---
title: Asynchronous JavaScript 1 of 3: setTimeout, setInterval, clearInterval
slug: asynchronous-javascript-1-of-3-settimeout-setinterval-clearinterval
published_at: 2021-08-02 14:00:06 +0000
updated_at: 2026-03-04 20:14:02 +0000
summary: 
description: This is the first episode in a set of 3 where we'll talk about how JavaScript runs asynchronously. In this episode, we'll use setTimeout and setInterval to build some timers with some olympics inspired mini tasks.
tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, javascript, asynchronous javascript, setTimeout, clearTimeout, clearInterval, setInterval, How javascript runs setTimeout callback functions, Event loop, JavaScript event loop, callback queue]
views: 2206
author: CJ Avilla
url: https://www.cjav.dev/videos/asynchronous-javascript-1-of-3-settimeout-setinterval-clearinterval
youtube_url: https://www.youtube.com/watch?v=Fku_cRb6odc
youtube_id: Fku_cRb6odc
embed_url: https://www.youtube.com/embed/Fku_cRb6odc
thumbnail_url: https://i.ytimg.com/vi/Fku_cRb6odc/hqdefault.jpg
type: video
---

# Asynchronous JavaScript 1 of 3: setTimeout, setInterval, clearInterval

*Published: August 02, 2021*
*Views: 2206*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=Fku_cRb6odc)

[![Asynchronous JavaScript 1 of 3: setTimeout, setInterval, clearInterval](https://i.ytimg.com/vi/Fku_cRb6odc/hqdefault.jpg)](https://www.youtube.com/watch?v=Fku_cRb6odc)

## Description

This is the first episode in a set of 3 where we'll talk about how JavaScript runs asynchronously. In this episode, we'll use setTimeout and setInterval to build some timers with some olympics inspired mini tasks.

## Transcript

hey what's up welcome back in this episode you'll learn a little bit more about hey welcome to this three-part series about asynchronous programming with javascript in this first episode you'll learn about set timeout and set interval these are two tools that the browser web apis give us for working with timers and we're gonna also talk about the javascript event loop and start talking about how javascript executes asynchronously in the next episode we'll talk about promises and in the final part we'll talk about async and a weight so right now we are in the thick of it with the limp olympics for the uh that are in tokyo it's actually 2021 olympics here um and what i wanted to do was go through the process of maybe building out some like a shot clock for basketball or maybe like a countdown timer for boxing we'll use these as we work on practicing our asynchronous javascript programming i'm going to assume you have a little bit of experience with javascript but not a ton and it's a really common thing to be confused by how javascript promises work and how async await works and i think it's really useful to first understand how the javascript is executing things asynchronously so what i wanted to do was create here an index.html file and in it what i wanted to do first was show how the browser might block execution if it's doing a ton of work on the single thread so javascript is asynchronous and single threaded and we're going to talk about how that works and how we can take advantage of that with our code and what we might run into if we were to write some code that would block the browser and block the execution of the browser so first let's take a look on the codist there is a an excellent blog post that has some really cool animated gifs here i would have probably created this call stack gif a little bit differently so at the very bottom we have this this function call to print square so print square is executing and then or is print square is added to the call stack and then as we execute print square we see this first line that has console.log with a square of n and this is where i actually would have made the gif a little bit differently because square of n is really the next function that's being called and we're going to wait to call console.log and tell the return of square of n so then we call square of n and that comes up here square of n then calls multiply and when multiply is called that's added to the stack and then multiply executes there's no real uh as multiply is on the top of the stack that's really the top of the stack is is multiply there's no function call for a times b so that will return to this point and then when multiply returns that'll return here and then console.log will be pushed onto the stack with that value and then console.log will execute and then console.log will return down here there's another uh there's another couple really interesting ones where we're talking about how callbacks are being added to a queue in this example they use settimeout and that's what we're going to talk about today is set timeout and so as your functions are executing if there is a need to make an io or an api call or some expensive call that's going to take a long time we generally will do that as part of part of like it's almost like a background task so we're going to we're going to enqueue some job to run in a background task so when we call set timeout we pass it a reference to a function that we want to call later and we pass it some amount of time and under the hood there's going to be some reference to a list of set timeout functions that need to be called and that delay and as soon as that delay expires there's some code that's written like inside of the engine for running uh running asynchronous code it's part of the web api and it's part of the browser it's part of v8 the engine that runs javascript and that will say like uh at the very lowest level it'll wait for a signal that says that delay has executed and finished and when this when it when the operating system sends that signal up that says okay 3 000 milliseconds is over then the function that is related to that timeout is enqueued into a callback queue and executed as soon as the the primary thread of execution is done it'll pick up that first task that's in the queue and it will start executing the function from that point forward until it's done like sort of running through a pass of the queue so this is how sort of the event loop works all right so we've got this html page what i wanted to do is start out with let's say that we have a button on the page and this button is going to say like go or something like that and then we'll have an input box of type text so this input box maybe we want to like click the button go and we're going to type in a name of something it's a little convoluted but i'll i'll show you what i'm talking about here then we're going to create a script where maybe inside of our script we have reference to that button we're going to say btn dot add event listener click and when the button is clicked we want to execute some function later so we're going to pass in a function so maybe our function is called we'll say function do something and inside of our do something call we're gonna we're gonna like make this make an expensive call meaning when we run this do something function it's gonna do a lot of work to demonstrate how the browser might block if you did not have an asynchronous function or the tools for asynchronous programming so we have this button we're going to add an event listener we tell it what type of event to listen for on that object so here we're talking about the button go i can use btn here because that's the id of the button so when the button go is clicked we're going to execute some function so here i can pass in do something so do something will run when we click on the button so we define this function but it's not actually executing here so then when we say let's make an expensive call i'm just going to create a for loop that does a bunch of work so here we'll say like maybe like var a is a function that takes in some number and it returns like n times n and then we'll say uh same thing for b and c and we'll change the signs of these or we'll make them different operations and then what we can do is if we can make a for loop where we say for var i is equal to zero um while i is less than one ten ten million ten billion something like this a hundred is that a hundred that's a 100 million a billion um i plus plus so we're going to do this we're going to execute this for loop a bill a billion times and what we want to do is say we'll call a and then uh we'll call that with the result of b we'll call b with the result of c and then inside of c we'll pass in 1000 so that this this for loop will execute this many times calling these three functions this should make it like an expensive api call so let's go see um and we can also actually yeah let's console.log like running and then we'll console.log done here at the bottom and i'm writing all of this code just in an inline script tag for an html page and then i'm using the http server npm module that i've installed globally you can go find this http server node module over on npm and if you globally install this then you can just run http dash server in the terminal and that'll set up a local web server that'll just host those files statically so that's what i'm using here to go to lo so now i'm going to go to localhost 8080 and i'll refresh the page and we're missing a closing semicolon here i think or closing parentheses uh button.add event listener is not a function add event listener did i spell that wrong okay i must have all right so now what's going to happen is when we click the button right when we click on the button it's going to call do something so i'm going to actually click on the gutter here inside of the sources tab i'm going to click on the gutter on line 14. when we click go it starts executing do something okay and then it's going to console.log that it's running and then it's going to console.log that it's done so let's see what that hap what happens here okay so it's running and it actually takes a minute and what you can see is that the debugger is actually sort of frozen here while it's running because the browser isn't able to do a bunch of stuff until it's done executing so there's some things that the browser knows how to do while it's executing but there's a lot that you cannot do okay so it's still running it's still running it's still running and done okay so that took a little longer than i wanted so i'm going to take one of these zeros out all right so let's refresh this page i'm not going to add a breakpoint but i'm going to click go all right that was too fast i'll put it back in all right so when we click on go it starts running but now notice that like oops okay let's let's add another zero back all right so when i click go i'm going to try to so right now i can type into the text box no problem but if i hit go while this is running if i try to type in the text box i can't even like click on it to add focus so i'm clicking on the text box nothing is happening i'm hitting tab nothing is happening because as the browser is executing in that single thread to do our really expensive work here we're not able to interact with the browser okay so anytime we need to do something expensive so for example if we wanted to make an api call out to some web service it takes uh anywhere between i don't know 50 milliseconds to one second in order to send an http request to a remote server and receive a response back it could even be longer if it's a slow running server or if the server needs to do a lot of data crunching on the back end or is trying to send you back a ton of data and so if we were to wait in the browser for a whole second while we were making api calls in the background to some service then it would be it would be uh really annoying because you wouldn't be able to interact with the browser while it was refreshing and loading all those things if you remember back in like the 90s or something when you would use a web page and all the images would sort of like sit there and load and kind of have to wait for uh for a page to load it was very annoying um but this is one of the reasons why we have asynchronous javascript and why javascript runs asynchronously now you might actually want to do something like this where you're you're running some really expensive math equation that you need to run how much is this this is like uh okay so i think it's 10 10 billion times so if you needed to run some set of three calls 10 billion times when you clicked a button then you might want to actually put this into a web worker or just something in the background that can actually run it but that's to demonstrate what happens when the browser's thread is blocking execution because it's in the middle of doing something really really expensive and that actually like sort of locks the browser while it's trying to figure out what it's doing all right so let's talk about set timeout so maybe we have like a p tag with the id of i don't know update or something and or let's call it scoreboard and again back to our olympics example here right let's say that we are in a boxing match three minute rounds and or maybe we'll make our rounds three seconds or something and so uh what we want to do is we want to update the scoreboard to say that we are now like fighting or boxing so we'll say scoreboard dot inner text is equal to fight okay and then after some time elapses we want the fight to be over and we want the fight to finish and so what we might do is use this tool called set timeout now set timeout takes a couple different arguments usually the most common way to call set timeout is to pass a function as the first argument and then a delay as the second argument so the delay in the case of a boxing match is going to be like the match duration or something like that so let's say that for now we're just going to pass in like uh 3 000 so this is the number of milliseconds that the delay should take and so three thousand millisecond delay we'll say that our match takes three seconds basically and then we want to execute some function after those three seconds are over and so what we can do is we can say like we can make a new function here called match over and inside of our function we could maybe update the scoreboard again and say stop or something so scoreboard dot inner text is equal to stop okay and then we'll pass that as our first argument to set timeout so we'll say match match over so we don't want to actually execute it here we're not calling the function we're passing it as an argument so at this point like as we're executing our code right so we are setting the inner text of the scoreboard to fight we're defining a function we're defining a function that's not going to run it we're just creating a new function and then we're passing reference to that function so i'm not calling the function here i'm passing reference to the function if i were to call the function then set timeout would say that the function is whatever the return value is from match over so i would maybe if i had like return one or something then set timeout would try to execute some it would try to execute this number and it would fail because one isn't actually a number but i don't want to pass the return value of match over i want to pass reference to match over and by passing reference to it i'm saying hey here is a function that i want you to schedule to execute in three seconds from now so in three seconds from now run this thing all right let's go take a look at what this looks like so i'm going to refresh the page and we'll see fight and then in three seconds it switches to stop okay so you can see that here so let's make this a little bigger so i'm going to refresh the page again we see fight it's we're waiting for those three seconds and while we're waiting uh we have there's like a a a q of all of the set timeouts or there's some sort of data structure internally that has reference to the amount of time and to the function that needs to be called and when that amount of time elapses then that function is put onto a a call stack queue i'm sorry into into a queue that will be picked up by the event loop so inside of javascript it's always looking for new events that are being ready to execute so it's waiting for you to click a button if you were to click on a button that's going to trigger an operating system signal that says like oh a button was clicked and that will fire a signal that says okay take the function that was passed as the click handler for that button and add that to the callback queue and then it will start processing all of those things in the callback queue in order and pulling those off and starting a new thread of execution so when you're when you first load the browser it's going to execute your javascript sort of top to top to bottom and that's like the first pass-through is like when you load the browser it's going to take a first pass through all your javascript then any time set timeout or click handlers or any of these other things are happening it starts to build up reference to all of these like chunks of work that need to happen and every time the event loop finishes running through all the code so for instance when we when we load the browser and it finishes execution of all of this code then it looks back at that event queue and says oh are there any other events that i need to run were there any set timeout callbacks that were that finished were there any click handlers that happened form submissions ajax responses that we have all the data for any of those things that are ready to go they'll all be pushed onto a queue and then processed in the queue so if our match over function here for instance called another function we could say like um i don't know you know match over like check winner or something and then if we had another function here that said like function check winner and we could say console.log the winner is mayweather right then as soon as the set timeout occurs we'll enqueue match over into the callback queue and then the event loop when it finishes its tick it will pick up that match over function it will execute it and then it's just going to keep executing any of the function calls that happen directly as the result of match over until it hits some async function maybe where it needs to schedule something so here when we get down to set timeout we are actually executing set timeout and the function the job for set timeout is to schedule match over to run later all right that's probably good for set timeout you can also like decide that you want to cancel the timeout so maybe you want to cancel the timer or something so maybe if you were making like a cooking clock right for uh when you're baking cookies and you wanted to set the timer for 15 minutes while it's in the oven and you look in the oven and the cookies are actually like ready to go but you want to cancel the timer then what you could say is like const i is equal to set timeout and then you can say i or like clear timeout and pass in i that's how you can sort of stop the timer before it finishes and executes this function so you could you could manually stop the timer and do whatever you want and let's make another part of the scoreboard here that we'll call timer very simply i'm going to console.log line one and then set timeout another way that you can write set timeout is instead of passing in reference to a function you can just anonymously define a function anonymous functions look like this where you have the word function and then parentheses and then you have the curly braces this is the same thing as passing reference to a function and here we'll again say like 3000 for three seconds and then inside of our anonymous function here we can say console.log line 2 like inside set timeout and then down here we can say console.log line 3. okay so pause the video and think to yourself what order these console.logs are going to spit out all right so let's go check it out let's go check it out and see what happens so if we hard refresh this page we see in the terminal line 1 line three and then three seconds later line two inside of the set timeout so line one went first then line three executed then line three executed that's because when we're calling console.log here that is not an asynchronous call that is a function call that's happening like as part of this thread's execution same thing with uh line three but when we call so we're calling line one then we're calling set timeout and settimeout is going to do its thing and then keep go and then like allow javascript to keep going so settimeout is saying schedule this function where we're gonna console log line two schedule that for three seconds from now and then it's done it returns and the set timeout is done executing and when it's done executing it moves to the very next line console.log for line three it executes line three and then our first pass through the javascript when this page was loaded is done so the event loop is empty the callback queue is empty everything is empty and all that we have is there's a data structure internally that has the list of all the timers and the related functions and when this timer expires this function is added to that callback queue the callback queue is checked every turn of the event loop the events loop sees oh there's a function i need to call because it expired that timer it calls that function and prints out line two so again we see line one line three then line two so that hopefully that helps also clarify sort of the order of operations here all right let's let's add a another tool to your toolkit called set interval so set timeout lets you say do this function after some delay set interval says do some function aft or like a do some function after some delay but keep doing it every single time that delay expires do the function again so here instead of doing set timeout where it's just kind of like a set interval that after it expires it runs the function one time and it's done in this case when we say set interval it's going to do something every three seconds it's going to do something new so in this case let's say that our scoreboard board dot inner text is like fight and we'll say that like after every three seconds um maybe there's like a new punch that's thrown or something so we'll say scoreboard inner text plus equals punch all right so what's going to happen here is every time three seconds elapses our scoreboard p tag paragraph tag is going to get a new word added to it so we're using plus equals here so we're taking whatever inner text was before and we're adding to it the word punch so let's take a look at what this looks like let's actually make it one second and we'll refresh the page so fight punch punch punch ah so many punches okay cool so um in this olympic battle i think we're winning right one punch per second pretty good rate uh it would be even funnier if we made it like okay what about every 200 milliseconds refrench boom boom boom boom boom boom boom all right so this is this is interesting whatever right okay so this is one way that you can use set interval but a more common use case for set interval is to use it as part of displaying some sort of timer or something so let's update timer with some countdown timer maybe this is going to be like a shot clock for basketball or something like that okay so we'll say like shoot and maybe we have a variable for like holding reference to what the current time is so we'll say like uh var or like let time left equal 10 seconds so we'll say that our shot clock timer ends after 10 seconds and here we'll say every 1000 milliseconds so every second we want to decrease the time that's left so we're gonna say you know time left minus equals one so that'll decrease our number of seconds that are left by one and every single time that the timer goes down we want to update that shot clock so let's say timer dot inner text is equal to time left okay so we should see a countdown timer now so it's saying shoot 10 9 8 7 six five four oh no are they gonna make it are they gonna shoot ah they shoot and our timer just keeps going okay so now it's like negative time right so negative seven seconds the the game has been over now for nine seconds the game has been over for ten seconds did they win who knows but we could check we could check uh check the olympic scoreboard here in a bit for basketball and it looks like uh i don't know maybe these are not going to actually happen until august 7th or 8th maybe that's like the the ceremony that tells us when when they won but we have a problem with our with our shot clock right it should end at zero seconds when the buzzer goes off and so we want to stop after a certain amount of time and so the way that we can do that is with clear interval so this in the same way that you can sort of stop a timer you can also stop an interval from running and so when we are executing this code right we're going to set the inner text to shoot creating a variable to hold the number of seconds that we have for time left and then we're calling set interval and set interval is going to return an integer value and that integer value is reference to uh where like which which function is running in that other data structure that we don't actually know about it's just like sort of like a a handle for the function that is going to like keep running or for the interval that's executing and so what we can do is we can say like const timer interval is equal to set interval so this gives us reference to the return value and again i'm going to console.log this timer interval at the bottom and the reason is that what i wanted to just like see what that integer is all right so after we've already like set the time left equal to zero we want to stop the timer so as this function is executing inside of the function that's executing we want to call clear interval clear interval and we want to pass in the timer interval okay but we only want to clear the interval if we're down to the last second so if we if we were to run the code right now right so if we if we execute this we we see that the timer interval is 1 10 was printed out but the the timer isn't going down because the first the very first time that we executed this this callback function we cleared the interval and we like stopped the timer so we only want to stop the timer after the time left is equal to zero or like is less than or equal to zero less than zero because uh yeah okay let's actually move time left down here and then we'll say if time left is equal to zero then clear the interval and then we're done okay so let's refresh the page we see 10 9 8 7 6 5 4 3 2 1 0 okay so now time left is equal to zero oh also in the in the javascript console you can hover over these variables and they'll tell you what their current value is so time left is zero the timer interval we logged out was one so we're passing in 1 to clear interval and this is going to clear or like stop that interval so that it doesn't keep executing we are going to decrease time left and it will ultimately be negative 1 but that doesn't matter because the last time we updated our view we set it to zero and it should not update again all right so now our shot clock seems to be working we have uh we say shoot and then it's counting down by one second every single time and ultimately it stops and we are done so let's run it through one more time without clicking on clear interval and then we'll see kind of what happens here one i actually don't know how long the shot clock is in basketball for the olympics or whatever but uh and then the other thing we could do is maybe we add like an alert that says like game over right so this is another tool that we could use game over is actually oh we put it we put this uh so that it executed um every time so alert is a function that blocks the browser's execution so when an alert is open almost you should almost never use alert but for demonstration purposes it's nice okay so what's interesting here is that the alert is also blocking execution and you'll notice that the browser still says one that's because we haven't actually finished executing the javascript and part of the browser's job is after we do all the javascript then it's going to make the updates to the dom so even though we're setting inner text here we've already executed this line of javascript that sets the inner text of that javascript or of that p tag to be zero when we use alert the alert blocks execution of the browser and so when when this alert is open until we click ok it's sort of frozen at this point and there's nothing else we could do like we wouldn't be able to like interact with an input box or do anything else while this alert is open because that alert is sort of holding holding reference to that sort of that thread of execution it's not letting us do anything until we finish and one of the things that the browser is going to do after we finish all the javascript is actually update the browser so at this point it can't update the browser until we click ok so when we click ok our current existing thread of execution which was caused by the the latest turn of the event loop where it was running that set interval because it timed out for that one second as soon as we click ok it's going to finish that thread of execution because we're clearing the interval it's not going to run it again and when it's done it will also update the browser so let's click ok and now we see 0 there correctly we're not yeah it's not continuing to count down because we cleared the interval so hopefully that helps and makes sense another really common use case for set interval is games when you have a game that has some sort of game loop like maybe you have a scroller or you you're yeah you're building something like flappy birds or something where you need to sort of or asteroids where you're flashing different frames of the game or different every time the uh you have like a different tick of the game you want to like check how many bullets you have or shoot a bullet or move a bullet along some axes or something like that set interval is one of the most common ways that you're going to run like that game loop so you'll say you know every 50 milliseconds do a tick of the game loop so your something is moving along some axes or something or you know scrolling sideways and the way that you do that is like every single time the game ticks you just move some object just a tiny bit and when you move it a tiny bit uh and continue doing that every maybe like 10 milliseconds the game will start to feel more fluid even though it's just flashing so that's that's kind of interesting i think we're going to leave it at this in the next episode we're going to cover promises so promises are pretty interesting and they're one of the most confusing things about javascript when you're getting started but we're going to dive into it in the same depth and hopefully we'll clear it up for you thanks so much for watching and we'll see in the next one

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