Javascript's this and that, and few others:

The this keyword in javascript is perhaps the most ambiguous thing to follow through when we are writing code. It sometimes confuses even the most experienced javascript developers. This article aims at explaining all the concepts related to this in javascript, especially the parts were understanding the mechanism of this becomes tricky. Sit back and enjoy the journey.

this in javascript is used similarly to how pronouns are used in the English language. Usually, we write "Jensen is studying javascript because he wants to learn web development". In the given example, note the usage of pronoun he to refer to John. The pronoun he acts as a referent to the person John. Similarly in Javascript, the keyword this acts as a referent to the object which is being executed in the current context.

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Consider the following example:

 var person = {
    firstName: "Jensen",
    lastName: "Ackles",
    fullName: function () {
        ​//using this
        console.log(this.firstName + " " + this.lastName);
        ​// using object name
        console.log(person.firstName + " " + person.lastName);
    }
}

person.fullName();

In the following code both this and person works, however, using the keyword this to refer to object is favored for two reasons:

1. A global object with the same name person could be present, and the code might try to access the global object.
2. Sometimes the name of the object is not known to the code which is executing it, this keyword simplifies it by bringing in the reference to the object which holds the current context.

Note that, this keyword can be used globally, however, in strict mode, this holds the value of undefined in global functions and in anonymous functions that are not bound to any object.

Basics of this: Clarifying misunderstandings

All javascript functions have properties, just like objects contain properties. Whenever a function is executed, it gets this property assigned to it with the value of the object that invokes the function. The property this ALWAYS holds the reference to the object, and it is usually used inside the function to access the properties and methods on the object.

If we consider the above example, the fullName function is executed from the object person. The property this used inside the fullName function now refers to the object person.

Big principle: The biggest concept to understand about the property this is the fact that, the value of this is not assigned until an object invoked the function. Even though it appears as if the value of this should be equal to the value of the object inside which the function is defined, that is not true. Until an object invokes the function where this is used, the value is not assigned, and the value of this is equal to the value of the object invoking the function in most circumstances. However, there are certain exceptional scenarios when this doesn't have the value of the invoking object, and these scenarios will be explained later.

This in global scope:

The value of this as explained earlier contains the value of the invoking object. In the global scope, the code is executed in the browser, and all global variables and functions are defined on the window object. Therefore, when this is used inside global functions, it holds the value of window object. Consider the following example:

var firstName = "Dean";
var lastName = "Winchester";
function showFullName() {
    console.log(this.firstName + " " + this.lastName);
}

var person = {
    firstName: "Jensen",
    lastName: "Ackles",
    showFullName: function () {
        console.log(this.firstName + " " + this.lastName);
    }
}

showFullName (); // Dean Winchester - this refers to global window object
window.showFullName (); // Dean Winchester - this refers to window object
person.showFullName (); // Jensen Ackles - this refers to person object

The context of this can be changed

Though we said that the value of this usually holds the value of the object which invokes the function, it can be changed in many scenarios. The common scenarios when the value of these changes are:

• Borrowing a method that uses this
• Assign a method that uses this
• Callback function using this.
• this used inside a closure.

Let's look at an example which demonstrates the possibility and then tackle different scenarios where value of this acts differently:

var person1 = {
    firstName: "Jensen",
    lastName: "Ackles",
    showFullName: function () {
        console.log(this.firstName + " " + this.lastName);
    }
}

var person2 = {
    firstName: "Dean",
    lastName: "Winchester",
}

person1.showFullName (); // Jensen Ackles - this refers to person1 object
person1.showFullName.apply(person2); // Dean Winchester - this refers to person2 object

You can see how the value of this changed simply because we used to apply and forced the context of this to person2

this when borrowing methods:

Borrowing methods is a common practice in javascript development. Sometimes methods are defined in a different object, and we would want to use it on our object without repeating the code. Considering the above example:

var person1 = {
    firstName: "Jensen",
    lastName: "Ackles",
    showFullName: function () {
        return this.firstName + " " + this.lastName;
    }
}

var person2 = {
    firstName: "Dean",
    lastName: "Winchester",
}

person2.fullName = person1.showFullName();
console.log(person2.fullName); // Jensen Ackles - this refers to person1 object
person2.fullName = person1.showFullName.apply(person2); 
console.log(person2.fullName); // Dean Winchester - this refers to person2 object

Using apply during the execution of the method, and providing the object on which this value must be set will fix the problems when borrowing methods.

this when a method is assigned to a variable:

The value of this gets assigned to the context of a different object when we assign a method that uses this to a variable:

var person = {
    firstName: "Jensen",
    lastName: "Ackles",
    fullName: function () {
        return this.firstName + " " + this.lastName;
    }
}

var showFullName = person.fullName;
console.log(showFullName()); // undefined undefined

Instead of getting the value "Jensen Ackles", we got "undefined undefined". This is because, when the method was borrowed, its context is not set to the global variable. Since the global window object doesn't contain the value of firstName and lastName, they are printed as undefined. To fix this problem, we can set the value of this to a particular object indefinitely using bind:

var person = {
    firstName: "Jensen",
    lastName: "Ackles",
    fullName: function () {
        return this.firstName + " " + this.lastName;
    }
}

var showFullName = person.fullName.bind(person);
console.log(showFullName()); // Jensen Ackles

this when used inside a method as a callback:

The value of this goes out of context when it is used in a function provided as a callback:

var person = {
    firstName: "Jensen",
    lastName: "Ackles",
    fullName: function () {
        console.log(this.firstName + " " + this.lastName);
    }
}

setTimeout(person.fullName, 1000); // undefined undefined

The above code works similarly to how the context of this changes when it is assigned to a variable. One can assume that the setTimeOut function has a variable that takes the function as a parameter, and then it executes on the global object. Since the global object doesn't contain the definition to firstName and lastName, the value is printed as undefined.

To fix this, we need to bind the value of this to the object on which it should be applied. This is a pattern most commonly seen when we are defining callback functions to the javascript events.

setTimeout(person.fullName.bind(person), 1000); // Jensen Ackles

this when used inside a closure:

Another instance when the value of this is misunderstood is when this is used inside closures.

var person = {
    fullName: "Jensen Ackles",
    getDetails: function () {
        var closureFunction  =  function() {
            console.log(this.fullName); // undefined
            console.log(this); // global window object
        }
        closureFunction();
    }
}

person.getDetails();

The value of this doesn't refer to the person object, rather it refers to the global window object. We have two ways of mitigating this problem:

1. Assign the value of this to an explicit variable, and then use that variable to access the properties.

var person = {
    fullName: "Jensen Ackles",
    getDetails: function () {
        var personObj =  this;
        var closureFunction  =  function() {
            console.log(personObj.fullName); // Jensen Ackles
            console.log(personObj); // person Object
        }
        closureFunction();
    }
}

person.getDetails();

2. Use arrow function provided by **ES6: Arrow function preserves the context of this.**

var person = {
    fullName: "Jensen Ackles",
    getDetails: function () {
        var closureFunction  = () => {
            console.log(this.fullName); // Jensen Ackles
            console.log(this); // person Object
        }
        closureFunction();
    }
}

person.getDetails();

This article has aimed at providing a detailed understanding of how this is used in javascript and has warned about the pitfalls which can be avoided while using this. Using javascript's apply, call, bind and arrow functions, we can control the value of this to meet our needs. As a final note, please remember that the value of this is usually the object on which the function is called.