The this keyword

The keyword this refers to the value of the object that is bound to the function at the time of its call, meaning that its value is different depending on whether a function is called as a method, as a standalone function, or as a constructor.

When a function is called, it creates an instance of the keyword this behind the scenes as a reference to the object that contains that function, giving access to the properties and methods defined alongside it from within its scope. Working with this is similar in some ways to working with a variable declared with const. Like a constant, this can't be removed and its value can't be reassigned, but the methods and properties of the object that the this keyword contains can be altered.

Global binding

Outside a function or the context of an object, this refers to the globalThis property, which is a reference to the global object in most JavaScript environments. In the context of a script running in a web browser, the global object is the window object:

this;
> Window {0: Window, window: Window, self: Window, document: document, name: '', location: Location, ...}

In Node.js, globalThis is the global object:

$ node
Welcome to Node.js v20.10.0.
Type ".help" for more information.
> this
<ref *1> Object [global] {
...
}

Outside strict mode, this also refers to the global object inside a standalone function, because the parent Window is the object that effectively "owns" those functions.

function myFunction() {
    console.log( this );
}
myFunction();
> Window {...}

(function() {
    console.log( this );
}());
> Window {...}

When using strict mode, this has a value of undefined inside a standalone function:

(function() {
    "use strict";
    console.log( this );
}());
> undefined

Before the introduction of strict mode, a null or undefined value for this would be replaced by a reference to the global object. You might sometimes see global binding referred to as "default binding" because of this legacy behavior.

Implicit binding

When a function is called as a method of an object, an instance of this inside that method refers to the object that contains the method, giving access to the methods and properties that sit alongside it:

let myObject = {
    myValue: "This is my string.",
    myMethod() {
            console.log( this.myValue );
    }
};

myObject.myMethod();
> "This is my string."

It might look like the value of this depends on how a function and its enclosing object are defined. Instead, the context for the value of this is the current execution context. In this case, the execution context is that the myObject object is calling the myMethod method, so myObject is the value for this. This might seem like a technicality in the context of the previous examples, but for more advanced uses of this, it's an essential distinction to keep in mind.

In general, use this in ways that don't expect the surrounding code to have any particular structure. The exception to this rule is ES5 Arrow functions.

this in arrow functions

In arrow functions, this resolves to a binding in a lexically enclosing environment. This means that this in an arrow function refers to the value of this in that function's closest enclosing context:

let myObject = {
    myMethod() { console.log( this ); },
    myArrowFunction: () => console.log( this ),
    myEnclosingMethod: function () {
        this.myArrowFunction = () => { console.log(this) };
    }
};

myObject.myMethod();
> Object { myMethod: myMethod(), myArrowFunction: myArrowFunction() }

myObject.myArrowFunction();
> Window {...}

In the previous example, myObject.myMethod() logs myObject as the object that "owns" that method, but myObject.myArrowFunction() returns globalThis (or undefined), because the instance of this inside the arrow function refers instead to the highest enclosing scope.

In the following example, myEnclosingMethod creates an arrow function on the object that contains it when it's executed. The instance of this inside the arrow function now refers to the value of this inside the enclosing environment, which is the method that contains that arrow function. Because the value of this inside myEnclosingMethod refers to myObject, after you define the arrow function, this inside the arrow function also refers to myObject:

let myObject = {
    myMethod() { console.log( this ); },
    myEnclosingMethod: function () {
        this.myArrowFunction = () => { console.log(this) };
    }
};

myObject.myEnclosingMethod();
myObject.myArrowFunction();
> Object { myMethod: myMethod(), myArrowFunction: myArrowFunction() }

Explicit binding

Implicit binding handles most use cases for working with this. However, you might sometimes need the value of this to represent a specific execution context, instead of the assumed context. An illustrative, if slightly outdated, example is working with this within the callback function of a setTimeout, because this callback has a unique execution context:

var myObject = {
  myString: "This is my string.",
  myMethod() {
    console.log( this.myString );
  }
};
myObject.myMethod();
> "This is my string."

setTimeout( myObject.myMethod, 100 );
> undefined

Although this specific shortcoming of setTimeout has since been addressed by other features, similar issues of "losing" this have previously been addressed by creating an explicit reference to the value of this within the scope of the intended context. You might occasionally see instances of this being assigned to a variable using identifiers like that, self, or _this in legacy codebases. These are common identifier conventions for variables containing a passed this value.

When you call a function using the call(), bind(), or apply() methods, this explicitly references the object being called:

let myFunction = function() {
    console.log( this.myValue );
}

let myObject = {
   "myValue" : "This is my string."
 };

myFunction.call( myObject );
> "This is my string."

var myObject = {
  myString: "This is my string.",
  myMethod() {
    console.log( this.myString );
  }
};

setTimeout( myObject.myMethod.bind( myObject ), 100 );
> "This is my string."

Explicit binding overrides the this value provided by implicit binding.

let myObject = {
    "myValue" : "This string sits alongside myMethod.",
    myMethod() {
        console.log( this.myValue );
    }
};
let myOtherObject = {
    "myValue" : "This is a string in another object entirely.",
};

myObject.myMethod.call( myOtherObject );
> "This is a string in another object entirely."

If a function is called in a way that would set the value of this to undefined or null, that value is replaced by globalThis outside strict mode:

let myFunction = function() {
    console.log( this );
}

myFunction.call( null );
> Window {...}

Similarly, if a function is called in a way that would give this a primitive value, that value is substituted with the primitive value's wrapper object outside strict mode:

let myFunction = function() {
    console.log( this );
}

let myNumber = 10;

myFunction.call( myNumber );
> Number { 10 }

In strict mode, a passed this value isn't coerced to an object in any way, even if it's a primitive, null, or undefined value:

"use strict";
let myFunction = function() {
    console.log( this );
}

let myNumber = 10;

myFunction.call( myNumber );
> 10

myFunction.call( null );
> null

new binding

When a class is used as a constructor using the new keyword, this refers to the newly-created instance:

class MyClass {
    myString;
    constructor() {
        this.myString = "My string.";
    }
    logThis() {
        console.log( this );
    }
}
const thisClass = new MyClass();

thisClass.logThis();
> Object { myString: "My string." }

Similarly, the value of this inside a constructor function called using new refers to the object being created:

function MyFunction() {
  this.myString = "My string.";
  this.logThis = function() {
    console.log( this );
  }
}
const myObject = new MyFunction();

myObject.logThis();
> Object { myString: "My string.", logThis: logThis() }

Event handler binding

In the context of event handlers, the value of this references the object that invokes it. Inside an event handler's callback function, that means this references the element associated with the handler:

let button = document.querySelector( "button" );

button.addEventListener( "click", function( event ) { console.log( this ); } );

When a user interacts with the button in the previous snippet, the result is the element object containing the <button> itself:

> Button {}

When an arrow function is used as an event listener callback, the value of this is again provided by the closest enclosing execution context. At the top level, that means this inside an event handler callback function is globalThis:

let button = document.querySelector( "button" );

button.addEventListener( "click", ( event ) => { console.log( this ); } );
> undefined

As with any other object, when you use the call(), bind(), or apply() methods to reference the callback function of an event listener, this references the object explicitly:

let button = document.querySelector( "button" );
let myObject = {
    "myValue" : true
};
function handleClick() {
    console.log( this );
}

button.addEventListener( "click", handleClick.bind( myObject ) );
> Object { myValue: true }

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