--- title: Asynchronous JavaScript 3 of 3: async / await slug: asynchronous-javascript-3-of-3-async-await published_at: 2021-08-06 15:00:31 +0000 updated_at: 2026-03-04 20:14:00 +0000 summary: description: In this last episode of the three part mini-series, you'll learn about the async and await keywords and how to use those when working with promises. tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, javascript, async, await, async / await, javascript async, async function, must use await in an async function, await used outside of an async function] views: 281 author: CJ Avilla url: https://www.cjav.dev/videos/asynchronous-javascript-3-of-3-async-await youtube_url: https://www.youtube.com/watch?v=do2neAvj8hQ youtube_id: do2neAvj8hQ embed_url: https://www.youtube.com/embed/do2neAvj8hQ thumbnail_url: https://i.ytimg.com/vi/do2neAvj8hQ/hqdefault.jpg type: video --- # Asynchronous JavaScript 3 of 3: async / await *Published: August 06, 2021* *Views: 281* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=do2neAvj8hQ) [![Asynchronous JavaScript 3 of 3: async / await](https://i.ytimg.com/vi/do2neAvj8hQ/hqdefault.jpg)](https://www.youtube.com/watch?v=do2neAvj8hQ) ## Description In this last episode of the three part mini-series, you'll learn about the async and await keywords and how to use those when working with promises. ## Transcript hey what's up welcome back you are watching part three of three in a tiny mini-series about asynchronous programming with javascript in the first episode we talked about set timeout and set interval and how sort of javascript executes code if you wanted to head over you can take a look at that one now uh the in part two we talked about promises so we talked about how you can create your own promises using the promise constructor function and the promise object inside of the browser and how they start off as pending and they work their way to either fulfilled or rejected and in the the last episode we went over how to use the fetch api and calling.then uh to get the result of some uh promise in this episode i want to talk about async and await so async and await are two keywords that were added to the javascript language pretty recently i think these were in es7 or maybe even like in a more recent version and so what we're going to do is we're going to modify our current api calls so let's go through and recall how this works so we are making a call out to this api that gives us back some weather specifically air quality right now i'm based in northern nevada we have a fire that is just north of us called the bootleg fire we have another fire that's just south of us called the tamarack fire and i was walking my dog this morning and as we were walking there was just tons of ash a car drove by and i could see the ashes just sort of like fly up into the air there's ashes everywhere the the visibility is very low the air quality is just garbage but the dog's got to get out for a walk right so what i wanted to do was use this as an example for fetching some air quality results and displaying them on the page and so at this point right now what we've got is a little a simple little so what we've got right now is a simple little app where we can refresh the browser here we see loading and then we see the current air quality indication 239 is very high it's like a really uh very unhealthy air quality um so yeah super disappointing but it should give us but it should give us something to talk about here when we're going through javascript so what's happening is we have this url that where we pass in the city and state in an api key and we're making use of the fetch api so this is something that the browser gives us to make an api call or fetch some data with an http request and it's returning a promise and in the last episode we talked about how you can use then and catch to handle the results of that promise to uh to take some actions when that promise ultimately resolves or is rejected down the line some other time in the future so this is how we respond to an asynchronous event completing and so what we want to do now is let's refactor this to use async and await so when a function returns a promise we can put the await keyword in front of it so a wait means we're going to wait for the result of that promise at this point in the code and then we're going to continue execution so our our code is actually just going to stop at this point and wait for wait for the resolution the successful resolution of the promise that's inside of the fetch api by using a weight we don't actually pass any callbacks into dot then we're not passing any callbacks into dot catch we are instead going to use a try catch and we're also going to use just a more like imperative programming style where we can say do this then do this then do this so here we're going to call a wait and that should wait for the fetch to come back so this is actually going to fail and we'll see why in just a second so we'll refresh the page and here we see uncaught syntax error a weight is only valid in async functions right so in order to use a weight we need to be inside of an asynchronous function so the way that we can create an asynchronous function is we can create a normal function f or like let's say like fetch weather and uh we're going to put our awake call inside there so in addition to writing our function like this in order to make it async we need to use a modifier so the modifier is async so we put the word async in front of where we say function that modifies this function to become an asynchronous function now our asynchronous function here we can just call normally fetch weather and the reason why we want to call it is because if we don't call it then it doesn't actually execute the stuff that's in here so we need to wrap all of our code that's using a weight inside of this async function so we're going to put we're going to create a new function called fetch weather we're going to make it an async function and then inside of fetch weather we're going to we're going to say await and let's actually console.log so we'll say here we'll say console.log before defining fetch weather okay and then we'll say after defining fetch weather and we'll say before calling fetch weather and then we'll say after calling fetch weather so you can see sort of see the order of operations here and then inside here we'll say console.log fetching weather data and then we'll say done fetching okay so that we can see the order of these events that are going to happen so we'll say refresh and so we see before defining after defining before calling fetching the weather data before calling so we see before calling and then we see fetching so as soon as we call fetch weather it's actually going to execute that function normally as we expect so it's it's calling fetch weather data and then as soon as it gets to here this is where the asynchronous call happens okay so now we're going to see after calling fetch weather data so we see this this return first so this is printed to the console first because as soon as we call await this function sort of stops executing and it says anything that's after this consider that code that needs to run after this callback or i'm sorry after this promise resolves so consider anything after this await sort of as part of the success or part of the dot then or part of the the bits that need to re be called after the promise is resolved okay so because we're pausing here to wait for some data to come back from the api none of this code is going to execute and we're just going to say that okay everything after that is going to be completed in part of another another call stack execution from this point forward the rest of this code is going to be added to that call stack that says as soon as the promise for fetch is resolved pick up all the or pick up this function where we left off and continue executing it later okay so because we're saying like at this point we're we're not done resolving this this callback because we're saying at this point this promise is not resolved just continue execution after fetch weather so then here we see after calling fetch weather so we see after calling fetch weather then after the weather api resolves and that's going to resolve in some time so like if we look at the api here we see this is actually 372 milliseconds so it took 372 milliseconds later the api returns and then we call done fetching weather data at this point where we call await we are waiting for the data to come back from this api so that the promise can resolve and as soon as the promise is resolved then we'll continue execution for the rest of this function how do we actually use the data that's returned here well we can in front of a wait we can assign the return value from this fetch call into a value so we can actually say like const response is equal to a weight fetch url so this is going to be the value that the promise is resolved with so that's going to be the return value that the promise was resolved with down here this is sort of the same thing as results so let's actually call this results and that'll give us like a similar look in fact like let's also call like a fetch url because this is going to be basically the same thing in two different ways so that at the top here we'll do the async version async await version and down here we'll have the promise version so when when this promise is resolved with results those results will be stored into this variable all right so i'm going to remove the console logs for now okay so we're going to see results if we console.log results we are again going to see this is the response object so if we refresh the page we see this is the response object and again we can look at all of the stuff about the response object recall that the response object has another method on it called json and dot json returns yet another promise so we can actually await that call also so here we can say const weather data is equal to await results dot json so results.json is going to call return another promise that we can then await again and we can when results.json's underlying promise is resolved the value that the promise is resolved with will be set to weather data so whether data will contain the value that the prominent the json promise resolved with and then we can we can just console.log weather data here okay so we can refresh the page and now we see weather data at the bottom we see that it was successful we see the data for it etc etc etc so this is how we can actually dig in and pull out the aqi for the pollution from this api this makes it much more readable so by using async and await we can sort of just write like this very like step-by-step code as you walk down the execution of the code right so even though fetch is doing something asynchronous we can plop in a weight in front of it and then wait for those results and then only once this promise has resolved will it start executing this next step here where we're going to call results.json and again we can use that weather data to set the air quality index equal to its value and we can just do that like this okay so as soon as we get the weather data back we're going to drill into the object's data's current pollution aqi and we're going to set that value as the inner text for our our air quality paragraph tag that's at the top so now loading should show our value for for the weather data weather data okay refresh the page and now we see 239 so when we refresh we see loading for a split second because it's going to make that api call and then as soon as all those promises resolve and those awaits are uh are resolved and they get back the data from the api then it's going to update the ui with uh with our our return value now what happens in the case of using async await what happens if we have a failure again in the previous episode we just messed with the url here we added blah so that we can see what happens if it fails if we refresh this page now at the bottom here we see a bunch of errors but our loading screen did not update and we see an uncaught in promise type error fail to fetch so this is the error that we sort of want to handle when you're using a weight if there's a chance that the method you're calling will result in a failure then instead of using dot catch which we did with the promise version instead of using catch we're going to use try catch that is like the standard try catch tools that you have inside of javascript to try something and then handle an error so we're going to say try hatch e and inside of our try block we'll also just do these two things also so that we can print out um the result and if there was an exception then we'll update our air quality tag to say like failed to fetch weather data all right so let's refresh our page and now when it fails it says fails like first when we when we refresh it is loading and then as soon as that api request fails because the url is invalid we get this message fail to fetch weather data and again down here at the bottom we see the a more detailed error message but at the end of the day this is what we really wanted was using this try catch mechanism in order to handle any of the exceptions that are raised inside of these promises where we're using a weight so similar to our catch call down here where we're getting the reason back this exception here will have the reason why this promise failed so any time that you're using a weight we want to wrap it into try catch if there's an exception anywhere in any of these four lines our catch will uh we'll handle those here and then we can respond that we fail to fetch the weather data so this is how you use async await there's another little trick that i wanted to show and that is that often instead of having to like create your own async function like this another way you can do this is by creating an immediately invoked function expression so you can say okay let's let's if we wanted to create a new function called like f right and as soon as we've created this function if we want it to call itself what we can do is we can say let's wrap it in parentheses so that it evaluates the expression that's inside the parentheses so this is the same thing as using like parens around like math expressions if you're doing like a times b plus c and you want to make sure that one of those is going to evaluate first you can put parentheses around it and by putting parentheses around it you sort of force it to execute so we're going to use parentheses to evaluate the expression that defines the function and then after the parentheses we can put another set of parentheses and these actually call the function so this is called an iffy actually it should be like function function f so it's called an iffy um uh and no it's iife so immediately invoked function expression so this is called an immediately invoked function expression the reason why we have these parens on the outside is just to evaluate that function definition and the reason we have the parens on the inside is so that we actually execute the function and it there's like a couple different weird syntaxes you can put a bang in front of this so like not function that's another way to evaluate force a function to evaluate and so using this that will define the function and then we'll call it down here another way you can do this is by putting the parens on the outside and then these parens on the inside will execute the function a bunch of different ways to do it but with the arrow syntax you can also define an immediately invoked function expression like this and again we can use this async function modifier in front of it so we can say async like this and this creates an immediately invoked async function expression which we can use instead of our fetch weather situation so let's just like copy this and paste it in here and so this is an alternative to using fetch weather we're just wrapping it inside of this iffy and now when we refresh we see the same result if we fix our url here and put blah take blah back out and refresh again we see 239 that's the result we expect so this is one way is using an iffy i think if these are a little iffy a little bit confusing for for uh when you're just getting started so another alternative is waiting for the dom to be fully loaded and so there is a an event that you can listen for on the document so we can say document.ad event listener dom content loaded and when the dom content is all loaded for the page you generally will pass this a function or an event handler so we can say this is called e and e is going to have the event for the dom loading but you might also just put the async modifier on this callback and now you have an if or now you have a function expression that's going to evaluate when the dom is fully loaded and is async so what's nice about this is you always will have and you'll always be inside of an async function so a wait the await keyword will always work and also the dom will be fully loaded so i like using this way just because it it's sort of like two birds with one stone right you'll have all of the content all of the like the tags will be on the page for the dom and also you'll be able to use the await keyword but we should get the same result here add event listener is not a thing for document add event i can't spell today okay so now we see 239 that's what we expect and now yep so there's a couple different ways that you can create async functions when you just want to use a weight inside of the um the context of your you know simple script tag that's on the page all right so that's it for asynchronous programming we talked about set timeout set interval we talked about promises we talked about async and await thank you so much for 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