Let's get them right once and for all.
If I were to talk about this in the literal sense, it simply means — this very thing in front of me. “This Jacket is Cool”, claimed my sister holding that jacket in her hand. I replied back saying, “No, this one’s better!” I was referring to the jacket near to me and she was referring to the one near her and hence the use of this in respective statements.
Value of this depends on
1. The Execution Context
Execution Context is the environment in which the code runs. For example, the local variables of a function would be accessible in that function only and not outside of it. This is called as Functional Level Scoping or Local Execution Context. Variables declared in global scope (outside of any function) would be accessible in all the underlying functions. This is called the Global Execution Context.
Let’s consider this example to understand the difference between the two of them -
The value of this in Global Execution Context by default is window object.
The value of this in any Local Context depends on the way the function is called.
I. Calling a function in the global context would have its this value as
III. Calling a function that is inside an object
add is one of the properties on object
obj. When we call add with reference to
obj.add(), it gets called in the context of
b have definite value in the context of
obj, so that result of
obj.add() is defined.
cacheAdd variable stores the reference of
add function. If we execute this function at a later stage, it will run in the context from where it is called. In our example, we’re calling
cacheAdd in the global context, so it will have a global object as
this reference. Since
b are not defined in the global context, we’re getting the value of
To prove this point, let's define
b in the global context.
obj.add() evaluates to 3 as the value of
b defined in obj is 1 & 2 respectively. When we run
add after caching it in a variable and then executing it in the global context, it evaluates to 7 as the value of a & b in the global context is 3 & 4 respectively. So, a function that uses
this may give a different result depending on the context in which it is executed.
Let's try another interesting example —
In this case, add function returns another function and the value of
a+b is executed in this inner function.
obj.add() will return a function that has one
console statement. So, we’ve to execute this returned function in order to get the sum of a & b. But this returned function (returned on executing
obj.add()) will run in the global context and hence the value of a+b evaluates to 7 (Value of a & b in the global context is 3 & 4 respectively).
Here, add function prints the value of a & b after a timeout of 500ms.
setTimeout runs in the global context.
Why setTimeout runs in the global context? (I’ll explain about it in detail in my next post.)
this takes the reference of the immediate object calling the function
Here, parentObj has childObj as one of the properties and add function is defined on childObj. When we call add as parentObj.childObj.add(), add function takes the this reference of childObj and variables a & b are not accessible from the local context of childObj. Hence the value of a & b is undefined.
Linking to the prototype chain
If we instantiate parentObj to create childObj, its properties would be accessible in the childObj context.
When add function gets called in the context of childObj, it first tries to look for values of a, b & c in the context of childObj and if any of these values is undefined, it looks up to its parent context, which in this case is parentObj.
Using new operator
In this case, properties of parent are available in the context of the child. As we can see in the trailing console statement, the constructor of the child is defined as Parent. So, it first checks for values in the context it is called and then look up to its prototype chain.
This ain’t Rocket Science!
By default, the code runs in a global context i.e. it has access to the variables defined in the context of the
window object. In each of the above cases, we’re just restricting the context to obj object. All the operations related to this would be carried out in the exact same way as they would have if the context was
window. Even with global context, if any of the values is not defined, it looks up to the prototype chain.
As we can see,
toString function is defined in
__proto__ property of
obj, which means it is defined on the prototype of Object (Because
obj is created by applying the
new operator to Object). As
toString is not defined in the context of
obj, it looks up to its prototype chain.
Let’s solve an interesting issue here —
My sister stubbornly refuses to share the candy with me and to support her claim, she says, she is the owner of this candy.
This is her Candy —
So, somehow I’ve to modify
this reference that her function
Call, Apply & Bind — Here I go!
With these 3 beautiful functions, you can modify the value of this reference. In the above example,
this reference of
whosCandyIsThis function to
What is call all about?
call explicitly pass
this reference to a function. In this case, it tells
whosCandyisThis function to use
The first argument of call is
this reference and if we have to pass in any additional parameters, you can do something like this
Let’s consider one more example:
Here, we’re passing
this reference explicitly to the
displayMenu function defined in the hotel object to display the menu accordingly. (Isn’t this cool?)
But what if I want to display the menu based on the user’s location. For that, we’ll pass an additional parameter
location to the
displayMenu function. Let’s modify the above code to fit in our requirements.
The first parameter of
this reference and then you can pass in any number of arguments that you want. The
displayMenu function now very well shows the list of menu items as per the location parameter. (Sounds good?)
apply operates in the same way as of
call, the only difference being the way in which we pass arguments. Apply takes in the first parameter as this reference same as that of call but other parameters are to be passed as an array.
Another function that can modify
this reference is
apply that returns the result of the function,
bind returns a function. And this returned function can then be called at any later stage. It will run in the context of the explicitly passed bounded argument.
hotel.displayMenu.bind(ccd) returns a function that has
this reference of
ccd. When we run
boundFunction, it runs in the context of
ccd and hence prints Espresso for location A.
I was trying to experiment with my instincts. If I bind an already bounded function to some other reference, would that function run in the context of the later binding? If this is true, I can do this endlessly and keep changing this reference with all the power I have! This is not true. You can only bind a function once. In the above example, I’ve tried to bind the bounded function again with pizzaCentre but when I execute this function, it prints the previous result which means its reference still points to the first binding (
This is how it works. Plain & Easy.
Why are we even talking about arrow functions in this article? What’s so special about them?
In the arrow function, the value of
this is that of its enclosing lexical context
Lexical Context is block level scoping.
It does not change with the
Arrow functions maintain the binding of this of its enclosing lexical context.
Crape! This Fat Arrow took away my candy.
As you can see,
whosCandyIsThis is defined as an arrow function. So it will maintain the binding of its Lexical Context. On instantiating Candy,
niks sets the owner and flavor for her candy.
whosCandyIsThis function gets bounded to
So, When I try to call it explicitly by passing this reference using call, apply or bind, it doesn’t work. The bindings of
niks.whosCandyIsThis function cannot be changed, whatsoever!
You might like my previous articles on Service Workers —
Service Workers Fundamentals — https://hackernoon.com/service-workers-62a7b14aa63a
Building Pokemon App to evaluate the power of Berries & Service Worker — https://hackernoon.com/building-pokemon-app-to-evaluate-the-power-of-berries-service-worker-176d7c4e70e3