Common React Render Bug

Who this note is not for

In my experience, If you’re working in a greenfield environment, modern tooling, with an end-to-end type safe stack, this won’t apply to you too much.

It’s likely a thing you won’t have to think much about, your editor will scream at you for this sort of thing.

Looping through Data from API

It’s very common to map over values coming in from an api to render stuff.

  • you can’t guarantee 100% return of ‘clean’ data from an API - unless you write the backend yourself.
  • and it’s an asynchronous action, which will trigger re-renders - weird things can happen during these
    • clean data refers to data that’s shaped exactly how you expect
  • Sometimes data changes hands a few times, before you’re using it on your frontend. from component to component, different variables in the store, calculations etc. wider surface area for things to go wrong.

Ways you can help yourself

  • Good types - [] | undefined - this on its own is not enough in some cases, I experimented with it. It’s best paired with the lint rule linked below
  • typescript eslint no unsafe call rule

These are just safeguards, for extra resilience, I think it’s best to assume that any array you might interact with will definitely be undefined at some point in its lifecycle.

What this looks like, is writing defensive code, allowing for safe & gracious failure, as opposed to the white screen of death (crashed app!)

const data: number[] | undefined = [4, 2, 1]

data?.map((item) => console.log(item))

  • That data type is sometimes inherited from API response type definitions, remember to be honest with your types & declare *** | undefined
  • A final thought, tweet by Cory. “Types as anti-corruption layer” as Dan North put it.