Credits: Pixabay.com
Your knowledge of JavaScript can be pretty well judged by your understanding of this. The fact that people find it difficult to comprehend is, because of its use in multiple and varied scenarios.
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 on1. The Execution Context2. Invocation
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 **window**
II. Calling a function that runs JavaScript statements in strict mode would have this as undefined
III. Calling a function that is inside an object
Function add
is one of the properties on object obj
. When we call add with reference to obj
as obj.add()
, it gets called in the context of obj
. Properties a
& b
have definite value in the context of obj
, so that result of obj.add()
is defined.
The 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 a
and b
are not defined in the global context, we’re getting the value of a+b
as NaN
.
To prove this point, let's define a
and b
in the global context.
obj.add()
evaluates to 3 as the value of a
and 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.
Using Object.create
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 whosCandyIsThis
is using. The little toddler doesn’t know that JavaScript is an unconventional language. You can always mend it the way you want.
Call, Apply & Bind — Here I go!
With these 3 beautiful functions, you can modify the value of this reference. In the above example, candy.whosCandyIsThis.call(myCandy)
modifies this
reference of whosCandyIsThis
function to myCandy
.
What is call all about?call
explicitly pass this
reference to a function. In this case, it tells whosCandyisThis
function to use this
as myCandy
.
The first argument of call is this
reference and if we have to pass in any additional parameters, you can do something like this
candy.whosCandyisThis.call(myCandy, true)`
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 call
is 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 bind
. Unlike call
and 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 (ccd
).
Well again, JavaScript is an unconventional language. If there’s freedom to modify anything as you wish, there are guards who protect the integrity. One such masterpiece is our cute Arrow Function.
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 contextLexical Context is block level scoping.It does not change with the call
, apply
or bind
.
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 niks
reference.
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