Prefer Hyperscript (using the @macrostrat/hyper package) over JSX.
Prefer functional components over class components.
Prefer named exports over default exports (default exports make it more difficult to track names through the codebase).
Prefer relative imports (e.g., import { fetchHelper } from '../helpers') over absolute imports (e.g., import { fetchHelper } from '~/pages/page1/helpers'). Minimizing the number of up-directory imports or imports from other parts of the codebase is a good way to keep it maintainable.
Extract logic into hooks or utility functions to keep components straightforward.
Generally, try to make sure folders have an index.ts file that exports all the files that might be used from the outside.
Put the most important function in the file first, and the least important (ex., utility functions) last.
Prefer function foo() {} over const foo = () => {} for named functions.

Code format

Use Prettier to format your code. You can run yarn format to format the entire codebase, or set up your editor to format on save.
Sort and prune imports periodically to keep them organized.

Styling and interface design

Prefer CSS modules where possible, but don't be afraid to use semantically named global classes for straightforward cases.
Use sentence case for headings, button text, and other UI elements

This codebase has been modernized and kept maintainable by avoiding (or belatedly correcting) many anti-patterns:

Hard-coding specific colors
Creating URLs by concatenating strings
Deeply nested code blocks (often with confusing control flow or side effects instead of unambiguous returns)
Duplicated code of more than a few lines