}

const me = new Person('Willem');
// When using the new keyword the this in the constructor function will refer to the newly created object

console.log(me.name); 
// Therefore, the name property was placed on the object created with new keyword.

0
Lord 2020-05-20 23:43.

To understand "this" properly one must understand the context and scope and difference between them.

Scope: In javascript scope is related to the visibility of the variables, scope achieves through the use of the function. (Read more about scope)

Context: Context is related to objects. It refers to the object to which a function belongs. When you use the JavaScript “this” keyword, it refers to the object to which function belongs. For example, inside of a function, when you say: “this.accoutNumber”, you are referring to the property “accoutNumber”, that belongs to the object to which that function belongs.

If the object “myObj” has a method called “getMyName”, when the JavaScript keyword “this” is used inside of “getMyName”, it refers to “myObj”. If the function “getMyName” were executed in the global scope, then “this” refers to the window object (except in strict mode).

Now let's see some example:

    <script>
        console.log('What is this: '+this);
        console.log(this);
    </script>

Runnig abobve code in browser output will:

According to the output you are inside of the context of the window object, it is also visible that window prototype refers to the Object.

Now let's try inside of a function:

    <script>
        function myFunc(){
            console.log('What is this: '+this);
            console.log(this);
        }
        myFunc();
    </script>

Output:

The output is the same because we logged 'this' variable in the global scope and we logged it in functional scope, we didn't change the context. In both case context was same, related to widow object.

Now let's create our own object. In javascript, you can create an object in many ways.

 <script>
        var firstName = "Nora";
        var lastName = "Zaman";
        var myObj = {
            firstName:"Lord",
            lastName:'Baron',
            printNameGetContext:function(){
                console.log(firstName + " "+lastName);
                console.log(this.firstName +" "+this.lastName);
                return this;
            }
        }

      var context = myObj.printNameGetContext();
      console.log(context);
    </script>

Output:

So from the above example, we found that 'this' keyword is referring to a new context that is related to myObj, and myObject also has prototype chain to Object.

Let's go throw another example:

<body>
    <button class="btn">Click Me</button>
    <script>
        function printMe(){
            //Terminal2: this function declared inside window context so this function belongs to the window object.
            console.log(this);
        }
        document.querySelector('.btn').addEventListener('click', function(){
            //Terminal1: button context, this callback function belongs to DOM element 
            console.log(this);
            printMe();
        })
    </script>
</body>

output: Make sense right? (read comments)

If you having trouble to understand the above example let's try with our own callback;

<script>
        var myObj = {
            firstName:"Lord",
            lastName:'Baron',
            printName:function(callback1, callback2){
                //Attaching callback1 with this myObj context
                this.callback1 = callback1;
                this.callback1(this.firstName +" "+this.lastName)
                //We did not attached callback2 with myObj so, it's reamin with window context by default
                callback2();
                /*
                 //test bellow codes
                 this.callback2 = callback2;
                 this.callback2();
                */
            }
        }
        var callback2 = function (){
            console.log(this);
        }
        myObj.printName(function(data){
            console.log(data);
            console.log(this);
        }, callback2);
    </script>

output:

Now let's Understand Scope, Self, IIFE and THIS how behaves

       var color = 'red'; // property of window
       var obj = {
           color:'blue', // property of window
           printColor: function(){ // property of obj, attached with obj
               var self = this;
               console.log('In printColor -- this.color: '+this.color);
               console.log('In printColor -- self.color: '+self.color);
               (function(){ // decleard inside of printColor but not property of object, it will executed on window context.
                    console.log(this)
                    console.log('In IIFE -- this.color: '+this.color);
                    console.log('In IIFE -- self.color: '+self.color); 
               })();

               function nestedFunc(){// decleard inside of printColor but not property of object, it will executed on window context.
                    console.log('nested fun -- this.color: '+this.color);
                    console.log('nested fun -- self.color: '+self.color);
               }

               nestedFunc(); // executed on window context
               return nestedFunc;
           }
       };

       obj.printColor()(); // returned function executed on window context
   </script> 

Output is pretty awesome right?

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2021年は、世界的なベンチャーキャピタル(VC)の資金調達にとって記録的な年でした。DealStreetAsiaによると、東南アジアも例外ではなく、この地域では年間で記録的な25の新しいユニコーンが採掘されました。

ムーアの法則を超えて

ムーアの法則を超えて

計算に対する私たちの欲求とムーアの法則が提供できるものとの間には、指数関数的に増大するギャップがあります。私たちの文明は計算に基づいています—建築と想像力の現在の限界を超える技術を見つけなければなりません。

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