你好呀，掘金的各位小伙伴们！👋

今天我们要来聊一个老生常谈，但每次提起都能让人“掉几根头发”的话题 —— Event Loop。

你是不是也曾在面试中被面试官那迷离的眼神注视着。问道：

或者在写代码时发现打印出来的日志顺序和你想的完全不一样，怀疑人生？🤔

别担心！今天这篇文章，我们就把这些“玄学”问题一次性讲清楚！话说回来，我们将不再只是死记硬背“宏任务”、“微任务”的定义。而是结合浏览器底层原理用最轻松愉快的语气，带你啃下这块最硬的骨头！🍖

准备好了吗？我们要发车啦，🚗💨

🧐 第一部分：浏览器的“打工”日常 —— 进程与线程

在深入 Event Loop 之前，我们得先了解一下 JS 代码运行的“环境” —— 浏览器。

大家常说“JS 是单线程的”，但这其实是说 JS 的主线程 是单线程的。现代浏览器其实是一个多进程的架构。你可以把浏览器想象成一个大型工厂 🏭。

浏览器的主要“部门”

这个工厂里有几个主要部门，它们各司其职：

• Browser 进程 🧠：工厂的“厂长”。负责界面显示、使用者交互、子进程管理等。
• GPU 进程 🎨：负责 3D 绘制等图形工作。
• Network 进程 📡：负责网络资源加载。
• Plugin 进程 🧩：负责插件运行。老实说，
• Renderer 进程 🏗️：主要来了！ 这是我们要关注的主角。每个 Tab 页通常都有自己独立的渲染进程。

渲染进程的“繁忙”生活

渲染进程的主要职责是把 HTML、CSS、JS 变成使用者看得到的网页。这个部门里有一个超级忙碌` 的员工，名叫 "渲染主线程" .

This 主线程有多忙呢？来看看它的 To-Do List：

1. 解析 HTML 📄：生成 DOM 树。
2. 计算样式 💅：建立 CSSOM 树。
3. 布局  📐：计算元素的位置和大小，生成 Layout Tree。
4. `分层`  🍰：处理图层。
`绘制`  🖌️ ：生成绘制指令。怎么说呢，`执行 JavaScript` ⚡ ：处理业务逻辑、交互等。

划主要:⚠️ 渲染和 JS 执行是互斥!说白了主线在执行 JS 的时候，就不能进行渲染；在渲染的时候，就不能执行 JS。它就像一个单核 CPU，同一时间只能做一件事。怎么说呢，

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Hello everyone on Juejin!👋

This article dives deep into a classic yet hair‑pulling topic — The Event Loop.

You’ve probably faced interviewers staring at you and asking:

You might also have seen console logs appear in an unexpected order,making you question reality 🤔.

No worries!In this post we’ll untangle all those “mystical” questions once and for all,moving beyond rote memorization of “macro‑tasks” and “micro‑tasks”. We’ll explore browser’s underlying mechanics with a light‑hearted tone so you can finally master this tough nut!🍖🚀

🧐 Part 1: The Browser’s “Day Job” – Processes & Threads

A solid grasp of environment where JavaScript runs – browser – is essential before tackling event loop.

The common claim that “JavaScript is single‑threaded” actually refers only to its main thread. Modern browsers like Chrome employ a multi‑process architecture that you can picture as a massive factory 🏭.

The core “departments” in this factory:

• The Browser process 🧠:  “factory manager”,handling UI rendering,user interactions,and child‑process management.
• The GPU process 🎨: responsible for 3D graphics and or GPU‑heavy work.
• The Network process 📡: handles network resource loading.
• The Plugin process 🧩: executes browser plugins.
• The Renderer process 🏗️: *Focus point*. Each tab usually has its own renderer process.

The hectic life of Rendering Process

The renderer’s main job is turning HTML,CSS。and JS into something users can see. Inside it works a super‑busy employee called  "Render Main Thread" .

This main thread juggles many tasks—think of its To‑Do list as follows:

1. PARSING HTML 📄: building DOM tree.
2. CALCULATING STYLES 💅: creating CSSOM tree.
3. LAYERING/LAYOUT 📐: computing element positions & sizes → Layout Tree.
4. LAYERING 🍰: handling graphic layers.
5. DRAWING 🖌️: producing paint instructions.
6. SCRIPT EXECUTION ⚡: running business logic & interactivity code....

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