async useEffect ⚛️ React

Warning: useEffect must not return anything besides a function, which is used for clean-up. It looks like you wrote useEffect(async () => ...)

An async function was passed directly to useEffect, so it returns a Promise instead of a cleanup function.

Seen on: React

Meaning

Effects must return nothing or a cleanup function. Define the async function inside the effect and call it.

Common causes

  • useEffect(async () => { … })
  • Returning a value from the effect

⚡ Quick fix

  1. Declare an inner async function and call it
  2. Handle cancellation with an ignore flag or AbortController

Detailed fix by platform

React

  1. jsx
    useEffect(() => {
  let ignore = false;
  (async () => { const d = await load(); if (!ignore) setData(d); })();
  return () => { ignore = true; };
}, []);

How to diagnose

  1. Effect — Is the callback async?
  2. Recent change — Did it start after a deploy, upgrade, or configuration change? Compare with the last working version.

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