Getting error after I put Async function in useEffect

I used IIFE Like this, and it worked!!-

Previously it was this -

useEffect(async ()=>{
    const awesome_value = await AsyncStorage.getItem('awesome')
    setAwesome(awesome_value)
},[]);

Now I changed it to this -

useEffect( ()=>{
   (async() => {const awesome_value = await AsyncStorage.getItem('awesome')
   setAwesome(awesome_value)} ) ();
},[]);

You're returning the result of calling getResponse() from the useEffect function. If you return anything from useEffect, it has to be a function. Changing your code to this should fix it because you're no longer returning anything from the useEffect function.

useEffect(() => { 
  getResponse();
});

---

The useEffect Cleanup Function

If you return anything from the useEffect hook function, it must be a cleanup function. This function will run when the component unmounts. This can be thought of as roughly equivalent to the componentWillUnmount lifecycle method in class components.

useEffect(() => { 
  doSomething();

  return () => {
    console.log("This will be logged on unmount");
  }
});

The reason the simple code above is crashing your app is due to how the useEffect hook, async functions, and the shorthand arrow function syntax work.

One feature of the useEffect hook is a cleanup function. If you return anything from the useEffect hook function, it must be a cleanup function. This function will run when the component unmounts. This can be thought of as roughly equivalent to the componentWillUnmount lifecycle method in class components.

In JavaScript, functions marked with the async keyword enable the use of the await feature, which lets developers pause the execution of a function while waiting for an asynchronous task to finish. Async functions also always return a Promise; if the function doesn’t return one already, the return value is automatically wrapped in a Promise.

Finally, the shorthand arrow function syntax allows developers to omit the curly braces around the function body, which is useful for simple one-liners. The value of the function body automatically becomes the return value of the arrow function, removing the need of the return keyword. This functionality is called Implicit Return.

Now, how do these tidbits come together to cause such a cryptic error? Simply put, the value of getResponse, which is a Promise, becomes the return value of the arrow function in the useEffect hook. Remember that the useEffect hook expects a cleanup function to be returned? A Promise is not a function. so React stumbles and produces the error.

To fix your app, change the useEffect arrow function to add curly braces and remove the Implicit Return, as shown:

useEffect(() => { 
  getResponse();
});

Now, the arrow function in the useEffect hook returns undefined, which is acceptable and tells React that there is no cleanup function needed. This will solve the problem, but it would be great if React gave a more useful error message when this occurs!

An async function is really just syntax sugar for promises, so when you call an async function, it's returning a promise.

Instead, you can wrap your async function with an IIFE (Immediately-invoked Function Expression) like this, so nothing is returned to useEffect and used as a cleanup function:

useEffect(() => { 
  (async () => getResponse())();
});

Edit: or as @json pointed out in comment below. Just:

useEffect(() => { getResponse() });