`useEffect` lets a component **subscribe** to something React does not own. The render stays *pure*, and ***both*** the setup and the cleanup belong in the effect. The old habit of ~~setting state during render~~ waits until after the browser paints. A cleanup is required when the effect opens a socket, a timer, or a subscription. The dependency array is the second argument. For a room that connects once, that array is empty: `[]`. The reference is [useEffect](https://react.dev/reference/react/useEffect), and the same page also appears as a bare address: https://react.dev/reference/react/useEffect ## What the effect is for An effect synchronizes the component with an external system. - open a connection - keep it in step with props - compare the room id - reconnect when the id changes - close it from the returned function React's order of work is fixed: 1. React renders the component. 2. The browser paints. 3. The effect runs. - [x] return a cleanup - [ ] leave the socket open after unmount > Effects are an escape hatch. If the value can be calculated during render, it does not belong in an effect. --- ## Timing | Hook | Runs | Use it for | | --- | --- | --- | | `useEffect` | after paint | subscriptions, fetching, logging | | `useLayoutEffect` | before paint | measuring the DOM | ### A connection ```jsx filename="Room.jsx" import { useEffect } from "react"; export function Room({ roomId }) { useEffect(() => { const connection = createConnection(roomId); connection.connect(); return () => connection.disconnect(); }, [roomId]); } ``` The same steps, without a language tag: ```text connect disconnect ``` ![One photograph of a room](/photos/image1.jpg "The only JPEG in the gallery")

## The dependency list If the effect reads `roomId`, then `roomId` belongs in the array. React runs the cleanup before the next effect, and once more on unmount.[^1] #### Markup inside the paragraph Water is H₂O, and an effect that runs on every change grows like x². [^1]: The cleanup from the previous render runs before the next effect, then again when the component unmounts. ## MDX that this page does not compile This file is `.mdx`. The CMS stores the body as Markdown, so an import, an expression, and a component stay as text: import { useEffect } from "react"; The doubled count is {count \* 2}. {/\* cleanup belongs next to setup \*/} ```mdx Do not start a subscription during render. ```

About