How to fix the "cannot update a component while rendering a different component" error in React

A quick guide to how I solved a confusing React error

While working on a React / Next.js application I got this error:

Cannot update a component (`App`) while rendering a different component

I researched a bit how to solve this problem, but there was a lot of confusion in the material I found.

Here is what I was doing: I had a centralized state managed in the App component:

function MyApp({ Component, pageProps }) {
  const [lessonsRead, setLessonsRead] = useState()

  return (

and in a Next.js page component I called setLessonsRead to populate this state with data, based on the result of a SWR (fetch) call:

if (courseData && courseData.lessonsRead) {

I was doing this right inside the component.

To solve this problem I had to wrap this code in useEffect, to only run it when the data changed and not on every component props update:

useEffect(() => {
  if (courseData && courseData.lessonsRead) {
}, [courseData])

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