Just a few weeks until the 2021 JavaScript Full-Stack Bootcamp opens.
Signup to the waiting list!
- Introduction
- Blocking the event loop
- The call stack
- A simple event loop explanation
- Queuing function execution
- The Message Queue
- ES6 Job Queue
Introduction
The Event Loop is one of the most important aspects to understand about JavaScript.
I’ve programmed for years with JavaScript, yet I’ve never fully understood how things work under the hoods. It’s completely fine to not know this concept in detail, but as usual, it’s helpful to know how it works, and also you might just be a little curious at this point.
This post aims to explain the inner details of how JavaScript works with a single thread, and how it handles asynchronous functions.
Your JavaScript code runs single threaded. There is just one thing happening at a time.
This is a limitation that’s actually very helpful, as it simplifies a lot how you program without worrying about concurrency issues.
You just need to pay attention to how you write your code and avoid anything that could block the thread, like synchronous network calls or infinite loops.
In general, in most browsers there is an event loop for every browser tab, to make every process isolated and avoid a web page with infinite loops or heavy processing to block your entire browser.
The environment manages multiple concurrent event loops, to handle API calls for example. Web Workers run in their own event loop as well.
You mainly need to be concerned that your code will run on a single event loop, and write code with this thing in mind to avoid blocking it.
Blocking the event loop
Any JavaScript code that takes too long to return back control to the event loop will block the execution of any JavaScript code in the page, even block the UI thread, and the user cannot click around, scroll the page, and so on.
Almost all the I/O primitives in JavaScript are non-blocking. Network requests, Node.js filesystem operations, and so on. Being blocking is the exception, and this is why JavaScript is based so much on callbacks, and more recently on promises and async/await.
The call stack
The call stack is a LIFO queue (Last In, First Out).
The event loop continuously checks the call stack to see if there’s any function that needs to run.
While doing so, it adds any function call it finds to the call stack and executes each one in order.
You know the error stack trace you might be familiar with, in the debugger or in the browser console? The browser looks up the function names in the call stack to inform you which function originates the current call:
A simple event loop explanation
Let’s pick an example:
I use
foo
,bar
andbaz
as random names. Enter any kind of name to replace them
const bar = () => console.log('bar')
const baz = () => console.log('baz')
const foo = () => {
console.log('foo')
bar()
baz()
}
foo()
This code prints
foo
bar
baz
as expected.
When this code runs, first foo()
is called. Inside foo()
we first call bar()
, then we call baz()
.
At this point the call stack looks like this:
The event loop on every iteration looks if there’s something in the call stack, and executes it:
until the call stack is empty.
Queuing function execution
The above example looks normal, there’s nothing special about it: JavaScript finds things to execute, runs them in order.
Let’s see how to defer a function until the stack is clear.
The use case of setTimeout(() => {}), 0)
is to call a function, but execute it once every other function in the code has executed.
Take this example:
const bar = () => console.log('bar')
const baz = () => console.log('baz')
const foo = () => {
console.log('foo')
setTimeout(bar, 0)
baz()
}
foo()
This code prints, maybe surprisingly:
foo
baz
bar
When this code runs, first foo() is called. Inside foo() we first call setTimeout, passing bar
as an argument, and we instruct it to run immediately as fast as it can, passing 0 as the timer. Then we call baz().
At this point the call stack looks like this:
Here is the execution order for all the functions in our program:
Why is this happening?
The Message Queue
When setTimeout() is called, the Browser or Node.js start the timer. Once the timer expires, in this case immediately as we put 0 as the timeout, the callback function is put in the Message Queue.
The Message Queue is also where user-initiated events like click or keyboard events, or fetch responses are queued before your code has the opportunity to react to them. Or also DOM events like onLoad
.
The loop gives priority to the call stack, and it first processes everything it finds in the call stack, and once there’s nothing in there, it goes to pick up things in the message queue.
We don’t have to wait for functions like setTimeout
, fetch or other things to do their own work, because they are provided by the browser, and they live on their own threads. For example, if you set the setTimeout
timeout to 2 seconds, you don’t have to wait 2 seconds - the wait happens elsewhere.
ES6 Job Queue
ECMAScript 2015 introduced the concept of the Job Queue, which is used by Promises (also introduced in ES6/ES2015). It’s a way to execute the result of an async function as soon as possible, rather than being put at the end of the call stack.
Promises that resolve before the current function ends will be executed right after the current function.
I find nice the analogy of a rollercoaster ride at an amusement park: the message queue puts you at the back of the queue, behind all the other people, where you will have to wait for your turn, while the job queue is the fastpass ticket that lets you take another ride right after you finished the previous one.
Example:
const bar = () => console.log('bar')
const baz = () => console.log('baz')
const foo = () => {
console.log('foo')
setTimeout(bar, 0)
new Promise((resolve, reject) =>
resolve('should be right after baz, before bar')
).then(resolve => console.log(resolve))
baz()
}
foo()
This prints
foo
baz
should be right after baz, before bar
bar
That’s a big difference between Promises (and Async/await, which is built on promises) and plain old asynchronous functions through setTimeout()
or other platform APIs.
Download my free JavaScript Beginner's Handbook
The 2021 JavaScript Full-Stack Bootcamp will start at the end of March 2021. Don't miss this opportunity, signup to the waiting list!
More js tutorials:
- Things to avoid in JavaScript (the bad parts)
- Deferreds and Promises in JavaScript (+ Ember.js example)
- How to upload files to the server using JavaScript
- JavaScript Coding Style
- An introduction to JavaScript Arrays
- Introduction to the JavaScript Programming Language
- The Complete ECMAScript 2015-2019 Guide
- Understanding JavaScript Promises
- The Lexical Structure of JavaScript
- JavaScript Types
- JavaScript Variables
- A list of sample Web App Ideas
- An introduction to Functional Programming with JavaScript
- Modern Asynchronous JavaScript with Async and Await
- JavaScript Loops and Scope
- The Map JavaScript Data Structure
- The Set JavaScript Data Structure
- A guide to JavaScript Template Literals
- Roadmap to Learn JavaScript
- JavaScript Expressions
- Discover JavaScript Timers
- JavaScript Events Explained
- JavaScript Loops
- Write JavaScript loops using map, filter, reduce and find
- The JavaScript Event Loop
- JavaScript Functions
- The JavaScript Glossary
- JavaScript Closures explained
- A tutorial to JavaScript Arrow Functions
- A guide to JavaScript Regular Expressions
- How to check if a string contains a substring in JavaScript
- How to remove an item from an Array in JavaScript
- How to deep clone a JavaScript object
- Introduction to Unicode and UTF-8
- Unicode in JavaScript
- How to uppercase the first letter of a string in JavaScript
- How to format a number as a currency value in JavaScript
- How to convert a string to a number in JavaScript
- this in JavaScript
- How to get the current timestamp in JavaScript
- JavaScript Strict Mode
- JavaScript Immediately-invoked Function Expressions (IIFE)
- How to redirect to another web page using JavaScript
- How to remove a property from a JavaScript object
- How to append an item to an array in JavaScript
- How to check if a JavaScript object property is undefined
- Introduction to ES Modules
- Introduction to CommonJS
- JavaScript Asynchronous Programming and Callbacks
- How to replace all occurrences of a string in JavaScript
- A quick reference guide to Modern JavaScript Syntax
- How to trim the leading zero in a number in JavaScript
- How to inspect a JavaScript object
- The definitive guide to JavaScript Dates
- A Moment.js tutorial
- Semicolons in JavaScript
- The JavaScript Arithmetic operators
- The JavaScript Math object
- Generate random and unique strings in JavaScript
- How to make your JavaScript functions sleep
- JavaScript Prototypal Inheritance
- JavaScript Exceptions
- How to use JavaScript Classes
- The JavaScript Cookbook
- Quotes in JavaScript
- How to validate an email address in JavaScript
- How to get the unique properties of a set of objects in a JavaScript array
- How to check if a string starts with another in JavaScript
- How to create a multiline string in JavaScript
- The ES6 Guide
- How to get the current URL in JavaScript
- The ES2016 Guide
- How to initialize a new array with values in JavaScript
- The ES2017 Guide
- The ES2018 Guide
- How to use Async and Await with Array.prototype.map()
- Async vs sync code
- How to generate a random number between two numbers in JavaScript
- HTML Canvas API Tutorial
- How to get the index of an iteration in a for-of loop in JavaScript
- What is a Single Page Application?
- An introduction to WebAssembly
- Introduction to JSON
- The JSONP Guide
- Should you use or learn jQuery in 2020?
- How to hide a DOM element using plain JavaScript
- How to merge two objects in JavaScript
- How to empty a JavaScript array
- How to encode a URL with JavaScript
- How to set default parameter values in JavaScript
- How to sort an array of objects by a property value in JavaScript
- How to count the number of properties in a JavaScript object
- call() and apply() in JavaScript
- Introduction to PeerJS, the WebRTC library
- Work with objects and arrays using Rest and Spread
- Destructuring Objects and Arrays in JavaScript
- The definitive guide to debugging JavaScript
- The TypeScript Guide
- Dynamically select a method of an object in JavaScript
- Passing undefined to JavaScript Immediately-invoked Function Expressions
- Loosely typed vs strongly typed languages
- How to style DOM elements using JavaScript
- Casting in JavaScript
- JavaScript Generators Tutorial
- The node_modules folder size is not a problem. It's a privilege
- How to solve the unexpected identifier error when importing modules in JavaScript
- How to list all methods of an object in JavaScript
- The String replace() method
- The String search() method
- How I run little JavaScript snippets
- The ES2019 Guide
- The String charAt() method
- The String charCodeAt() method
- The String codePointAt() method
- The String concat() method
- The String endsWith() method
- The String includes() method
- The String indexOf() method
- The String lastIndexOf() method
- The String localeCompare() method
- The String match() method
- The String normalize() method
- The String padEnd() method
- The String padStart() method
- The String repeat() method
- The String slice() method
- The String split() method
- The String startsWith() method
- The String substring() method
- The String toLocaleLowerCase() method
- The String toLocaleUpperCase() method
- The String toLowerCase() method
- The String toString() method
- The String toUpperCase() method
- The String trim() method
- The String trimEnd() method
- The String trimStart() method
- Memoization in JavaScript
- The String valueOf() method
- JavaScript Reference: String
- The Number isInteger() method
- The Number isNaN() method
- The Number isSafeInteger() method
- The Number parseFloat() method
- The Number parseInt() method
- The Number toString() method
- The Number valueOf() method
- The Number toPrecision() method
- The Number toExponential() method
- The Number toLocaleString() method
- The Number toFixed() method
- The Number isFinite() method
- JavaScript Reference: Number
- JavaScript Property Descriptors
- The Object assign() method
- The Object create() method
- The Object defineProperties() method
- The Object defineProperty() method
- The Object entries() method
- The Object freeze() method
- The Object getOwnPropertyDescriptor() method
- The Object getOwnPropertyDescriptors() method
- The Object getOwnPropertyNames() method
- The Object getOwnPropertySymbols() method
- The Object getPrototypeOf() method
- The Object is() method
- The Object isExtensible() method
- The Object isFrozen() method
- The Object isSealed() method
- The Object keys() method
- The Object preventExtensions() method
- The Object seal() method
- The Object setPrototypeOf() method
- The Object values() method
- The Object hasOwnProperty() method
- The Object isPrototypeOf() method
- The Object propertyIsEnumerable() method
- The Object toLocaleString() method
- The Object toString() method
- The Object valueOf() method
- JavaScript Reference: Object
- JavaScript Assignment Operator
- JavaScript Internationalization
- JavaScript typeof Operator
- JavaScript new Operator
- JavaScript Comparison Operators
- JavaScript Operators Precedence Rules
- JavaScript instanceof Operator
- JavaScript Statements
- JavaScript Scope
- JavaScript Type Conversions (casting)
- JavaScript Equality Operators
- The JavaScript if/else conditional
- The JavaScript Switch Conditional
- The JavaScript delete Operator
- JavaScript Function Parameters
- The JavaScript Spread Operator
- JavaScript Return Values
- JavaScript Logical Operators
- JavaScript Ternary Operator
- JavaScript Recursion
- JavaScript Object Properties
- JavaScript Error Objects
- The JavaScript Global Object
- The JavaScript filter() Function
- The JavaScript map() Function
- The JavaScript reduce() Function
- The JavaScript `in` operator
- JavaScript Operators
- How to get the value of a CSS property in JavaScript
- How to add an event listener to multiple elements in JavaScript
- JavaScript Private Class Fields
- How to sort an array by date value in JavaScript
- JavaScript Public Class Fields
- JavaScript Symbols
- How to use the JavaScript bcrypt library
- How to rename fields when using object destructuring
- How to check types in JavaScript without using TypeScript
- How to check if a JavaScript array contains a specific value
- What does the double negation operator !! do in JavaScript?
- Which equal operator should be used in JavaScript comparisons? == vs ===
- Is JavaScript still worth learning?
- How to return the result of an asynchronous function in JavaScript
- How to check if an object is empty in JavaScript
- How to break out of a for loop in JavaScript
- How to add item to an array at a specific index in JavaScript
- Why you should not modify a JavaScript object prototype
- What's the difference between using let and var in JavaScript?
- Links used to activate JavaScript functions
- How to join two strings in JavaScript
- How to join two arrays in JavaScript
- How to check if a JavaScript value is an array?
- How to get last element of an array in JavaScript?
- How to send urlencoded data using Axios
- How to get tomorrow's date using JavaScript
- How to get yesterday's date using JavaScript
- How to get the month name from a JavaScript date
- How to check if two dates are the same day in JavaScript
- How to check if a date refers to a day in the past in JavaScript
- JavaScript labeled statements
- How to wait for 2 or more promises to resolve in JavaScript
- How to get the days between 2 dates in JavaScript
- How to upload a file using Fetch
- How to format a date in JavaScript
- How to iterate over object properties in JavaScript
- How to calculate the number of days between 2 dates in JavaScript
- How to use top-level await in ES Modules
- JavaScript Dynamic Imports
- JavaScript Optional Chaining
- How to replace white space inside a string in JavaScript
- JavaScript Nullish Coalescing
- How to flatten an array in JavaScript
- This decade in JavaScript
- How to send the authorization header using Axios
- List of keywords and reserved words in JavaScript
- How to convert an Array to a String in JavaScript
- How to remove all the node_modules folders content
- How to remove duplicates from a JavaScript array
- Let vs Const in JavaScript
- The same POST API call in various JavaScript libraries
- How to get the first n items in an array in JS
- How to divide an array in multiple equal parts in JS
- How to slow down a loop in JavaScript
- How to load an image in an HTML canvas
- How to cut a string into words in JavaScript
- How to divide an array in half in JavaScript
- How to write text into to an HTML canvas
- How to remove the last character of a string in JavaScript
- How to remove the first character of a string in JavaScript
- How to fix the TypeError: Cannot assign to read only property 'exports' of object '#<Object>' error
- How to create an exit intent popup
- How to check if an element is a descendant of another
- How to force credentials to every Axios request
- How to solve the "is not a function" error in JavaScript
- Gatsby, how to change the favicon
- Loading an external JS file using Gatsby
- How to detect dark mode using JavaScript
- Parcel, how to fix the `regeneratorRuntime is not defined` error
- How to detect if an Adblocker is being used with JavaScript
- Object destructuring with types in TypeScript
- The Deno Handbook: a concise introduction to Deno 🦕
- How to get the last segment of a path or URL using JavaScript
- How to shuffle elements in a JavaScript array
- How to check if a key exists in a JavaScript object
- Event bubbling and event capturing
- event.stopPropagation vs event.preventDefault() vs. return false in DOM events
- Primitive types vs objects in JavaScript
- How can you tell what type a value is, in JavaScript?
- How to return multiple values from a function in JavaScript
- Arrow functions vs regular functions in JavaScript
- In which ways can we access the value of a property of an object?
- What is the difference between null and undefined in JavaScript?
- What's the difference between a method and a function?
- What are the ways we can break out of a loop in JavaScript?
- The JavaScript for..of loop
- What is object destructuring in JavaScript?
- What is hoisting in JavaScript?
- How to change commas into dots with JavaScript
- The importance of timing when working with the DOM
- How to reverse a JavaScript array
- How to check if a value is a number in JavaScript
- How to accept unlimited parameters in a JavaScript function
- JavaScript Proxy Objects
- Event delegation in the browser using vanilla JavaScript
- The JavaScript super keyword
- Introduction to XState
- Are values passed by reference or by value in JavaScript?
- Custom events in JavaScript
- Custom errors in JavaScript
- Namespaces in JavaScript
- A curious usage of commas in JavaScript
- Chaining method calls in JavaScript
- How to handle promise rejections
- How to swap two array elements in JavaScript
- How I fixed a "cb.apply is not a function" error while using Gitbook
- How to add an item at the beginning of an array in JavaScript
- Gatsby, fix the "cannot find module gatsby-cli/lib/reporter" error
- How to get the index of an item in a JavaScript array
- How to test for an empty object in JavaScript
- How to destructure an object to existing variables in JavaScript
- The Array JavaScript Data Structure
- The Stack JavaScript Data Structure
- JavaScript Data Structures: Queue
- JavaScript Data Structures: Set
- JavaScript Data Structures: Dictionaries
- JavaScript Data Structures: Linked lists
- JavaScript, how to export a function
- JavaScript, how to export multiple functions
- JavaScript, how to exit a function
- JavaScript, how to find a character in a string
- JavaScript, how to filter an array
- JavaScript, how to extend a class
- JavaScript, how to find duplicates in an array
- JavaScript, how to replace an item of an array
- JavaScript Algorithms: Linear Search
- JavaScript Algorithms: Binary Search
- JavaScript Algorithms: Selection Sort
- JavaScript Algorithms: Quicksort
- JavaScript Algorithms: Merge Sort
- JavaScript Algorithms: Bubble Sort
- Wait for all promises to resolve in JavaScript