useEffect Pet Peeve: Side Effects, Not Initialization

React’s useEffect API is a fantastically useful hook that allows for “side effect” logic to be run in function components. useEffect has many uses ranging from asynchronous data fetching to interacting with browser APIs.

However! Overusing useEffect can cause crashes or confusing behavior in your components. In particular I’ve developed a pet peeve around code unnecessarily wrapping synchronous variable initialization code in a useEffect hook. Allow me to explain why…

Bug: Initially Missing Data

Take a look at this WaitingQueue component that renders a queue of names. It stores a list of names in a queue with a button to pop to the next one if there are any left.

import { useEffect, useState } from "react";

function getNames() {
    return ["Abed", "Annie", "Britta", "Jeff", "Pierce", "Shirley", "Troy"];
}

function WaitingQueue() {
    const [remaining, setRemaining] = useState(); // 😬

    useEffect(() => {
        setRemaining(getNames());
    }, []);

    if (!remaining.length) {
        return <div>Nobody left!</div>;
    }

    const next = () => {
        setRemaining((previous) => previous.slice(1));
    };

    return (
        <button onClick={next} role="button">
            {remaining[0]} is first.
        </button>
    );
}

WaitingQueue’s state management works by:

1. Initially starting remaining off with no value: undefined
2. Running a useEffect to setRemaining to the result of getNames()

This version of the WaitingQueue component will crash on first render! Because the remaining piece of state starts off as undefined, asking for remaining.length throws an error.

// TypeError: cannot read property 'length' of undefined
if (!remaining.length) {
    return <div>Nobody left!</div>;
}

useEffect does not run functions immediately. Its functions are run after render.

Creating a piece of state without an initial value and only giving it a value in a useEffect hook means it that it will be undefined for at least one render during the function.

Fun fact: this bug would have been caught by TypeScript! TypeScript would have seen that remaining is type undefined and complained about retrieving property length on an undefined value. [Playground link]

Bug: Initially Wrong Data

I’ve also seen a lot of gut reactions on how to fix the above crash. One common strategy is to provide some default stub value for the state to be used until the useEffect hook runs.

import { useEffect, useState } from "react";

function getNames() {
    return ["Abed", "Annie", "Britta", "Jeff", "Pierce", "Shirley", "Troy"];
}

function WaitingQueue() {
    const [remaining, setRemaining] = useState([]); // 🤔

    useEffect(() => {
        setRemaining(getNames());
    }, []);

    if (!remaining.length) {
        return <div>Nobody left!</div>;
    }

    const next = () => {
        setRemaining((previous) => previous.slice(1));
    };

    return (
        <button onClick={next} role="button">
            {remaining[0]} is first.
        </button>
    );
}

Since remaining is an [] empty array in the first render pass of the component, it will include a <div>Nobody left!</div> message instead of the message including Abed. We fixed the crash! 🎉

Any version of React running on the client will update the DOM extremely quickly after first render. If your app only runs on the client browser, you likely will never notice any flicker of the initial, wrong message on your screen.

Unfortunately for us, the first render of a React component can be quite important for a production webiste. Static site generation (SSG) and server-side rendering (SSR) are popular and important performance solutions for code. This first-render bug means the statically rendered version of the website will have the wrong first value. So this bug is still something you’ll probably want to fix.

<!-- view-source:https://localhost:3000 -->
<div>Nobody left!</div>

Solution: Initial Data

The bugfix is my favorite kind. It involves deleting code and simplifying logic.

We can remove the useEffect altogether and start remaining off equal to getNames().

import { useState } from "react";

function getNames() {
    return ["Abed", "Annie", "Britta", "Jeff", "Pierce", "Shirley", "Troy"];
}

function WaitingQueue() {
    const [remaining, setRemaining] = useState(getNames());

    if (!remaining.length) {
        return <div>Nobody left!</div>;
    }

    const next = () => {
        setRemaining((previous) => previous.slice(1));
    };

    return (
        <button onClick={next} role="button">
            {remaining[0]} is first.
        </button>
    );
}

<!-- view-source:https://localhost:3000 -->
<div>Abed is first.</div>

Running a useEffect to update the remaining state from an initial value to an updated one was unnecessary.

Cool. Cool cool cool.

Moral of the Story

Don’t run useEffect updates you don’t need.

If a piece of state is meant to start off pointing to data you always have access to, you can use that data as the initial value for the state.

Instead of this:

const [data, setData] = useState();

useEffect(() => {
    setData(initialValue);
}, []);

…most of the time, probably do this:

const [data, setData] = useState(initialValue);

What About Other Data Behaviors?

Everything I’ve said so far about how you shouldn’t use useEffect to initialize a synchronous value indeed only applies to synchronous values initialized once.

If the data is asynchronously loaded then sure, you’re not going to be able to start off with it available synchronously. You might want to use a useEffect that initializes the call to load the data:

function MyComponent({ value }) {
    const [data, setData] = useState();

    useEffect(() => {
        loadData().then(setData).catch(setData);
    }, []);

    // ...
}

Even if the data is loaded synchronously and immediately available, you may still wish to add a useEffect to reset the value. For example, if your component is meant to reset the display whenever a prop changes, you can both pass the value as an initial state and reset it on change:

function MyComponent({ value }) {
    const [data, setData] = useState(value);

    useEffect(() => {
        setData(value);
    }, [value]);

    // ...
}

Updated August 21st, 2022: @TkDodo on Twitter mentions that this could often be better solved with a key. ✨

Understand Your Effects

Built-in React hook APIs such as useEffect are intentionally flexible and can be used in a variety of ways. Understanding how they work can greatly help you avoid common pitfalls and bugs when using them.

I hope this post was useful to you in at least showing one of the ways useEffect can blow up. Let me know on Twitter!