Everything You Need to Know About “this” in JavaScript

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 :

1. By default :

this

refers to the global object.

2. Inside a function :

this

refers to the global object. In

strict

mode, however,

this

will be

undefined

.

3. 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.)

4. In an event :

this

refers to the element on which the event was triggered.

5. Inside an Immediately Invoked Function Expression (IIFE) :

this

refers to the global object. In strict mode,

this

will be

undefined

, just like any other function in a global context.

6. Inside a Fat-Arrow function : When a fat arrow

()=>

is used to define a function, it doesn’t create a new value for

this

, 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

baseSpeed

value 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.

1. A brand new object is created (aka constructed) out of thin air.
2. The newly constructed object is

   [[Prototype]]

   -linked. (Out of the scope of this article)
3. The newly constructed object is set as the

   this

   binding for that function call.
4. 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 :

1. Is the function called with

   new

    ? If so, this is the newly constructed object (New binding). Example,

   var attempt = new catchPokémon("Pidgey");

2. Is the function called with

   call

   or

   apply

    , even hidden inside a

   bind

   hard-binding? If so, this is the explicitly specified object (Explicit binding). Example,

   var attempt = catchPokémon.call("Pidgeotto");

3. 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");

4. Otherwise, this defaults to the

   global

   object, or

   undefined

   in

   strict

   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.

1. Called with

   new

   ? Use the newly constructed object.
2. Called with

   call

   or

   apply

   or

   bind

   ? Use the specified object.
3. Called with a context object owning the call? Use that context object.
4. Default :

   undefined

   in

   strict

   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.

Previously published at https://anmshpndy.com/how-well-do-you-know-this