How well do you know “this”?
An in-depth explanation of the underlying concepts behind what the value of "this" in JavaScript stands for, in any given context.
What’s “this”?
In the simplest of terms, the JavaScript keyword this
refers to the object it belongs to on runtime, depending upon its call-site (where it is called).
However, understanding what it would refer to in any given context, requires a slightly deeper understanding of some relevant concepts, which will be covered in this article.
Just to start with, this
can have the following values depending upon where it is accessed :
By default :
this
refers to theglobal
object.Inside a function :
this
refers to theglobal
object. Instrict
mode, however,this
will beundefined
.Inside a method :
this
refers to the owner object. (A method is a function that belongs inside an object. In other words, it’s a function that’s an object’s property.)In an event:
this
refers to the element on which the event was triggered.Inside an Immediately Invoked Function Expression (IIFE) :
this
refers to theglobal
object. Instrict
mode,this
will beundefined
, just like any other function in a global context.Inside a Fat-Arrow function: When a fat arrow
()=>
is used to define a function, it doesn’t create a new value forthis
, instead, it keeps referring to the same object it was referring to outside of the function.
This article hopes to give you an understanding of how these values are assigned to this
, and how this knowledge can be utilized to suit our requirements.
Call-Site and Call-Stack
As discussed in the last section, we got to know that this is a runtime-binding made for each function invocation, which entirely depends upon where exactly it was called.
This location in the code where the concerned function was called, is called the call-site. An understanding of determining the call-site is crucial towards understanding what this would be bound to, at any given point of the execution.
While finding the call-site is generally as simple as locating where a function was called from, it might not always be that clear because of certain coding patterns that might obscure it.
Hence, it’s important to think about the call-stack, the stack of functions that have been called to get us to the current stage of the execution which we’re concerned with.
Let us take a simple example to illustrate how a call-stack and call-site could be determined.
By following the chain of function calls in order, you can determine the call-stack and call-sites.
* Tip for determining call-stack
Utilize the built-in JS debugger
provided with any modern browser’s developer tools.
In the execution of any JS code, you can set a breakpoint by using the keyword debugger
, to stop the execution at that point in the browser.
Let’s say, we add a breakpoint when thunderbolt()
was called.
The debugger stops the execution at the custom breakpoint, and the function call-stack at that point can be viewed on the right side.
In the image above, we can see that the execution was stopped at the point where we mentioned the debugger
keyword, as soon as thunderbolt()
is called. At this point, we will not observe any execution of code that comes after the debugger
(just the thunderbolt()
log, in this case).
Our primary point of interest right now, is the call-stack which is clearly illustrated on the right-hand side, same as we determined in the example above. (anonymous)
at the bottom of the stack, refers to the initial global call to choosePikachu()
.
Binding rules for “this”
Now that we understand what a call-site and a call-stack is, we can learn about how a call-site determines what this will hold during execution.
There are four general rules which apply. First, let’s understand them independently, and then, their order of precedence when multiple rules can apply to the call-site.
1. Default Binding
This is the default catch-all rule, when none others apply. It comes from the most common case of a function invocation, which a standalone function call.
Let’s look at the below example.
The variable ultraBall
declared in global
scope is the same as declaring a property on the global
object of the same name.
Inside getPokemon()
, the reference to this defaults to the global
object. Hence, we would see the value of this.ultraBall
getting logged.
However, if strict
mode is in effect globally or inside getPokemon
, the global
object is not permitted default binding. In that case, we will see the error TypeError : 'this' is 'undefined'
.
2. Implicit Binding
If the call-site has a context object (if a function is called through an owning or containing object, as its property), implicit binding applies.
The rule states that, when there is a context object for a function reference, it’s that object that should be used for its method calls’ this
binding.
Let’s look at a few examples to illustrate the different cases which can arise.
Since the object pikachu
is the this
for the getBaseSpeed
call, this.baseSpeed
is synonymous to pikachu.baseSpeed
.
Let’s look at another example to see how only the top or last level of an Object property reference chain matters to the call-site for implicit this
binding.
As we can see, the baseSpeed
value is still 90
. That’s because the call to getBaseSpeed
is bound to its direct caller, pikachu
, which serves as its this
binding. In this context, the baseSpeedvalue
is 90
.
Let’s look at a few more examples to show common cases where implicit binding can seem unexpected.
In this example, we have lost our implicit this
binding to pikachu
in case of assigning pikachu.getBaseSpeed
to a different variable baseSpeedFunction
. Now, for baseSpeedFunction
, this
refers to the global
object (default binding takes place). Hence, for the call, this.baseSpeed
will be 50
.
Now, a more common and not-so-obvious way this loss of implicit binding can occur is when we pass a callback function. Consider the following example :
Once again, inside the callback function executor executeFunction
, we are effectively passing a reference to pikachu.getBaseSpeedfunction
. Upon execution, this
will be bound to the global
object again (or throw a TypeError
, if strict
mode is enabled), instead of pikachu
.
It’s quite common for function callbacks to lose their this
binding. Another unexpected outcome can arise when the function we’ve passed our callback to, intentionally alters the this
for the call. For example, Event handlers in popular JavaScript libraries often modify this
to point to the DOM element
that triggered the event.
You are not really in control of how your callback function reference will be executed. So far, you don’t have any way of controlling the call-site to assign the binding you intended. This is where explicit binding comes into play.
3. Explicit Binding
To resolve the unintended loss of this
with implicit binding, we can explicitly set the value of this
to a given object for a function call.
There are several in-built methods that can help us achieve explicit binding, like :
The bind() method
bind()
is a method of the Function.prototype
property. This means bind()
can be used by every single function.
The bind()
method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
In other words, bind()
returns a new function that is hardcoded to call the original function with the this
context set as specified.
The call() and apply() methods
call()
and apply()
are also methods of the Function.prototype
property, with similar but slightly different usage.
The call()
method calls a function with a given this
value and arguments provided individually.
Whereas, the apply()
method calls a function with a given this
value, and arguments provided as an array (or an array-like object).
Invoking Pokémon
with explicit binding by Pokémon.call()
or Pokémon.apply()
allows us to force its this
to be the this
of function PokémonExtension
.
Also, a noteworthy aspect of the above example is that all instances of PokémonExtension
will bind their respective this
to the execution of Pokémon
within them. Such an explicit binding is also called hard binding.
4. new Binding
In JavaScript, there is really no such thing as “constructor functions”, but rather construction call of functions.
When a function is invoked with new
in front of it, otherwise known as a constructor call, the following things are done automatically.
A brand new object is created (aka constructed) out of thin air.
The newly constructed object is
[[Prototype]]
-linked. (Out of the scope of this article)The newly constructed object is set as the
this
binding for that function call.Unless the function returns its own alternate object, the new invoked function call will automatically return the newly constructed object.
All binding rules in action
It should be clear that the default binding is the lowest priority rule of the four.
Let’s compare implicit binding, explicit binding, and new binding with each other.
Implicit versus Explicit
As we saw, the explicit binding of firstAttempt.catchPokémon
with secondAttempt
took precedence over its own implicit binding, as it did for the second case as well.
Hence, explicit binding is of higher precedence than implicit binding.
Implicit versus new
So, new binding is more precedent than implicit binding.
Explicit versus new?
new
and call
or apply
cannot be used together, so something like var fourthAttempt = new catchPokémon.call(firstAttempt);
is not allowed to test new binding directly against explicit binding. But, we can still use a hard binding to test the precedence of the two.
attemptBinder
is hard-bound against firstAttempt
, but new attemptBinder(“Steelix”)
did not change firstAttempt.name
to "Steelix"
, as we may have expected, but it remained "Onix"
.
Instead, the hard-bound call to attemptBinder("Steelix")
is able to be overridden with new
. Since new
was applied, we got the newly created object back, which we named secondAttempt
, and we see that secondAttempt.name
indeed has the value "Steelix"
.
Thus, the newly created this is used, rather than the previously specified hard-binding for this. Effectively, new
is able to override hard-binding.
The primary reason for this behaviour is to create a function that essentially ignores the this hard-binding, and presets some or all of the function’s arguments.
Finally, determining “this”
We can summarize the rules for determining this from a function call’s call-site, in their order of precedence.
Here they are :
Is the function called with
new
? If so, this is the newly constructed object (New binding). Example,var attempt = new catchPokémon("Pidgey");
Is the function called with
call
orapply
, even hidden inside abind
hard-binding? If so, this is the explicitly specified object (Explicit binding). Example,var attempt = catchPokémon.call("Pidgeotto");
Is the function called with a context, otherwise known as an owning or containing object? If so,
this
is that context object (Implicit binding). Example,var attempt = firstAttempt.catchPokémon("Pidgeot");
Otherwise,
this
defaults to theglobal
object, orundefined
instrict
mode (Default binding).
Summary
Determining the this
binding for an executing function requires finding the direct call-site of that function.
Once examined, four rules can be applied to the call site, in this order of precedence.
Called with
new
? Use the newly constructed object.Called with
call
orapply
orbind
? Use the specified object.Called with a context object owning the call? Use that context object.
Default :
undefined
instrict
mode,global
object otherwise.
Credits
Official documentation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this
You Don’t Know JS: this and Object Prototypes, by Kyle Simpson.