In most cases, the value of this is determined by how a function is called. It can’t be set by assignment during execution, and it may be different each time the function is called. You can change this context through .call(), .apply() and .bind(). Value of this is equal to the value of the object which invokes the function. this is not assigned a value until an object invokes the function where this is defined.
Global Context
When functions are executed in the global scope, value of this is window object. This is because when we call a function in global scope by default they are invoked on the Window object. In strict mode, value of this in the global context will be undefined. Consider the below example:
As an object method
When a function is called as a method of an object, it’s this is set to the object the method is called on.
Example:
In the above example, we have defined two val variables, one in the global scope with value 37 and another inside myObj object with value 10.
When we call myObj.someFunction(), as someFunction() was called by myObj object, value of this inside someFunction will become equal to myObj.
Hence, inside someFunction when we do console.log(this.val), it outputs myObj’s val variable value i.e 10.
When we do window.val, we get value 37 as global value for val is 37.
Another example:
In the above example, we have defined someFunction outside myObj object.
First we call someFunction() on myObj object, hence inside someFunction value of this is equal to myObj object.
In second case, we have called someFunction() in the global scope which is same as writing window.someFunction(). Here, as someFunction() has been called on window object, value of this is equal to window object.
First we call someFunction() in the context of myObj. Later we called the function in the global context and the value of this inside the someFunction() changed accordingly. This shows that the value of this was determined dynamically on the basis of execution context.
As a constructor
When a function is used as a constructor (with the new keyword), its this is bound to the new object being constructed. To understand what constructor function are, please go through this article.
Here, we have defined ConstructorFunc to create new objects. When
var obj1 = new ConstructorFunction(20) is executed, value of this will be equal to the new object that is created.
‘this’ in Immediately Invoked Function expression (IIFE)
In IIFE, value of this is always equal to Window object. Let’s see an example:
Console Output:
First example:
In the first example, IIFE outside any function, value of this inside IIFE will be Window object.
Second Example:
In the second example, when we have called IIFE inside someFunc still the value of this is Window object not obj object.
This is because, value of this inside a function is equal to the object on which it is called. someFunc is called on obj, hence value of this inside someFunc is obj. But, IIFE is self-invoked, it has not been called by any object. Hence, the value of this inside IIFE is Window object.
Event handler in JavaScript
Inside event handler, value of this is equal to the element on which the event is fired. Let’s see this with an example of click event.
In the above example, we have added an on click event handler on the div element which calls the clickMe function.
When we click on the div, from the above console output we can see that value of this is equal to the div element which we clicked.
We can change the value of this at run time using call, apply and bind function. We will discuss call, apply and bind in another article.
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