---
title: Asynchronous JavaScript 2 of 3: JavaScript Promises
slug: asynchronous-javascript-2-of-3-javascript-promises
published_at: 2021-08-04 15:00:13 +0000
updated_at: 2026-03-04 20:14:01 +0000
summary: 
description: This is the second of three episodes in a mini series about working with asynchronous javascript. You&#39;ll learn about how to create your own promises and how to work with browser and library APIs that return promises.
tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, javascript, Promises, async, asynchronous, JS, new Promise, new Promise()]
views: 353
author: CJ Avilla
url: https://www.cjav.dev/videos/asynchronous-javascript-2-of-3-javascript-promises
youtube_url: https://www.youtube.com/watch?v=YDL4yvbEb50
youtube_id: YDL4yvbEb50
embed_url: https://www.youtube.com/embed/YDL4yvbEb50
thumbnail_url: https://i.ytimg.com/vi/YDL4yvbEb50/hqdefault.jpg
type: video
---

# Asynchronous JavaScript 2 of 3: JavaScript Promises

*Published: August 04, 2021*
*Views: 353*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=YDL4yvbEb50)

[![Asynchronous JavaScript 2 of 3: JavaScript Promises](https://i.ytimg.com/vi/YDL4yvbEb50/hqdefault.jpg)](https://www.youtube.com/watch?v=YDL4yvbEb50)

## Description

This is the second of three episodes in a mini series about working with asynchronous javascript. You&#39;ll learn about how to create your own promises and how to work with browser and library APIs that return promises.

## Transcript

hey what&#39;s up welcome back you&#39;re watching part two of three in a little mini series about asynchronous javascript programming and in this episode you&#39;ll learn all about promises we&#39;re gonna talk about the promise object so let&#39;s open a brand new index.html file and here we&#39;re just going to scaffold out some html and i&#39;m going to start off with just a script tag down at the bottom here where we&#39;re going to talk about this promise object so a promise is an is a type of object it&#39;s a class just the same way that we have a type of object for an array or a string or a map this is another type of object that is available in the javascript api and this gives us the ability to track the state of a job that is going to be either resolved or rejected so a promise has three possible states it&#39;s like pending fulfilled or rejected so when a promise is pending that&#39;s like it&#39;s it&#39;s an outstanding promise we&#39;re waiting for something to happen and in this example i wanted to talk about this promise that i made to my kids they want to go to this smoothie store that&#39;s right down the down the street and they want to get some some smoothies or these like blended drinks and so i said i promise that i will take you to the smoothie store and while that promise is waiting for us to actually go to the smoothie store there&#39;s a bunch of other stuff that&#39;s happening maybe we&#39;re working maybe we&#39;re you know sleeping exercising whatever uh during all of that time the pen the the promise is pending okay and then if we actually go and get smoothies then the promise is fulfilled if we don&#39;t actually go get smoothies and i changed my mind and i&#39;m like oh you guys have been awful kids and you don&#39;t deserve smoothies then the promise might be rejected it&#39;s it&#39;s good to keep your promises if you are an adult and you&#39;re making promises to children you don&#39;t want to like give them issues but in that case your promise might be rejected okay so while a promise is pending that means we&#39;re sort of waiting for something to come back when a promise is fulfilled that means the the thing has been done and when a promise is rejected that means that some error happened something failed it didn&#39;t actually resolve in the way that you want uh in the way that you wanted now fulfilled and rejected sometimes we&#39;ll talk about promises either being fulfilled or rejected and calling that settled so settled is the same thing as fulfilled fulfilled or rejected so if the promise is settled then it was either it like something happened party&#39;s over the promise was either broken or it was kept and that&#39;s it when you call the constructor function for the promise so when we&#39;re saying new promise this accepts an argument and the argument to this function is another function that will execute immediately and this function takes in two arguments so it takes in a resolve or a reject and these two functions resolve and reject are functions that you would call inside of your execution of this function so we&#39;re going to call either resolve or reject or both inside of this function if we call resolve that means the promise was fulfilled if we call reject that means the promise failed for some reason so here we could just say resolve with like hello and that would mean that as soon as the promise is created it&#39;s resolved and it gets some value so you will call resolve with the data that you want to resolve with the data that is like the resolution of the promise you will call reject with some reason failed to blah blah blah okay so we can only call either resolve or reject you can&#39;t call like both as soon as you call one of them the promise will be either fulfilled or it will be rejected you can&#39;t have like both states so maybe we say like if like true equals false then we say hello else we say reject so what this will do is because true is not equal to false it will reject the promise and then if we say is true equal to true then it will resolve and then otherwise if true for some reason is not equal to true then it will reject the promise usually you would only use this new promise sort of syntax if you are authoring a library a javascript library that other people are going to use you typically would not actually write your own new promise function like this but this is how the promise how promise functions actually work and this gives us back this promise this instance of a promise so we&#39;ll say promise like promise one and for now we&#39;ll console.log promise one and see what we get back see what this promise object looks like so let&#39;s go refresh our browser and open up the console and so we&#39;re going to refresh the page and here inside of the console we see that the promise this is the status of the promise again so this is it says fulfilled and it gives us the data that it was fulfilled with right here so we are right now we are successfully executing our promise now if we were to comment out all of this content here and create a new promise that&#39;s not actually resolving or rejecting anything and the function right now is just a no op a no op means that it&#39;s a function that does nothing so if we refresh the page now we see a promise that has the state pending so the promise is waiting to be resolved or rejected so we actually have reference to promise one because we defined it at like sort of the global level so from the terminal here i can actually mess around with promise one so if i wanted to i could say promise1.then or promise1.catch and these are ways that we can add functionality that says once the promise is fulfilled execute something or when the promise is rejected do something else so let&#39;s go back over here and say if age is greater than 21 then we&#39;re going to resolve with like a fancy smoothie if uh let&#39;s see like else if age is less than or if the age is greater than 11 then we&#39;re gonna resolve with just a normal smoothie otherwise we&#39;re going to reject and like not allowed to have a smoothie not no smoothies for you okay and then we need some age some reference to an age so const age is we&#39;ll start off with someone who deserves a fancy smoothie and that&#39;s me and i&#39;m 31 i mean i&#39;m 34. uh i don&#39;t deserve a fancy smoothie but uh that sounds delicious right now this the smoothie store is like i call it the smoothie store it&#39;s like the smoothie shop or whatever is seriously like a stone&#39;s throw or like a par three or i don&#39;t know it&#39;s like right around the corner from us and we&#39;ve been living here for like 10 years and haven&#39;t actually gone and explored this shopping center and so now this is like a new a new thing that we&#39;ve been doing lately anyways as soon as we execute this this promise and create this promise uh we will resolve with fancy smoothie and we should see that um that we are resolving with the fancy smoothie but we need to talk about how we can add functionality that will respond to success or failure of this promise the brand new javascript api that was released with promises sometime between 2015 and now has the dot then in dot catch syntax so we can say promise one dot then and dot then is a function that we&#39;re going to call and we&#39;re going to pass in another function and that will be the function that&#39;s called if the promise is resolved successfully so we&#39;re going to pass in another function here so we&#39;re going to say this is our success function also now i&#39;m like totally mixing and matching these tiny little sidebar if you have function f that looks like this and it does stuff that&#39;s the same as like or that is very similar to having some variable f where we use this arrow syntax so this this parenthesis with an arrow pointing at curly braces is another way to write a function call and that&#39;s what i&#39;m passing here so this is like parentheses where it&#39;s accepting the parameters and arguments for a function then we&#39;re using the arrow function to pass in this so this whole thing here is the body of the function that we&#39;re passing into promise and that&#39;s going to execute again immediately and then usually a promise is going to do some asynchronous thing and then later on it will resolve or reject but in this case we are going to immediately resolve or immediately reject based on the age so what we can do is down here we can pass in another function to then and then we can say inside we&#39;ll say console.log and as part of our resolution recall that we&#39;re passing in some data that is like the result of this promise that we made so in the case of a promise to take the kids to the smoothie store we might resolve with the type of smoothie here so we&#39;re going to say fancy smoothie or normal smoothie and that&#39;s going to be passed as an argument into dot then so here we&#39;re going to get like the like smoothie type or something so we can console.log smoothie type and that we should see that con this this log to the console immediately because as soon as our promise executes it will be resolved so it&#39;s going to be immediately resolved and only later are we attaching this dot then so it turns out that you can call dot then on a promise that is already resolved that&#39;s already settled so if you if you have a promise that&#39;s already fulfilled so in fact like if we if we recall that promise one at this point was already fulfilled see when we console log that we see fulfilled here in the terminal so that promise was already fulfilled and we&#39;re then attaching some callback later that&#39;s executing when we called then so there&#39;s a couple different syntaxes when you call.then you can pass it one function or you can pass it two functions so if we pass it two functions the second function will execute if there was a rejection so we&#39;ll say reason here and then we&#39;ll say console.error and we&#39;ll pass in the reason okay console.error will just make it red in the console so in order to make this fail let&#39;s say that we are actually four years old we&#39;re not allowed to have smoothies we refresh the browser and now we see no smoothies for you and this is this is red because we used console.error and that is the reason of our rejection was this the reason that was passed to reject is what we receive as an argument to this second function that we&#39;re passing into then so uh it turns out that you can continue to chain these promise these uh the the methods for then and catch okay this this style of calling then where you pass in the first argument as the success callback and the second argument as the failure callback is actually not that common and there&#39;s a few different reasons for it but a more common pattern is to instead of calling instead of passing a second argument you you call another function and you chain that onto.then and we call dot catch and this is where we pass in our our error handler console we&#39;ll say console.error with our reason and we&#39;ll talk about that in just a moment so if we refresh now we get the same result we&#39;re still logging out no smoothies for you but the difference is that usually when you&#39;re working with promises it&#39;s nice to chain a bunch of different calls so here we&#39;re saying like we have promise one and when promise one resolves then do something so we&#39;re console logging the smoothie type and then you can say then do something else so maybe we want to say then like i don&#39;t know you&#39;ve gotta like after you drink your smoothie you&#39;re going to um i don&#39;t know we&#39;ll like console log uh then you&#39;re gonna take a nap or something okay so if we refresh the page no smoothies for you because we did not successfully resolve let&#39;s say that we are 11 and we&#39;ll refresh uh oh we have to be older than 11 so 12. we&#39;ll refresh so we received a a normal smoothie so we drank a normal smoothie and then we took a nap and then it like finished resolving all these promises so you can sort of chain these on and what&#39;s really nice is with this arrow syntax we can actually just remove a lot of these lines because these are all one-liners and we can just do this here and this makes it read very nicely right so you have your promise you have your dot then and again recall that like this dot then is uh is executing when we successfully resolve and then this that then was going to be called after this then runs so you can you can continue chaining these on and have as many thens as you want and then you usually will have a catch at the end all right so you&#39;ve got all these then calls but if at some point during the execution of any of those those subsequent calls results in an error then our catch is going to be called so for instance if at this line for some reason we said throw new error like failed to fall asleep or something then then the execution is going to stop and we&#39;re going to go into our catch so this makes it really nice to add a bunch of a bunch of uh thens and then automatically sort of handle the failure cases as you&#39;re working through a process of of going through a list of errors so here we see our we drank a normal smoothie then we tried taking a nap taking a nap taking a nap and then we failed to fall asleep that error was raised and then we went down here to console.error that&#39;s where we actually like failed and printed out failed to fall asleep so that is pretty handy now there&#39;s a couple other things that i wanted to talk about so usually again when you&#39;re using this promise it is not going to be resolving immediately like ours is now usually it&#39;s going to resolve in some time so we&#39;re going to look back and use what we learned in the last episode about set timeout and we&#39;re going to say that you know let&#39;s actually use a set timeout here and we&#39;re going to pass in another function that will run and it takes some time to like actually go to the smoothie store so let&#39;s say that we actually want this to happen after three seconds so we&#39;ll console.log that we are like on our way okay and then as we drive we&#39;re gonna it&#39;s gonna take us three seconds to drive to the smoothie store and then we should see some output after that so we&#39;re going to refresh our page we see that are on our way and then we count three seconds and then we get our normal smoothie we take a nap take a nap take a nap and then we fail to fall asleep okay so let&#39;s take out our failure here and let&#39;s take out some of these naps and then refresh the page so we&#39;re on our way while we&#39;re on our way we&#39;re counting down that three second timer when the three seconds elapses then we call resolve when we call resolve that ends up executing all of the dot thens that were successfully added to our promise and then ultimately if there was any exceptions it&#39;s going to call catch what&#39;s cool about this is we still have access to promise one and it&#39;s fulfilled but we could just in the console or anytime at some later date we could say then like you know we&#39;re going to console.log i want another smoothie so i don&#39;t know how long it takes for you to want another smoothie but we could add a dot then to our fulfilled promise at a later date and it would continue to to call the dot dense you can attach these later so that&#39;s kind of handy alright let&#39;s take a look at a more real world example of using a promise that&#39;s already built in so the fetch api i&#39;m actually just going to comment all this out so that we can keep it for later but so fetch is a method that&#39;s built into the browser that will return a promise now fetch takes in some arguments to make an api call or to make a an http request from the client on the on the browser to some server to some remote server and when it gets the data back from that server then it will resolve the promise if it successfully got some data back or it will reject the promise if that request failed so let&#39;s go take a look at some apis here so here i&#39;ve got an api pulled up for checking air quality the air quality in northern nevada right now is awful we&#39;ve got fires to the north we&#39;ve got fires to the south i&#39;m like a little bit congested because there&#39;s just so much smoke i took the dog for a walk this morning and there was you could see ashes and particles of ashes just flying around everywhere i&#39;ve seen like you know chunks here and there flying around but as a car drove by me today there was just like a bunch of ashes sort of in the gutter that swept up and you could see just tons of ash it&#39;s really sad um there&#39;s just yeah the the environment is uh is the air qual is just not healthy right now it&#39;s it&#39;s wild so this is an api for checking the air quality for a given city so i&#39;m gonna copy the url there and the first argument to fetch is the url to which you want to send your api call i don&#39;t need this curl stuff i&#39;m just going to actually pass the the api call and here we can actually say const city is equal to sparks and the state is equal to nevada and i&#39;m gonna i&#39;m gonna use string interpolation to put those into our url here so the city is going to be the city and the state is going to be the state and the country yeah i will just stick with the u.s country or u.s cities because i don&#39;t know what other cities uh or which other countries this api supports and then we need to pass our api key and so we&#39;re going to say const api key is equal to and then i have an api key here that i&#39;m going to roll right after this video so you don&#39;t need to copy this okay so this is going to be our our call to fetch let&#39;s actually make this the url const url is equal to um this thing all right so we have our fetch call here and this is going to take in a couple different arguments so the very first argument is the url to which we want to send this api call i also had to switch the state to the fully the full name of the state here otherwise this doesn&#39;t work so i&#39;m going to pass in the url to fetch and this is going to return a promise so we&#39;re going to say const p is equal to this and for now we&#39;ll just console.log p and we&#39;ll take a look at what p looks like in the in the terminal so if we refresh the page here we see that the status of the promise is pending so this promise is still waiting to resolve um when we logged it out right and so if we look at this this network tab of the terminal and refresh the page we can see for like a split second all right so thinking again about how javascript is executing right so it&#39;s going to execute top to bottom as soon as it makes that fetch call that is sort of saying like go out and fetch some data and it&#39;s in the background it&#39;s going to go fetch the data and that&#39;s going to happen with the browser but our the browser is going to continue executing the javascript because fetch is an asynchronous api an asynchronous api call so it&#39;s going to return that promise and continue executing the benefit is that it&#39;s not going to lock the browser or prevent us from doing any other actions inside of the browser while it&#39;s waiting for that response from the api as soon as it gets the response from the api it&#39;s going to call that resolve function it&#39;s going to call the resolve function for the promise passing in any data that came back from the api and that will cause any of the dot then or dot catch methods that were that were attached to the promise to call so the same way that we had our promise that we called resolve with manually under the hood fetch&#39;s promise the resolve method for the promise that fetch is using will be called and that&#39;s going to result in calling.then so now what we can do is this this p this promise that was returned from fetch we can now say p dot then and we can pass in a function that we want to call with the results of what came back from the api so here we might say results and we can console.log results okay so as soon as that 180 millisecond or whatever the duration is for that api call to happen as soon as that returns we&#39;re going to console.log results before we log results i&#39;m going to say console.log like this is before fetch then we&#39;re going to say after fetch then we&#39;re going to say before then and after then and then here we&#39;re going to say like uh got results so you can sort of see the order in which things are happening here okay so we&#39;re gonna see we&#39;re let&#39;s talk through the asynchronous like flow for these calls right so we&#39;re going to console.log before fetch then we&#39;re going to kick off a call we&#39;re going to tell the browser hey go make an api call to this url and go get some data from that server and the browser is going to start doing that independently of our code executing so we&#39;re just sort of saying schedule some job to run and that job is to go fetch some data and then when you&#39;re done scheduling it come back like immediately so don&#39;t wait for the data to come back come back immediately so it&#39;s going to come back immediately then we&#39;re going to log after fetch then we&#39;re going to log before then then we&#39;re going to attach this callback or we&#39;re going to attach this success handler to the promise that&#39;s gonna say if as soon as the promise is resolved call this function but it&#39;s not gonna actually execute the function at that time okay it&#39;s just gonna say as soon as that&#39;s resolved we&#39;re attaching some code to run as soon as fetch finishes and resolves so we&#39;re still continuing to execute so we&#39;re calling dot then and we&#39;re passing it this function and that function is going to be set up as like a a thing to do as soon as fetch is done but it&#39;s not going to execute yet then we&#39;re going to call after then so it&#39;s going to say before fetch after fetch before then after then and then 100 milli 180 milliseconds later might as well be a year later or like an hour later right the data is going to come back from the api and that promises resolve function is going to get called internally and when resolve is called we&#39;re going to call all of the callbacks that we have passed to thens okay so let&#39;s go take a look so if we refresh this page so we see before fetch after fetch before then after then and then only after 100 180 milliseconds later we see got results and this is the data that we got back okay so this is results this response object is the results from the api so if we look at this response object it has a few things in here it has the url that we called it has an http status code this is the 200 status code everything was great thumbs up we were not redirected to another uh endpoint okay status okay 200 these all mean the same thing they&#39;re all just like helpful ways of breaking down that response we get some http headers back from from the response and then we have this body now in order to get access to this body so the body is a readable stream it turns out that this response object has a method on it called dot json so we can call json on the response and that will read all the data that the browser got back but it turns out that dot json also returns a promise so let&#39;s take a look at that so here we got some results back we&#39;ll say const p2 p2 is equal to results.json if we console.log p2 you&#39;ll see what i mean so we got some results and we&#39;re going to see what p2 is i&#39;m going to put this inside of curly braces this is a shortcut so that we can see p2 points at and then whatever p2 is it&#39;s a this is like a handy trick for console logging so if we refresh again now we see p2 points at a promise and the promise is fulfilled immediately right so this is just telling us that when we call dot json on the results that were passed to us from the very first fetch call this json returned a promise and that promise will then again resolve and it&#39;s going to resolve with the json object with the actual javascript object okay let&#39;s look at this way first so we can say results.json.then and then pass in some other thing like obj so this is going to be our ultimate like weather object and that weather object is going to have some data on it so we&#39;ll console.log weather object okay so in this case i&#39;m chaining.then directly onto the results.json because we know that results.json when you call results.json so here we know that results.json is going to return a promise so as soon as this bit executes that&#39;s going to give us back a promise that we can call.then on so this is sort of the same as if we had called if we said like const p2 is equal to results.json and then you know p2.then so a shortcut is to just chain.then onto the end of this json call because we know json returns a promise so now we&#39;re chaining on.then and as soon as j the promise return from json resolves which will be immediate because we have the the data there it will execute this function that we&#39;re passing to then to log out the weather object so i&#39;m going to again put this inside of these curly braces and refresh the page so now we see the weather object and the weather object has the data inside of the data we see you know the location we see the current weather inside of the weather we see a bunch of oh here we go i think this is the air quality indicator um is 242 which is huge usually i think this is supposed to be down like in the tens or something and i think it&#39;s if it&#39;s over a hundred then it&#39;s unhealthy if it&#39;s over 200 it&#39;s like very unhealthy or i don&#39;t know something like that but this gives us like an air quality indication um but this is the data that we wanted back from the api all right great so that at least that&#39;s working so we&#39;ve got we&#39;ve got this fetch call it&#39;s returning a promise p and we talked about here how we can say results.json.then we can just chain.then directly onto the end so let&#39;s actually refactor our code here so that instead of using p we&#39;re just going to chain dot then onto the end of this fetch call and i&#39;m going to use indentation so that i know that this is sort of like a chain method call this is a common practice so there&#39;s another trick that we can do though because because then returns a promise remember that we can sort of chain a bunch of thens down here we said like dot then dot then dot then take a nap take a nap take a nap so they all return promises so what we can do is we can return results.json as the return value for the first call to then and then we can call.then again we can chain it on there and print out the weather object i&#39;ve got a missing u here so because we&#39;re returning results.json we&#39;re returning the promise that comes from here and that promise is what will be used for the next dot then okay so the the resolution of json is going to be what&#39;s passed in as the object the weather object here so i&#39;m going to remove this console log and we should just see the weather object now so now we see the weather object that&#39;s great we can expand it we see this success we see all the data et cetera et cetera okay now when you&#39;re using this arrow function style of function or this arrow function yeah like arrow functions inside of javascript one shortcut you can use is a one liner where you don&#39;t have these curly braces and we talked about this earlier but when you don&#39;t use curly braces you actually don&#39;t have to even pass return there&#39;s an implicit return so if it&#39;s a one liner like this so you take in results and then it&#39;s gonna because we don&#39;t have curly braces it knows that it should return whatever this thing is on the right hand side so results.json is going to return the promise for results.json is going to return to this then and then we&#39;ll have access to it down here again we can sort of clean this up and remove these curly braces so we&#39;ll console.log the weather result there so now we get back the weather object again we see all the data very cool let&#39;s actually do something with the data real quick so let&#39;s add maybe we&#39;ll add like a a p tag here that has the um error quality error and maybe we&#39;ll start off with like zero and then so we should see it show zero and then it&#39;ll just flash for just 180 milliseconds we should be able to detect it with the naked eye and then as soon as we we get this result we can update that air quality with this weather object so let&#39;s actually like go back to a multi-line stream or a multi-line function here and in order to get the data out of this object too so we might say like here we have a json object i&#39;m exploring it sort of from the from the javascript console and i know that weather object here i&#39;m expanding it and i can see data and then i can expand current and i see pollution and then i expand pollution and i see aq aqi or whatever we can also expand weather and we can see a bunch of stuff about the weather the temperature etc what i want to do is talk about how we dig through this object and call all the different properties to know where we&#39;re going so inside of this data we want to get the current pollution so we&#39;re going to call data.current.pollution so here on the weather object we should be able to console.log weatherobject.data.pollution or i missed current current.pollution and this should give us back just pollution and it didn&#39;t so let&#39;s see if we can work our way back up so let&#39;s narrow this down so we&#39;ll say whether data whether object.data that gives us that our our data and then if we say dot current refresh we see the current and then we see dot weather and then refresh we see the weather we actually want pollution maybe i spelled it wrong okay so there&#39;s the pollution and then if we want this value here for a q u aqi i just copied it because i don&#39;t know a q i u s then we can refresh and now that&#39;s the value that we see so that&#39;s the value that we want to use for our for our air quality indicator here so we&#39;re going to say document.query selector error quality dot inner text is equal to aq and this should work all right so when we refresh the page you&#39;ll you&#39;ll be able to see zero for just a fraction of a second and then it&#39;s going to populate whatever with whatever the current air quality is so we see zero boom it&#39;s just it&#39;s super quick and in fact like we can look at the the api calls here and tell exactly how quick it is so that one took 172 milliseconds let&#39;s refresh again okay that one took 186 milliseconds and so the naked eye can detect that but if you have it i think if it drops below like 50 milliseconds or something it&#39;s like not detectable with the naked eye but um here we&#39;re definitely seeing a flash of zero followed by the actual value so in a lot of cases what you might do is actually type in like loading dot dot and then when you refresh you&#39;ll see loading while the thing is loading this is a common practice where if you need to make an ajax call so this is sometimes referred to an ajax call an asynchronous javascript and xml call uh it&#39;s an asynchronous call out to some web service we&#39;re gonna get back some data when that term was first coined xml was what you would get back a lot of times instead of json xml is just another format for data that&#39;s another like yeah structure of structure of data json is another object notation or another structure for the data that we get back now we mostly work with json but no one has updated it to be aged so it is what it is but so all to say when working with promises you can chain these dot thens and you can also chain a dot cache so it&#39;s a common practice to add a dot catch and have your reason and then maybe you want to like actually add that also into the query selector so we&#39;ll say like reason here so if your api call failed for some reason then you might want to display that to the user so instead of just constantly showing loading or something if you messed up the api request and it failed or if the server was down or if for whatever reason we didn&#39;t get back the data that we expected we might want to tell the user the reason for that so let&#39;s actually just like mess up the url here so we&#39;ll say v i don&#39;t know v2 blah that&#39;s not a valid url so now when we refresh we should get a 404 why are we not getting any errors from oh whoop i&#39;m missing a i&#39;m missing a closed parent oh i missed my arrow function here okay refresh the page okay here we go here we go here we go so now our error is showing so failed to fetch so we can see this course error here this is failing v2 block is not a valid url so this is the error that we&#39;re going to see so you could potentially say you know like request failed or something like that so we see loading and then we see request failed so that is where you would use catch i think it is a best practice to always add catch onto all of your your promises if we were to not have this then we should see unhandled rejection unhandled promise rejection so we see uncaught inside of promise if we don&#39;t have the catch there when something is failing all right so this is how you use promises again generally you would only use this new promise constructor function to create a new promise if you&#39;re like authoring a library or you need to wrap up some asynchronous code that does not already expose a promise a lot of the times you&#39;re going to work with libraries or browser apis that are going to return promises and knowing how to handle the results of those promises by adding.then and catch can be really handy alright so that&#39;s it for promises if you found this episode useful at all i would really appreciate a thumbs up so that other people can find the content thank you so much for watching and we&#39;ll see you next time you

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