/conventions

description: PostHog coding conventions for frontend and backend development

PostHog Coding Conventions

When writing code, follow these PostHog coding conventions.

> **Source of truth**: These conventions are maintained at: > > - https://posthog.com/handbook/engineering/conventions/frontend-coding > - https://posthog.com/handbook/engineering/conventions/backend-coding > > If this file gets out of sync, update it from the source.

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Frontend Coding Conventions

Two layers: Kea -> React

Our frontend webapp is written with [Kea](https://keajs.org/) and [React](https://reactjs.org/) as two separate layers. Kea is used to organise the app's data for rendering (we call this the _data_ or _state_ layer), and React is used to render the computed state (this is the _view_ or _template_ layer).

We try to be very explicit about this separation, and avoid local React state wherever possible, with exceptions for the `lib/` folder. Having all our data in one layer makes for code that's easier to [test](https://keajs.org/docs/intro/testing), and observe. Basically, getting your [data layer](https://keajs.org/blog/data-first-frontend-revolution) right is hard enough. We aim to not make it harder by constraining your data to a DOM-style hierarchy.

Hence the explicit separation between the data and view layers.

General tips

• Think data first: get [your mental model of the data flowing through the app](https://acco.io/i-escaped-node) right, and then everything else will be simpler.
• Be practical, yet remember that you are balancing speed of delivery with ease of maintainability. If you have to choose: code should be easier to understand than it was to write.

Do-s & Don't-s

• General
• Write all new code with TypeScript and proper typing.
• Write your frontend data handling code first, and write it in a Kea `logic`.
• Don't use `useState` or `useEffect` to store local state. It's false convenience. Take the extra 3 minutes and change it to a `logic` early on in the development.
• Logics still have a tiny initialization cost. Hence this rule doesn't apply to library components in the `lib/` folder, which might be rendered hundreds of times on a page with different sets of data. Still feel free to write a logic for a complicated `lib/` component when needed.
• Use named exports (`export const DashboardMenu = () => <div />`), and avoid `default` exports.
• Naming things:
• Always look around the codebase for naming conventions, and follow the best practices of the environment (e.g. use `camelCase` variables in JS, `snake_case` in Python).
• Use clear, yet functional names (`searchResults` vs `data`).
• Logics are camelCase (`dashboardLogic`)
• React components are PascalCase (`DashboardMenu`).
• Props for both logics and components are PascalCase and end with `Props` (`DashboardLogicProps` & `DashboardMenuProps`)
• Name the `.ts` file according to its main export: `DashboardMenu.ts` or `DashboardMenu.tsx` or `dashboardLogic.ts` or `Dashboard.scss`. Pay attention to the case.
• Avoid `index.ts`, `styles.css`, and other generic names, even if this is the only file in a directory.
• Scenes
• Our app is built of _scenes_, managed through a scene router in `sceneLogic`.
• A scene is the smallest unit in the router and for code splitting. Usually we split scenes by resource type (dashboard, insight) and function (edit, index).
• Each scene (e.g. Dashboards) exports an object of type `SceneExport`, containing the scene's root `logic` and its React `component`.
• The scene's logic is automatically mounted and receives the scene's URL params as props (via `paramsToProps`).
• Use `urlToAction` and `actionToUrl` on the scene's logic to sync state with the URL. Try to only use them on the scene's logic, not in any deeper logics.
• Logics mounted by React components through the view layer unmount when the component unmounts. Use `useAttachedLogic(dataNodeLogic(propsFromComponent), mySceneLogic())` to attach a logic to the scene's logic so it persists until the scene's logic is unmounted, surviving React component remounts.
• You can control what's shown on the tab via the `breadcrumbs` selector in your scene's logic. The last breadcrumb controls the title and the icon, the one before that controls the back button. If there are more breadcrumbs, they will be ignored.
• Kea
• It's worth repeating: think of the data flow. Then work to simplify it. Derive as much state as possible via selectors, update the source via cascading actions, and avoid complex loops where a value triggers a subscription which calls an action which changes the value which triggers the subscription, ...
• Use `subscriptions` and `propsChanged` sparingly, only if you can't find any other way. These have a high chance of leading to messy, cyclic or slow data flows.
• Try to write your code such that you only use `urlToAction` in your scene's logic (e.g. `insightSceneLogic`), and never deeper down in e.g. `propertyFilterLogic`.
• Take the time and read through [the Kea docs](https://keajs.org/) until you can explain how all the various operations (actions, reducers, selectors, listeners, subscriptions, props, events, hooks, etc) work behind the scenes. It's worth knowing your tools.
• CSS
• We use Tailwind CSS wherever possible
• Where it's not possible
• We use regular SCSS files for styling to keep things simple and maintainable in the long run, as opposed to supporting the CSS-in-JS flavour of the month.
• Inside `MyBlogComponent.tsx` import `MyBlogComponent.scss`
• Namespace all your CSS rules under globally unique classes that match the component's name and case, for example `.DashboardMenu { put everything here }`
• We loosely follow BEM conventions. If an element can't be namespaced inside a container class (e.g. modals that break out of the containing DOM element), use BEM style names like `.DashboardMenu__modal` to keep things namespaced.

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