---
title: How does this work in JavaScript?
slug: how-does-this-work-in-javascript
published_at: 2021-01-19 00:00:26 +0000
updated_at: 2026-03-04 20:14:35 +0000
summary: 
description: You're probably wondering how the `this` keyword is set in JavaScript. There are a couple rules of thumb and things to look for that will help you understand what `this` is. We'll cover 4 different ways that methods are executed and how `this` will be bound for each. We'll also talk briefly about the differences with arrow functions  ### Table of contents  Ways to call a function:  00:52 Function style overview - global object a.k.a. window in the browser 02:34 Call / Apply style overview - explicit binding 05:30 Constructor style overview - new keyword 06:14 Method style overview - dot operator 08:14 Function style demo 09:55 Call / Apply style demo 12:57 Constructor style demo 13:50 Method style demo 24:50 Class vs. prototype 29:03 function vs arrow fn 33:37 strict mode gotchas 35:15 bind 38:00 Implement myBind to see how bind might work under the hood  ### Resources  MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this Code: https://gist.github.com/cjavdev/867ba2aecb695001ebdc3613d494f185 Let's connect: https://twitter.com/cjav_dev
tags: [this, javascript, js, function, method, call, apply, bind, myBind, how does this work, how does this work in javascript, how is this set, ways to call a function, JS, JavaScript, prototype, class, arrow functions]
views: 235
author: CJ Avilla
url: https://www.cjav.dev/videos/how-does-this-work-in-javascript
youtube_url: https://www.youtube.com/watch?v=q66M2hYiEcY
youtube_id: q66M2hYiEcY
embed_url: https://www.youtube.com/embed/q66M2hYiEcY
thumbnail_url: https://i.ytimg.com/vi/q66M2hYiEcY/hqdefault.jpg
type: video
---

# How does this work in JavaScript?

*Published: January 19, 2021*
*Views: 235*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=q66M2hYiEcY)

[![How does this work in JavaScript?](https://i.ytimg.com/vi/q66M2hYiEcY/hqdefault.jpg)](https://www.youtube.com/watch?v=q66M2hYiEcY)

## Description

You're probably wondering how the `this` keyword is set in JavaScript. There are a couple rules of thumb and things to look for that will help you understand what `this` is. We'll cover 4 different ways that methods are executed and how `this` will be bound for each. We'll also talk briefly about the differences with arrow functions

### Table of contents

Ways to call a function:

00:52 Function style overview - global object a.k.a. window in the browser
02:34 Call / Apply style overview - explicit binding
05:30 Constructor style overview - new keyword
06:14 Method style overview - dot operator
08:14 Function style demo
09:55 Call / Apply style demo
12:57 Constructor style demo
13:50 Method style demo
24:50 Class vs. prototype
29:03 function vs arrow fn
33:37 strict mode gotchas
35:15 bind
38:00 Implement myBind to see how bind might work under the hood

### Resources

MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this
Code: https://gist.github.com/cjavdev/867ba2aecb695001ebdc3613d494f185
Let's connect: https://twitter.com/cjav_dev

## Transcript

hey what's up everyone today i'm going to do my best to try to explain how this works in javascript this keyword how this is bound uh when you're executing functions so at the in most cases this is bound based on how the function is called so there's four different ways i like to categorize it in four different ways function style call and apply style constructor style or method style now some other differences between classes versus using like a prototypes and there's differences between defining the function with the function keyword versus using an arrow function there's also differences for strict mode uh strict mode um and so we'll go over kind of each of these uh in detail and try to explain the best i can what the difference is between each of these so with function style and by function style i mean when you're calling a function this way so we call fn so if fn is the name of the function i'll just like create an empty function here at the top we'll call it function fn and then maybe we have like a b c it'll take in three arguments here and if we were to call like what's uh this so we're going to talk about what this that this keyword is bound to so for function style here we're using the fn the name fn and the name fn here fn refers to the name of the function so the name of this function is fn the name of this function is fn now we use two parentheses and open parentheses in a closed parenthesis to invoke the function if we were to just write fn like this and that was our entire line then that would just be referring to the function we're not that would not actually call the function so we need to invoke the function with parentheses so when a function is called function style uh this is bound to the global object so by default this is bound to the global object now when you're in node that global object is going to either be some global object or if you're at the very top level that's like the module exports thing so but by default just know that when you call something function style when you call a function function style then this is bound to the global object now here's a couple other examples we might pass in you know one two uh three as our arguments to the function so that is function style and that is how this is bound all right let's talk about call and apply style it turns out that when you define a function as we have here at the top that is creating an instance of a function object so it's creating an instance of a class and the class is a function and the class function has a couple of methods two of them are call and apply so if we wanted to another way that we could invoke this function or call this function is using the call or apply method so we could say fn.call here so fn notice that we're saying fn but we're not invoking fn we don't have parentheses right after fn instead we're using a method so we're calling the call method on the function object and what we can do now is the first argument to call is some object maybe we say like you know const this is my this and that can refer to any object and if we push if we pass that in as the first argument to call this is my this then when fn is executed this the this keyword inside of fn when it executes will be bound to whatever we pass in as the first argument to call same thing with apply so call and apply work the same way in that there are two options for invoking a function and call and apply here are called and the first argument is the object we want to bind to this explicitly and then in the case of call it can accept numerous other arguments you know you can just like pass in one two three that would be the sort of the this line would be the equivalent to this line up here where we're uh we're executing the function we're explicitly binding this and then we're passing in the arguments one two three so here we're invoking the function we're passing in one two three this way or like when we call the function function style here this is bound to the global scope when we call the function call style down here this will be bound to the object this is my this all right the difference between call and apply is that call takes the arguments kind of separated out one comma two comma three and apply accepts the arguments one two three but these ones are inside of an array and the way that i can the way i remember which one is which is that apply starts with an a array starts with an a and so you just remember that apply accepts its arguments in an array in a second argument to apply so apply takes two arguments the first is what you want to bind to this the second argument is an array of arguments that should be passed to our function so that we're when we invoke the function this way this is bound and these arguments are passed so these two are also equivalent all right so that's call style that's like one way that you can explicitly set or bind this we'll talk about binding a little bit all right so the third way i want to talk about is constructor style so if we if we call the function like so right we know this is function style but if we were to prepend this with the new keyword then the resulting object is uh or the resulting this that's bound when this function is called will be a new instance of an object it's like a brand new object that you can create and generally you're constructing an instance of something so constructor style is when we use the new keyword at the beginning so we'll talk about how how that works and then finally we have method style so method style looks like this we might have dog.bark okay where bark is a method on the dog class and dog is an instance of the dog class so here the way this works is that we have an object dog and then we have the dot operator and the dot operator is how we can look up properties that's how we can do property accessing so when we say dog dot bark these this expression here dog dot bark is going to look up the bark function or look up the bark property on the dog object and then by using the parentheses here we are invoking bark and when we invoke bark we invoke a function that was just looked up on an object then this when this function is called this will be bound to the object that's on the left hand side that's by default now there's ways that you can explicitly change what this is bound to however this uh when you call the function this way this is called method style notice that fn.call and fn.ply dot apply are methods on the function object so the function itself is an instance of an object where you can execute a method or a function called call method style and in this case you're calling the method and this by default inside of the call function when the call executes is is the function but then when it's actually run this is bound there's no there's no way to implement call or apply with just writing javascript these are parts of the language that are built in when you are running javascript so here's the three different ways we've got function style call apply style constructor style and method style so these are all sort of the default ways and i wanted to go through some examples for all of these so i'm going to start down here and we'll talk about function style first so if we have that function fn and let's say it takes a b c and let's console.log this and we'll console.log and then we'll also just like say console.log a b and c so a b and c are the arguments and then this is going to be the keyword that's bound when we execute the function so we're going to call the function function style here then by default this inside of here is going to be the global object so let's run this node this dot js so i'm just running a script locally here and that's what we get so uh this in this case like this big huge object that we're seeing printed out is the uh is the global object that's inside of node okay so this is some global object if we were to run the same code inside of the browser okay so we're going to paste this code this will be bound to the window so when we're working in the context of a browser which is a lot of folks start out and the writing code that's on the front end it's in javascript it's written on the front end in the browser then this the global object on the browser is the window so here when we execute this function this is bound to the window now note that we get undefined undefined undefined that's because we didn't pass any arguments to fn if we pass one to and i don't know test then we should see those printed out at the bottom also so now we're seeing one two and test printed out so this again this stuff is how we execute a function function style so we'll just put that here all right now let's do the same thing but call and apply style so we're going to use our same function actually i'll just leave it there yeah well let's call it fn2 so we'll say fn2 okay but instead of invoking it directly here function style let's use call or apply style so we're going to say dot call and now we need to pass in some object that is the first object here so let's create a new object we'll just call it like const uh person and um here maybe they have like a name and that's uh yeah cj all right cool whatever so now we're going to use the call style where we pass in uh person as what we want to explicitly bind to this so in this case this in here should be bound to person so we should see this this object printed out rather than this huge global object printed out because now we're going to call it call style and we're going to explicitly pass in this all right so let's run it again okay cool and uh i think you can see that let's see uh so oh gosh uh all right so at the end of our function that was executed oh let's comment out this x this invocation um okay all right so what we get back is this was this object right because person is what we're bound to uh and then we also have one two and test so if we had maybe if we had uh the age and we'll just say 21 and then we'll say console.log this dot age plus a plus b right so now we're going to take in two arguments one and two and then we're going to add those to the person object so we should get 24. all right so we've printed out 24 and here we're accessing the age property on the person object that we explicitly passed in that's why we can call this.h now if we were to call if we were to try to call fn2 explicitly just like one to test right this is gonna fail because age is undefined right so or we get nan which is uh fun uh okay so same thing if we if we execute fn2 with apply we can pass in person and then the only difference is that we want to pass these arguments as an array all right so let's run this again again we get 24 again we get our object that's correctly bound so that is call and apply style this might be useful if you're writing a function that is taking in some list of arguments and then you or it's taking in a function and a list of arguments and you want to execute that sometime later um but it's it's a little bit less common than you know just calling the function that as you as you see all right so i'm going to comment those out so we don't execute fn2 let's go talk about constructor style um all right so i'm actually going to just comment all of these out okay so for constructor style this is when we're constructing a new instance of an object so if let's say we have a class or we want to we want to define a class like an old school es5 class like this where we say function whoops function cat and the cat is going to have a name so we're defining a new function that takes in a name as a variable notice that i used a capital c here that's not import like it's it's not required javascript doesn't care if you name it with a capital c or a lowercase c but generally when you're creating a function that is a constructor function one of the ways that you can signal that it's a constructor function is by capitalizing the name of it so here if we were to just like console or let's say console.log name now let's let's let's say this dot name equals name all right so in this case right when we call it constructor style so if we say if we were to just say cat name um all right so let's yeah let's let's make it uh console. no yeah i'm gonna i'm gonna add some methods to the cat so we're gonna say cat.prototype.meow is equal to a function and here in the function we'll console.log this.name we'll just start with this.name and then we'll we'll work out from there so if we say uh const cat equals new cat and we'll pass in the name garfield or something and then if we say cat.meow right and we execute it and we run this code we should get back garfield and we see garfield right so what this is okay so we wrote out a lot of stuff right so here is the actual invocation we're calling new garfield or we're using the new keyword and passing in garfield if we were to not use the new keyword then cat this function is not actually returning any there's no like return value from cat right so cat is going to be returning undefined by default so if we if we run this again we're going to see an error that says cannot read property meow of undefined that's because our cat function doesn't actually return anything by default okay remember that now so when we call it function style the return value is whatever the return value from cat is and in this case we didn't return anything if we were to return this then this this should work so if we were uh no i'm sorry it's not going to work because this is still not created as an instance of a cat so in order to make this work right we need to use the new keyword and by using the new keyword there is a default return value from the cat constructor function and that is going to be the instance the new instance of the cat so that is going to return the new the brand new this so if we execute this again right then we see garfield that's because we're able to look up meow on the cat object and find garfield so what is this prototype thing that's going on here right so when we're creating classes or so in javascript it uses prototypical inheritance so you can build up the prototype chain uh to define or to like work with inheritance in javascript this is how you used to define classes before the class keyword became available in javascript is you would have to add properties to this prototype object okay so in this case what happens is we're creating a new instance of a cat we get back a new instance of a cat and then when we do when we do the property lookup if we say cat.meow that looks on the cat object itself and says huh does the cat object itself have a meow property and it does not have a meow property so then it says okay if it doesn't have a meow property so if is there like a is there a a cat dot meow no then it says like okay is there a cat dot underscore underscore proto underscore underscore dot meow uh yes so when we create this instance um using this constructor style it is cons it is setting this secret and private underscore underscore proto value on the cat object that when we go to do property access on the cat it's going to look up and it will follow through those protos all the way up until it encounters a meow or an it encounters the name of this property so in this case meow happens to be on the ins on the class that cat is an instance of however you could have like multiple chains of inheritance so you could have maybe meow was defined on the animal object and cat inherited from animal we're not going to go over inheritance right now but this is kind of what's happening is we're looking up and trying to find meow and we happen to find it on this prototype and so cat.meow just cat topping out is a function and we're able to invoke that function method style here so we're invoking the function method style and so when we invoke method style on the left hand side we have the object that is bound to this when meow is executed so constructor style is really just this bit here that is creating the new instance of the cat and creating the new instance of this that is bound inside of um our cat function so this is constructor style so i'm going to put this here and we'll put these two comments above that and then let's go talk about method style i'm going to move this down here a little bit so we'll just put down here all right so again if we have uh we have our new instance of a cat right um the new instance of the cat here is going to be let's say cat 2 and then when when we say cat two dot meow let's actually console.log this function we're not gonna actually execute cat2 dot meow in this case because we're not we didn't put parentheses at the end we're just going to console.log what meow is all right so meow itself is an instance of a function so this is like javascript's kind of like console log for the tostring of a function object it prints out the square brackets and then the function so because we were not actually executing meow we're not printing out like the return value or anything of meow we're actually just returning the function and you'll notice here when we're console logging we just see function and really no other details that's because meow is defined as an anonymous function so this function here we said function and then the open close paren and then like the curly braces right to define the function body whereas if we look up higher our other functions that we've been working with have names so here we said function and then we had a space and then we had the name of the function and then parentheses so that named the function here same thing right function and then we had a space then we had cat which was the name of the function in this case we're saying function and then parentheses there's no name for the function but that we can change that we can say meow we can give it a name if we want and now when we console.log the function we see the name of the function also so by default a very common like pattern is to use this cat.prototype.meow right another way that you might see this manifest is or okay so let's just let's just go through this so like if we were to execute cat 2 dot meow right then inside of the meow function when we when we execute when we execute meow this will be bound to the instance of the cat that's on the left hand side so if we if we say console.log this we should see the instance of the cat and we do we see an instance of a cat and its name is garfield let's comment this out garfield v2 and well well sorry we need the cat constructor still okay so for cat we uh so we see an instance of a cat and its name is garfield v2 now this same method style you'll see it in other ways right so like you don't necessarily need to use the constructor in order to create an instance of an object that has a meow or has a property that is a method right so here's another way we could do this we could say like const person is equal to an object we can just say it's a json object and their name maybe their name is uh you know jenny rosen and they have a say hi method and that is a function that when they when they say hi they say console.log you know hi my name is and then this.name so it's going to be their name all right so then if we say person dot say hi we're executing say hi method style right and when we execute say hi method style this will be bound to the person object so let's also console.log this all right so all right so the first thing we're printing out is the object so this is the object and we see the name and the name is jenny rosen and we also see say hi and that is a function here and in this case say hi actually has a name also so when we're defining functions directly like this the function will be named for you by javascript and here we say hi my name is or we see hi my name is this dot name or jenny rosen right so it's able to access that property now here's where things get kind of interesting right because you could have an instance of a cat right so we're going to construct an instance of a cat and what we could do is we could say i'm going to look up the say hi function on person but i'm not going to invoke it the default way instead what i want to do is call say hi but i'm going to use the call apply way and i'm going to say hi but as a cat so we can say person.sayhigh.call and we can pass in cat2 in this case right we're looking up the function on the person object that's going to give us back this say high function it's going to give us back this function and because we're using the dot call method on the say high function we're able to explicitly bind this cat object which also has a name it has garfield v2 uh to this or we're explicitly binding this to the the cat2 object so if we run this we should see hi my name is garfield v2 and you see that when we console.log this we're seeing the name is garfield v2 so we're explicitly binding this to cat 2 by executing say hi call style here so this is a this is just like another way of uh sort of changing what this is bound to so i'm going to go back to this because or back to uh method style here like so um all right so that's all fun that's kind of cool stuff all right let's go let's go a little bit further and talk about classes versus prototype so we talked about prototype before there's another way to define instances of classes and that's using the class keyword so we can say class dog and if we want to initialize the dog instances of the dog with some data we're going to use a constructor a constructor function which might also take in the name and inside of the constructor function we have reference to the this keyword and when we're defining a class it's a little bit more clear that this constructor is like an initializer or is uh you know like an underscore underscore a knit underscore underscore in python or like def initialize in ruby or you know this is this is a really common pattern in other languages where you have a class and that has some constructor function and that's how you create instances of that class so here we can say this dot name equals name and then let's uh okay so we'll say const d equals new dog and we can pass in the name stan lee here and let's just uh we'll just console.log d and then we'll see an instance of a new dog all right so we see a new dog and its name is stanley and it's bound to um to the new instance of the object so d dot name is going to be stanley so we could just say like d.name and that prints out stanley okay so it's very similar to the prototype style but it's so much cleaner so if we wanted to add a method we can just say bark and then we can you know console.log maybe this.name says bark okay so if we can if we wanted to we could call bark method style so we're going to say d.bark and execute it so we're using the parentheses we're looking it up on d that means that this will be bound to the instance of the dog inside of the bark function so if we execute this again stanley says bark okay so that's super cool uh again we could create like you could create an instance of another dog like dog two and maybe that dog's name is max and uh similarly we could mess around and say like dog dot bar d dot bark dot uh dot apply right and then pass in d2 and that would execute the bark function but bound to the other the other dog so if we run this we see max says bark even though we didn't explicitly call d2.bark we're still seeing max because we're executing the bark function here and we explicitly bound this to d2 all right so that's another way that we can that you can do this so i'm going to say d.bark i'm going to put this back to just invoking it method style the other thing that's kind of interesting is if you console.log d dot bark in the clay so in the in the case of a class then use it'll use this name so you don't have to use the function keyword anywhere it's just going to create the uh it's just going to create the bark function for you and name it for you so that's kind of handy and a nice convenience and to be honest in the last couple years i've seen a lot less use of the prototype style and prototypical inheritance in general in javascript because folks just prefer the class syntax and it's just a little bit cleaner if you're writing object oriented javascript that is um okay so that is what i wanted to talk about i think for class versus prototype there's also an interest there's like a bunch of interesting like little caveats like um if your if your subclasses don't return an object then there might be some failure but uh you can check out mdn for some some edge cases i i really want to just go through like the most common use cases which again function style call apply style constructor style and method style all right so let's talk about function versus um this new arrow function thing that was released in es2015. so if we have a function f fn right and we say console.log this and we invoke fn then this again will be bound to the global object so if we run this we should see the global object is this big messy blob and node that's totally cool but if we separately created another function const fn2 and we're going to set that equal to a new arrow function that just returns this then if we execute fn2 this will be bound to the enclosing scope so in node by default when you're like working at the very top level so out here so if i this is you know x equals one or something when you're working at the very top level scope in node this is bound to module.exports um so in this case we're going to see a little bit of a difference between this when we're talking about fn versus this when we're talking about fn2 so console.log this all right so if we run this you see that we get back an empty object that's because in the arrow case arrow functions don't bind to this they don't have a they don't change what this is in their context so when we're in the body of an arrow function this is whatever this was when we were just outside the body of the arrow function so this does not have a way to um or i'm sorry arrow functions don't have a way to bind this okay so even if we were to call or explicitly try to pass in some of this to fn2 right so like with with fn with with a full function that we've defined this way we can bind this to whatever we want so we say const bind or like this is my this is a new object that has like x is pointing to one all right so if we said fn dot call this is my this right then when we execute the function then fn1 is binding this explicitly but if we try to do the same thing with fn2 it won't allow us to change anything it doesn't actually bind anything right so we're still getting back that empty object okay so we're still getting back this empty object that points to this module.exports thing because arrow functions when a function is defined like this or when a function is done defined this way this is again it's bound to the scope that's enclosing the function in this case the scope that encloses the function is the module.export so this top level thing inside of node so when we're talking about arrow functions one thing to be careful of is they don't behave the same way as functions this defined with the function keyword so if you're if you're writing um that same kind of like person object that we had here right and you have this say hi function but if you define it as an arrow function instead then person.say hi is not going to work as you expect so if we were to let's clean these up if you were to run this like this hi my name is undefined because this dot name is not defined um on or yeah the module.exports object which is bound to the this keyword inside of the arrow function here does not have a name property and so when we call personnel say hi here because it's a an arrow function this is not bound to anything so we need to use the function keyword if we want this bound to the object that we're talking about when we're calling it method style so the arrow arrow style functions don't have binding to this there's a couple other things they don't bind to that are nice to haves for normal functions like the arguments keyword and a couple other things so just something to keep in mind when you're using those functions all right now when we're talking about strict mode when you have a function that's defined like so if you return if you uh if you use if you return this um in strict mode when strict mode was added if you were not explicitly binding this to anything and you were calling it function style like so right um there was something in the spec that's in the spec that said if you use strict mode then this should be undefined inside of the function if you're calling a function style this should be undefined but there was a mistake when browsers implemented uh when they implemented uh strict mode they didn't follow this pattern and so they kept returning the window or the global scope even when this was um i'm sorry even when the function was called function style they would continue to return and give access to the window even in strict mode so that is uh one of the differences so if we go to run this oh yeah let's see um console.log let's let's console.log the result of calling that function and we'll see that it is the the global scope again in practice that should have been undefined but browsers didn't follow that pattern so you might get a little bit of a difference between strict mode and not strict mode when you're when you're working um with this and executing the function function style okay the one final thing that i wanted to cover is the bind keyword because like call and apply there's another method that's available on the function object that is called bind and bind allows you again to explicitly bind this the difference between call and apply and bind is that bind does not execute the function so if we have fn we can say fn.bind and then we can pass in some this so if we say const my bound this is a new object where you know um a points at b okay so if we say dot bind my bound this this is not actually going to execute the function right we're getting back nothing when we run our code nothing happens nothing prints out because fn.bind does not actually execute our function instead it returns a new function that can be called so here we might say like const fn2 equals fn.bind then we can say fn 2 and we can execute it any of the ways we talked about before method style um or i'm sorry function style or call apply style or whatever right like most commonly you're going to execute something that was recently bound by function style but now because we're calling it function style we are changing what the default was by using bind we are changing what the default behavior is because before when we called something function style remember that this was bound to whatever the global object was but because we're explicitly binding here then when we execute fn2 it's going to use the this that we passed in so if we say node this um oops let's see what do we want this to do return this let's say console.log this so we can see what's inside all right so now what we're getting is we're getting the printout of my bound this and that's because when we're executing fn2 we're executing a function that was explicitly bound already to an object recall that call and apply those methods can't be implemented in javascript but i wanted to go through and implement bind so we're gonna we're gonna add a new function we're gonna add a new method to the function class that will accept an argument um and then it will return a new function and that new function when called will be using this new object that we talked about and this is like just to kind of experience and see what it looks like inside of or how it might be implemented inside of javascript right so let's say that we have a new we're gonna we're gonna use um monkey patching we're just gonna it's called like reopening we're gonna open the function object and add stuff to it by saying here we're going to say function.prototype.mybind is a new function right okay so we're adding a new function or we're adding a new method to the function class so now we should be able to say like function fn and uh you know this is gonna console.log you know this is my this and we'll console.log this okay now we uh by adding this method to the prototype on the function class when we create new functions later like this function when we create a new instance of it here and then we go to we can we can work with that instance of the function so we can say fn.bind or apply or call and then we can also access our mybind so we can say bind and recall that this is possible because we're going to look up does fn my bind exist no does fn dot underscore underscore proto underscore underscore dot my bind exists yes it does now because we just defined it right we just defined it here so it will look up through the the prototype for the function and find this new object or this new function or this new method on the function class called mybind right now uh back here the default bind that comes with javascript takes uh an argument and that argument is it actually technically takes a couple of arguments you can you can bind um you can bind this and then you can also bind arguments that are like will be explicitly passed in so we'll we'll start with uh my this so we'll start with passing in a new object that we want to be bound explicitly when our function is ultimately called okay so when we say fn.mybind we're going to pass in some object so const bind to this okay google stop and in here i'm going to say message success all right so if we pass in bind to this we expect to be able to use the result of that of calling my bind and store that value in a new function fn2 and then we expect to be able to call fn 2 and that should log out this is my this and it should print out message success so if we run it right now nothing is going to happen this doesn't work so it's saying fn2 is not a function that's because our mybind implementation does not return a function yet so we can return a function from another function so we can say return function and that's we'll start there okay and then we'll build up so now we have a function that returns another function and by default nothing is being printed out yet and but it is executing so now we have fn.mybind we're calling fn.mybind here right so we have an instance of a function class we're executing it and it's returning this function that function is being stored in fn2 then we're executing fn2 but nothing is happening because the function that we're returning from fn.mybind is doing nothing right now okay so instead what we want to do is we're going to return this function but when that function is called so when fn2 is called we want to execute fn and we want to execute fn call style or apply style and we want to pass in the arguments that were passed to my bind so we'll just start easy we're going to say here when we're in the function my bind where we have reference to fn so we want to say const fn equals this right so i'm going to make it a little bit more explicit so like original function is this now when function is executed we want to return the result of original function original function dot call and pass in my this right because here we're able to like actually execute the original function which again was fn so here we're going to execute it and we're because we're using the call or apply style we're able to pass in my this so it's going to be bound to this when original function is executed so let's run this again this is my this and we get object object that's because it's being converted to an object we'll just say console.log this and then we'll just say this is my message this dot message okay all right so let's run this again cool this is my message success so we console.logged this which was explicitly bound to our our object that we passed in so bind to this was the object that we wanted to bind to we got back a new function and when we executed that function function style note we had explicitly bound this by returning a function or currying and then binding this directly to the original function and executing it so now let's make this even more complex okay so let's call let's change the name of our function to sum alright and it's going to take in a b and c and uh we want to we're going to accept an argument that has an x value so we'll for now we'll just console.log this and let's also um console.log a b and c all right so we're going to change this up so actually yeah let's uh okay so i'm going to comment this out uh coming out that code i keep saying this or trying to refer to this without saying this all right so let's update our code here so now we're going to have a function called sum which takes in a b and c and we're going to return our console dot log a b c and console.log uh this all right so um right now sum is not executing if we wanted to we could just say sum and then pass in one two three and again by default this will be bound to the global object um and above the global object somewhere here we should see our one two and three so boom okay so there's our there's our um our output for the sum of one two three right we got our abc and we got this giant global object that we don't really care about so if we wanted to we can say you know maybe let's just say console.log a plus b plus c so we should get back six right we got back six and we're still getting back this global object now if we wanted to make it so that we could bind some let's say like let's make a new function that's called like add 2 and that's going to equal i'll say const add 2 is equal to sum dot bind 2 or like yeah some object that we want to explicitly bind to this and then the number two okay and then if we say add two by passing a second argument to bind this to will be explicitly bound to a and in the future we will only be able to pass in b and c so if we pass in like 10 and 10 then we're going to get back 22. so it should be like the function that is returned from sum.bind should be a function that accepts two arguments or whatever however many arguments but when we pass it two arguments the ultimate function add two will be executing the function the original function sum and it will first pass in two as the first argument and then the the remaining arguments that were actually executed in real time when we called add two those will be passed in as the second and third argument so if we execute this we see 22. now that's a little bit advanced and this is where it might get a little confusing but i wanted to like extend our my bind function to work just like bind and there's a couple of interesting things that fall out of that so let's keep building on this so that some uh or so this add two function works with our my bind and let's actually simplify a little bit so that it's only a plus b and so if we execute this we should get back um 12. right okay so we got back 12. that's cool so now we're saying like we have this new function called add2 we pass in some number to it and it adds it adds two to it right actually don't know where that's console oh console.logging okay cool all right so let's extend our our my bind function so that it accepts this and then any optional arguments right and in in javascript when you have a function not an arrow function but a function that's defined with the function keyword there's another thing that's bound in addition to this you will also get an arguments keyword so we can say const args is arguments so arguments is another keyword similar to this that will be bound when the function is executed so in this case arguments is going to be the list of arguments that this function was called with so if we change mybind actually let's just like console.log args args and we'll change this to uh my bind okay so now we see the args are it's an instance of arguments in the zero place we have the object that we're tr that this should be and then in the first place we see two so we technically want all of arguments except for the first argument so um in this case we want arguments and i think i might do it like this like original function comma and then we'll use the splat operator to get the rest um so original function should still be this that'll still be the first argument and um actually no i'm sorry we still need this and then we want the first argument is my this right yeah okay so then we don't actually have to pass any arguments or we don't have to like accept any parameters in the function definition here instead we can use the arguments keyword to bind directly to the arguments that are being passed in and the first one will be my this and then the rest of the arguments will be should be extracted let's see if that works i don't know if the argument's keyword works like that okay cool [Music] um great so now we have args and args looks like it is an array and recall that we have this function here that if we wanted to we can change this to apply and we can bind my this and we can pass args all right so now we have because we're switching to the apply method we can pass what we want to bind to and then we can pass a an array that contains the list of arguments so if we execute this uh here we are getting man as the result and that's because uh as it stands we're only passing in the args that were passed in the first function right so when we're executing add two we're calling add two that's executing this function it's calling original function which was sum when it calls sum it's up it's binding it to my this which is this empty object so let's just say like uh name my this so it's binding it to my this but then the only other argument that it's getting is this two it's not getting a a b right so it's executing with just the array with the list 2 and so it's only passing in a it's not passing in b because b this 10 right would be is an argument to this function that that was returned from mybind so if we were to take you know this and say like uh b and say args.push b and then call it that should work right we get back 12. cool but we want to make it a little bit more generic so that we we don't have to say like b c d e and then like push push push push push right we can say arguments or i'm sorry we can use again we can use the arguments keyword the arguments keyword here is going to be bound when my bind is executed and then the arguments keyword inside of this function will be bound when when this function is executed so we can say args.concat arguments i think we might be able to just call call it like this all right let's see what we got two plus okay so concat did not work um i remember there being like an odd thing that you have to do in order to get the the argument so um let's say um it's like array.protocol type dot slice arguments zero args2 and let's see args2 is that gonna work no uh array dot slice dot call no yeah call let's see ah slice of undefined um okay so the arguments keyword like this arguments object is an instance of arguments it's not an instance of array which is like a really weird thing so it doesn't um it doesn't act like an array and so in order to do array-like things on it you have to convert it to an array and this is one of the ways that you can convert it to an array you call array.prototype.slice.call and then you pass in arguments in xero so that it's like creating uh it's creating a slice of the argument's object and turning it into an array so then we get args2 which is going to be the list of arguments passed when we actually executed the returned function so in this case args2 is going to be just like the array with 10 in it so if we say console. args2 args2 then we get back args2 is the array 10. so we see args concat args2 so we're combining those two arrays and we're using apply so the resulting array is going to be separated out into little individual arguments and then we'll be binding to this so when we actually execute the the my sum function this is bound to uh the object that we passed in my this and then we have add two which is passing in this curried or my bind is creating this argument two and then we call add two that is a a function that was explicitly bound to this my this object this is probably like way more detail than we need to go into but it's kind of cool to see how this is bound and i wanted to go like pretty in depth here so there's a couple other like odd things around what this is bound to when you're working in the browser so from the dom right from each dom element if you're executing functions um depending on which function it is if it's an event listener for instance then this will be bound to the target element that was uh that was executing on but i'm hopeful that this explanation was useful again going back to what this is bound to i think if you follow the pattern of four ways of calling a function function style call apply style constructor style and method style then you're going to go pretty far in understanding and being able to work with this also just kind of watching out for the trip hazards that are the anonymous function and then finally knowing that you can explicitly bind this if you want to so hopefully that was useful i will see you in the next one

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