Javascript This Keyword

Definition of Javascript This Keyword

The value of 'This' is always determined by its execution context. It can't be set manually, it will always point to an object, and that object might not always be what you expect. Here's a simple example:

let object = {
    greeting: 'Hello, ',
    speak() {
        return this.greeting + 'world!';
    }
}

console.log(object.speak());  // returns 'Hello, world!'.

In this example, 'This' inside the speak() method refers to the object. Hence, 'this.greeting' retrieves the greeting property of the object.

Role of This Keyword in Javascript

Let’s consider another example to illustrate the role of this keyword:

let student = {
    firstName: 'John',
    lastName: 'Doe',
    getFullName: function(){
        return this.firstName + " " + this.lastName;
    }
}

console.log(student.getFullName());  // returns 'John Doe'.

Here, 'this.firstName' retrieves the firstName property of the student object and 'this.lastName' fetches the lastName property of the student object.

Importance of This Keyword in Computer Programming

Employing the 'this' keyword can often lead to more concise and readable code, as you can refer to properties and methods of the current object without needing to know its name:

let car = {
    speed: 0,
    accelerate: function(amount) {
        this.speed += amount;
    }
}

car.accelerate(10);
console.log(car.speed);  // returns 10.

Here, 'this.speed' refers to the speed property of the car object, allowing flexible manipulation without directly referring to the car by name.

How to Use This Keyword in Javascript

Having understood what 'This' keyword is in the world of JavaScript, it's now time to delve deeper into its applications. How do you use this incredibly versatile tool to maximise your coding potential? Let's explore!

An Overview on the Application of This Keyword in Javascript

The 'This' keyword's main applications lie in object constructors, prototype methods, and object methods. The value of 'This' remains initially undefined until a function where it is defined executes. Here's a basic summary:

• When a function operates globally, 'This' refers to the global object.
• For a function that is part of an object, 'This' refers to the object it is part of.
• In 'strict mode', 'This' remains at whatever it's set to when entering the execution context. If it's not defined, it remains undefined.
• When a function is called as an event handler, 'This' becomes the element that fired the event.

The context in which the code is running determines the value of 'This'. Think of it as the subject of the context - very similar to how 'I' is used in English. Here's a practical example to describe the behaviour:

function globalCheck(){
  return this;
}

console.log(globalCheck());  //returns the global Window object.

In this case, the function is executed in a global context, hence 'This' refers to the window object.

Detailed Guide on How to Use This Keyword

To effectively use the 'This' keyword in your JavaScript undertakings, here's a detailed guide to three specific scenarios where 'This' shows its true potential: Object Methods, Constructor Functions, and Prototypal Inheritance.

            let person = {
                name: 'Alice',
                greet: function() {
                    console.log('Hi, ' + this.name);
                }
            }

            person.greet(); //returns 'Hi, Alice'.
            

            function Person(name) {
                this.name = name;
                this.greet = function() {
                    console.log('Hi, ' + this.name);
                }
            }

            let bob = new Person('Bob');
            bob.greet(); //returns 'Hi, Bob'.
            

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

            Person.prototype.greet = function() {
                console.log('Hi, ' + this.name);
            }

            let charlie = new Person('Charlie');
            charlie.greet(); //returns 'Hi, Charlie'.
            

In conclusion, the semantics of 'This' in Javascript cater to the various scopes and contexts that a function can execute within, providing a versatile and robust tool for programming any task.

Javascript Arrow Function and This Keyword

An innovative concept introduced with ES6, arrow functions are a concise syntax for writing function expressions in JavaScript. They are anonymous and change the way 'This' binds in functions. Arrow functions are syntactically similar to regular ones, but the key difference lies in the context-specific behaviour of 'This' keyword.

Interaction of This Keyword with Javascript Arrow Function

In regular JavaScript functions, every new function defines its own 'This' value based on how the function is called. However, arrow functions are designed to behave differently. Rather than establishing their own 'This' context, arrow functions instead capture the 'This' context of the enclosing lexical context; in other words, they 'inherit' the 'This' context from their parent.

let car = {
    speed: 0,
    accelerate: function (amount) {
        setTimeout(() => {
            this.speed += amount;
            console.log(this.speed);
        }, 1000);
    }
};

car.accelerate(10);    // prints 10.

In this example, 'This' inside the arrow function used for the setTimeout callback refers to the car object, which is enclosing the scope of this function. Unlike regular functions, the 'This' context does not change because of the setTimeout. Please note again, arrow function does not create its own 'This', it captures the 'This' context from its parent scope.

This context inheritance discussed above is actually a very useful feature of arrow function, as it saves from dealing with unexpected 'This' value in callbacks and reduces the use of 'bind()' or 'self = this' phrases.

A key thing to remember is that

• In an arrow function, the 'This' context isn’t influenced by how and where they are called, but only by containing scope.
• A new, standalone arrow function cannot bind 'This'; it remains in the state of the enclosing context.

Therefore, you should keep in mind that if you’re trying to apply '.call()', '.apply()', '.bind()' to an Arrow function and pass a new 'This' context, it simply won’t work.

let speed = function () {
    return this.isFast ? 'fast' : 'slow';
};

console.log(speed.call({isFast: true}));  // prints 'fast'.

In this example, 'This' inside the speed function refers to the object passed in by using call. However, if speed is an arrow function, the following will occur:

let speed = () => this.isFast ? 'fast' : 'slow';

console.log(speed.call({isFast: true}));  // ReferenceError: isFast is not defined.

Here, 'This' inside speed function remains at its Lexical scope and is not influenced by call method, resulting in an error message because 'isFast' is not defined in the global context.

Understanding the Dynamics of Arrow Functions and This Keyword

Below is a detailed exploration of how 'This' behaves differently in Arrow Functions as compared to regular function expressions when it comes to various scenarios:

1. Global Scope - In both Arrow functions and normal functions, 'This' at the global level points to the Window Object.
2. Object Method - 'This' in a standard function within an object points to that object. However, in an Arrow function enclosed within an object, 'This' points to the window/global object.
3. Event Listeners - In an event listener, 'This' in a regular function refers to the object that the event is being applied to. In contrast, 'This' in an Arrow function within an event listener points to the Window Object.
4. Constructor Functions - Here is where things diverge significantly. You can't create a constructor using an Arrow function because they can't be used with 'new'. They also lack the prototype property, which brings us to the fact they can't handle such features of object-oriented programming as 'super' or 'new.target'.

In conclusion, while Arrow functions offer more concise syntax and fix some JavaScript quirks, they also have distinct characteristics pertaining to 'This' keyword, which you need to be mindful of when designing your JavaScript applications.

This Keyword In Classes Javascript

Familiarity with 'This' keyword can be a major game-changer when working with classes in JavaScript. It is a special keyword that represents the context in which current code is executing, for example, it can represent an object that invoked the current method or a new instance of a class among other things.

The Connection Between This Keyword and Javascript Classes

In JavaScript classes, 'This' keyword is fundamentally used to access and manipulate properties of an instance of the class within the class methods. In other words, it represents an instance of the class where it’s currently being used.

Typically, when using the class to create objects, 'This' is assigned the new object being made. By using 'This' keyword you’re able to set or change properties on the object, as well as access them for some computation or processing.

To illustrate, consider a simple 'Person' class:

class Person {
    constructor(name, age) {
        this.name = name;
        this.age = age;
    }

    greet() {
        console.log(`Hello, my name is ${this.name}.`);
    }
}

In the Person class, the constructor function assigns the name and age arguments to the 'This' keyword so that these properties are set on objects created from the Person class. 'This' inside class methods refers to the instance that the method has been called on.

In the greet method, 'This' is once again used, this time to refer to the name of the person object being greeted. So if we create a new person and call greet:

let john = new Person('John', 25);
john.greet();  // prints 'Hello, my name is John.'

'This' keyword is referring to the 'john' instance of the 'Person' class, and retrieves the 'name' property of 'john'.

Examples of Using This Keyword in Javascript Classes

Here are some more concrete examples of how the 'This' keyword can be used in JavaScript Classes:

class Car {
    constructor(brand) {
        this.carName = brand;
    }

    present() {
        return 'I have a ' + this.carName;
    }
}

let myCar = new Car("Ford");
console.log(myCar.present());  // prints 'I have a Ford'

class Animal {
    constructor(type, name) {
        this.type = type;
        this.name = name;
    }

    introduce() {
        console.log(`Hello, I am a ${this.type} and my name is ${this.name}.`);
    }
}

let myPet = new Animal('Dog', 'Bruno');
myPet.introduce();  // prints 'Hello, I am a Dog and my name is Bruno.'

Working with JavaScript classes, understanding and mastering 'This' keyword is absolutely crucial. However, understanding 'This' doesn't stop at classes. It’s useful when dealing with regular functions, event handlers and even array methods. It offers a more flexible and effective way to access properties and methods of objects, enhancing the potential of your code.

Examining Examples and Syntax of This Keyword in Javascript

The 'This' keyword in JavaScript possesses critical value in the realm of object-oriented programming. It enables methods to access the objects that invoked them, and consequently increases the efficiency and readability of the code. Let's look at 'This' keyword in more detail, exploring prime examples and understanding its syntax.

Popular Examples of This Keyword Utilisation in Javascript

'This' keyword finds its main use in accessing properties of an object within its method. When 'This' is used inside an object method, it references the 'owner' of the method.

For illustrating this, let's consider an object representing a person, with properties 'firstName' and 'lastName' and a method 'fullName' that combines these properties:

let person = {
    firstName: "John",
    lastName: "Doe",
    fullName: function() {
        return this.firstName + " " + this.lastName;
    }
}

In the above example, 'This' inside the 'fullName' function refers to the 'person' object. It accesses the 'firstName' and 'lastName' properties of the 'person' object and concatenates them to generate a string representing the full name.

Additionally, 'This' keyword can be used with the new keyword to create new objects:

function Person(first, last) {
    this.firstName = first;
    this.lastName = last;
}

let myFather = new Person("John", "Doe");

In this case, 'This' refers to the newly created object. Thus, when the function 'Person' is invoked with 'new', the 'This' inside 'Person' is bound to the newborn object. As a result, 'myFather' becomes an instance of 'Person' with 'first' and 'last' properties.

Understanding the Syntax of This Keyword in Javascript

'This' keyword is not declared. It is automatically defined by the JavaScript runtime environment and is accessible inside all functions (though how it behaves in arrow functions differs from standard functions). The syntax is as simple as invoking it using "this" in your code.

You can access an object's property using the "this" keyword followed by a dot (.) and the property name:

let dog = {
    name: "Max",
    getDescription: function() {
        return "The dog's name is " + this.name;
    }
};

Using Call and Apply Methods with This Keyword in Javascript

Both 'call' and 'apply' are pre-defined JavaScript methods that fundamentally allow you to write a method that can be used on different objects, without needing to write the entire method for each object individually.

These functions can be used to set the value of 'This' explicitly and call a function with an owner object as an argument (parameter).

Here are some basic utilisations:

let person1 = {fullName: function() {return this.firstName + " " + this.lastName}};
let person2 = {firstName:"John", lastName: "Doe"};

person1.fullName.call(person2);  // Returns "John Doe"

Understanding Call Method in Javascript This Keyword

The call() method is a predefined JavaScript method. It can be used to invoke a function with a given 'This' value and arguments provided explicitly.

It takes arguments as follows:

call([thisArg[, arg1, arg2, ...argN]] )

The 'thisArg' is an argument that will replace 'This' inside the called function, whereas 'arg1, arg2, ...argN' will be arguments for the function being executed.

An example:

function Product(name, price) {
  this.name = name;
  this.price = price;
}

function Toy(name, price) {
  Product.call(this, name, price);
  this.category = 'toy';
}

let jumper = new Toy('jumper', 29.99);

console.log(jumper); // Outputs: {name: "jumper", price: 29.99, category: "toy"}

Applying Apply Method with Javascript This Keyword

The apply() method is quite similar to call(), with a noteworthy difference that apply() accepts a single array of arguments, unlike call() which takes a list of arguments.

This method calls a function with a given 'This' value, and arguments provided as an array (or an array-like object).

Here is its general syntax:

apply([thisArg[, argsArray]] )

An example would be:

let numbers = [5, 6, 2, 3, 7];
let max = Math.max.apply(null, numbers); 

console.log(max);  // Outputs: 7

In this example, apply() is used with 'This' keyword set to null and 'numbers' array as arguments to find the maximum number in the 'numbers' array.