How well do you know “this”?
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 :
thisrefers to the
Inside a function :
thisrefers to the
Inside a method :
thisrefers 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:
thisrefers to the element on which the event was triggered.
Inside an Immediately Invoked Function Expression (IIFE) :
thisrefers to the
undefined, 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 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
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.
ultraBall declared in
global scope is the same as declaring a property on the
global object of the same name.
getPokemon(), the reference to this defaults to the
global object. Hence, we would see the value of
this.ultraBall getting logged.
strict mode is in effect globally or inside
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’
Let’s look at a few examples to illustrate the different cases which can arise.
Since the object
pikachu is the
this for the
this.baseSpeed is synonymous to
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
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
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
this refers to the
global object (default binding takes place). Hence, for the call,
this.baseSpeed will be
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
strict mode is enabled), instead of
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 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.
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
apply() are also methods of the
Function.prototype property, with similar but slightly different usage.
call() method calls a function with a given
this value and arguments provided individually.
apply() method calls a function with a given
this value, and arguments provided as an array (or an array-like object).
Pokémon with explicit binding by
Pokémon.apply() allows us to force its
this to be the
this of function
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
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
thisbinding 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
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?
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
new attemptBinder(“Steelix”) did not change
"Steelix", as we may have expected, but it remained
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
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
apply, even hidden inside a
bindhard-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,
thisis that context object (Implicit binding). Example,
var attempt = firstAttempt.catchPokémon("Pidgeot");
thisdefaults to the
strictmode (Default binding).
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.
new? Use the newly constructed object.
bind? Use the specified object.
Called with a context object owning the call? Use that context object.
You Don’t Know JS: this and Object Prototypes, by Kyle Simpson.