---
title: "Main thread - Glossary | MDN"
chunk: 1/3
source: "https://developer.mozilla.org/en-US/docs/Glossary/Main_thread"
category: "reference"
tags: "web, html, css, javascript, documentation"
date_saved: "2026-05-05T05:37:20.344673+00:00"
instance: "kb-cron"
---

  * [Skip to main content](https://developer.mozilla.org/en-US/docs/Glossary/Main_thread#content)
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[ MDN ](https://developer.mozilla.org/en-US/)
HTML
[HTML: Markup language](https://developer.mozilla.org/en-US/docs/Web/HTML) 

HTML reference
    
  * [Elements](https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements)
  * [Global attributes](https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Global_attributes)
  * [Attributes](https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Attributes)
  * [See all…](https://developer.mozilla.org/en-US/docs/Web/HTML/Reference "See all HTML references")

HTML guides
    
  * [Responsive images](https://developer.mozilla.org/en-US/docs/Web/HTML/Guides/Responsive_images)
  * [HTML cheatsheet](https://developer.mozilla.org/en-US/docs/Web/HTML/Guides/Cheatsheet)
  * [Date & time formats](https://developer.mozilla.org/en-US/docs/Web/HTML/Guides/Date_and_time_formats)
  * [See all…](https://developer.mozilla.org/en-US/docs/Web/HTML/Guides "See all HTML guides")

Markup languages
    
  * [SVG](https://developer.mozilla.org/en-US/docs/Web/SVG)
  * [MathML](https://developer.mozilla.org/en-US/docs/Web/MathML)
  * [XML](https://developer.mozilla.org/en-US/docs/Web/XML)

CSS
[CSS: Styling language](https://developer.mozilla.org/en-US/docs/Web/CSS) 

CSS reference
    
  * [Properties](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties)
  * [Selectors](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors)
  * [At-rules](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/At-rules)
  * [Values](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Values)
  * [See all…](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference "See all CSS references")

CSS guides
    
  * [Box model](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Box_model/Introduction)
  * [Animations](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Animations/Using)
  * [Flexbox](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Flexible_box_layout/Basic_concepts)
  * [Colors](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Colors/Applying_color)
  * [See all…](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides "See all CSS guides")

Layout cookbook
    
  * [Column layouts](https://developer.mozilla.org/en-US/docs/Web/CSS/How_to/Layout_cookbook/Column_layouts)
  * [Centering an element](https://developer.mozilla.org/en-US/docs/Web/CSS/How_to/Layout_cookbook/Center_an_element)
  * [Card component](https://developer.mozilla.org/en-US/docs/Web/CSS/How_to/Layout_cookbook/Card)
  * [See all…](https://developer.mozilla.org/en-US/docs/Web/CSS/How_to/Layout_cookbook)

JavaScriptJS
[JavaScript: Scripting language](https://developer.mozilla.org/en-US/docs/Web/JavaScript) 

JS reference
    
  * [Standard built-in objects](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects)
  * [Expressions & operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators)
  * [Statements & declarations](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements)
  * [Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions)
  * [See all…](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference "See all JavaScript references")

JS guides
    
  * [Control flow & error handing](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Control_flow_and_error_handling)
  * [Loops and iteration](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Loops_and_iteration)
  * [Working with objects](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Working_with_objects)
  * [Using classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_classes)
  * [See all…](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide "See all JavaScript guides")

Web APIs
[Web APIs: Programming interfaces](https://developer.mozilla.org/en-US/docs/Web/API) 

Web API reference
    
  * [File system API](https://developer.mozilla.org/en-US/docs/Web/API/File_System_API)
  * [Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API)
  * [Geolocation API](https://developer.mozilla.org/en-US/docs/Web/API/Geolocation_API)
  * [HTML DOM API](https://developer.mozilla.org/en-US/docs/Web/API/HTML_DOM_API)
  * [Push API](https://developer.mozilla.org/en-US/docs/Web/API/Push_API)
  * [Service worker API](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API)
  * [See all…](https://developer.mozilla.org/en-US/docs/Web/API "See all Web API guides")

Web API guides
    
  * [Using the Web animation API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Animations_API/Using_the_Web_Animations_API)
  * [Using the Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch)
  * [Working with the History API](https://developer.mozilla.org/en-US/docs/Web/API/History_API/Working_with_the_History_API)
  * [Using the Web speech API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Speech_API/Using_the_Web_Speech_API)
  * [Using web workers](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers)

All
[All web technology](https://developer.mozilla.org/en-US/docs/Web) 

Technologies
    
  * [Accessibility](https://developer.mozilla.org/en-US/docs/Web/Accessibility)
  * [HTTP](https://developer.mozilla.org/en-US/docs/Web/HTTP)
  * [URI](https://developer.mozilla.org/en-US/docs/Web/URI)
  * [Web extensions](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions)
  * [WebAssembly](https://developer.mozilla.org/en-US/docs/WebAssembly)
  * [WebDriver](https://developer.mozilla.org/en-US/docs/Web/WebDriver)
  * [See all…](https://developer.mozilla.org/en-US/docs/Web "See all web technology references")

Topics
    
  * [Media](https://developer.mozilla.org/en-US/docs/Web/Media)
  * [Performance](https://developer.mozilla.org/en-US/docs/Web/Performance)
  * [Privacy](https://developer.mozilla.org/en-US/docs/Web/Privacy)
  * [Security](https://developer.mozilla.org/en-US/docs/Web/Security)
  * [Progressive web apps](https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps)

Learn
[Learn web development](https://developer.mozilla.org/en-US/docs/Learn_web_development) 

Frontend developer course
    
  * [Getting started modules](https://developer.mozilla.org/en-US/docs/Learn_web_development/Getting_started)
  * [Core modules](https://developer.mozilla.org/en-US/docs/Learn_web_development/Core)
  * [MDN Curriculum](https://developer.mozilla.org/en-US/curriculum/)
  * [Check out the video course from Scrimba, our partner](https://scrimba.com/frontend-path-c0j?via=mdn-learn-navbar)

Learn HTML
    
  * [Structuring content with HTML module](https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Structuring_content)

Learn CSS
    
  * [CSS styling basics module](https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Styling_basics)
  * [CSS layout module](https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/CSS_layout)

Learn JavaScript
    
  * [Dynamic scripting with JavaScript module](https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting)

Tools
Discover our tools 
  * [Playground](https://developer.mozilla.org/en-US/play)
  * [HTTP Observatory](https://developer.mozilla.org/en-US/observatory)

  * [Border-image generator](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Backgrounds_and_borders/Border-image_generator)
  * [Border-radius generator](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Backgrounds_and_borders/Border-radius_generator)
  * [Box-shadow generator](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Backgrounds_and_borders/Box-shadow_generator)
  * [Color format converter](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Colors/Color_format_converter)
  * [Color mixer](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Colors/Color_mixer)
  * [Shape generator](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Shapes/Shape_generator)

About
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[Blog](https://developer.mozilla.org/en-US/blog/)
  1. [Glossary](https://developer.mozilla.org/en-US/docs/Glossary)
  2. [Main thread](https://developer.mozilla.org/en-US/docs/Glossary/Main_thread)

# Main thread
The **main thread** is where a browser processes user events and paints. By default, the browser uses a single thread to run all the JavaScript in your page, as well as to perform layout, reflows, and garbage collection. This means that long-running JavaScript functions can block the thread, leading to an unresponsive page and a bad user experience.
Unless intentionally using a [web worker](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers), such as a [service worker](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API/Using_Service_Workers), JavaScript runs on the main thread, so it's easy for a script to cause delays in event processing or painting. The less work required of the main thread, the more that thread can respond to user events, paint, and generally be responsive to the user.
## In this article
  * [See also](https://developer.mozilla.org/en-US/docs/Glossary/Main_thread#see_also)

##  [See also](https://developer.mozilla.org/en-US/docs/Glossary/Main_thread#see_also)
  * [Asynchronous JavaScript](https://developer.mozilla.org/en-US/docs/Learn_web_development/Extensions/Async_JS)
  * [Web worker API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API)
  * [Service worker API](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API)
  * Related glossary terms: 
    * [Thread](https://developer.mozilla.org/en-US/docs/Glossary/Thread)