Introduction

Certain components in React need to interact with things outside themselves. These things can be anything from querying data from a server to finding/changing the position of the component on the webpage or even sending some data to a server when necessary. This interaction with the outside world is called a side-effect.

While we are already familiar with rendering code and adding event handlers, it is not enough for all uses, like when you want to connect to your server and fetch messages to show to a user. Effects let you run some code to synchronize your component as necessary, on rendering or a reactive/state value change rather than on a particular event.

Similar to how we have the useState hook, React offers us a handy useEffect hook to use effects in our components.

Lesson overview

This section contains a general overview of topics that you will learn in this lesson.

• What are effects?
• How are effects used in React?
• What are the different parts of a useEffect hook?
• When should I use an effect?

Using effect saves the day

The useEffect hook

Let us take a component in question. We want to make a Clock component that shows how many seconds have passed since the user has loaded the webpage. To update it every second, we can use our nifty setInterval function to add one to the counter state variable, every second. Let’s try putting it in the body of our component.

import React, { useState } from "react";

export default function Clock() {
  const [counter, setCounter] = useState(0);

  setInterval(() => {
    setCounter(count => count + 1)
  }, 1000);

  return (
    <p>{counter} seconds have passed.</p>
  );
}

Alas, we see our counter going berserk. This happens because the setInterval function is being called not once, but at every state render.

When our component first renders, it calls our initial setInterval function. That interval updates the state every second, triggering the component to re-render. But every re-render calls setInterval again, which triggers more frequent state updates, which each spawn new intervals, and everything quickly spirals out of control.

This is where the useEffect hook swoops in to save us. We can wrap this calculation inside a useEffect hook to move it outside the rendering calculation. It accepts a callback function with all the calculations.

import React, { useEffect, useState } from "react";

export default function Clock() {
  const [counter, setCounter] = useState(0);

  useEffect(() => {
    setInterval(() => {
      setCounter(count => count + 1)
    }, 1000);
  })

  return (
    <p>{counter} seconds have passed.</p>
  );
}

But, it still keeps growing too fast! This is where another argument of the useEffect comes in: the dependency array.

The dependency array

By default, useEffect hook runs on every render. Since setting state tears the component down, we still get multiple setter calls on every render, which doesn’t help us.

Fortunately, the second argument accepts an array of dependencies allowing the hook to re-render only when those dependencies are changed. So if you have a state variable and want to have some side-effect occur any time the state changes, you can use this hook and mention the state variable in the dependency array.

We pass an empty array in this example because we do not want the useEffect hook to run anytime other than the initial component render.

import React, { useEffect, useState } from "react";

export default function Clock() {
  const [counter, setCounter] = useState(0);

  useEffect(() => {
    setInterval(() => {
      setCounter(count => count + 1)
    }, 1000);
  }, [])

  return (
    <p>{counter} seconds have passed.</p>
  );
}

useEffect(() => {
  // This runs after every render
});

useEffect(() => {
  // This runs only on mount (when the component appears)
}, []);

useEffect(() => {
  // This runs on mount *and also* if either a or b have changed since the last render
}, [a, b]);

The clean-up function

Oh, it’s not going berserk anymore! We still have an issue with the counter updating twice every second though. That can be understood as a behavior caused by the React StrictMode. It is supposed to help us catch bugs, so what is that bug here?

Notice that every time the useEffect hook runs, a new setInterval is used. When the component is unmounted, setInterval is not stopped, it keeps incrementing. This unnecessary behavior can be prevented by clearing the interval when the component is unmounted and that is where the third part of our useEffect hook comes in - the cleanup function.

You can return a function from the callback in the useEffect hook, which will be executed each time before the next effect is run, and one final time when the component is unmounted. In this case, let us clean up the interval with a cleanup function.

import React, { useEffect, useState } from "react";

export default function Clock() {
  const [counter, setCounter] = useState(0);

  useEffect(() => {
    const key = setInterval(() => {
      setCounter(count => count + 1)
    }, 1000);

    return () => {
      clearInterval(key);
    };
  }, [])

  return (
    <p>{counter} seconds have passed.</p>
  );
}

Phew! It finally works! As the counter keeps merrily ticking on, let us sum up what we know about the useEffect hook.

useEffect(
  () => {
    // execute side effect
    return () => {
      // cleanup function on unmounting or re-running effect
    }
  },
  // optional dependency array
  [/* 0 or more entries */]
)

But do we need the effect?

useEffect is a mechanism outside the concepts that React usually applies, allowing you to sync your component with various external systems like a server, API, or browser DOM. The single question that you can ask yourself before you use an effect is if there are any such external systems that need to be synced with, apart from props or state. Unnecessary useEffect hooks are code-smell, error-prone, and cause unnecessary performance issues.

Let us address a few cases where useEffect does not need to be used.

• You do not need to use an effect if you are only calculating something based on the state during rendering. For a change in a component, due to a change in the props, you can calculate and set it during rendering.

    import React, { useState } from "react";
  
    export default function AdditionDisplay() {
      const [number1, setNumber1] = useState(0);
      const [number2, setNumber2] = useState(0);
  
      // This is all unnecessary.
  
      // const [sum, setSum] = useState(0);
      // useEffect(() => {
      //   setSum(number1 + number2);
      // }, [number1, number2]);
  
      const sum = number1 + number2;
  
      return (
        <p>{number1} + {number2} = {sum}</p>
      );
    }
  

• You do not need effects for events. Code that runs when a component is displayed should be in effects, the rest should be in events.

    import React, { useState } from "react";
  
    export default function App() {
      const [input, setInput] = useState("");
  
      const handleInput = (e) => {
        setInput(e.target.value);
      };
  
      // You should avoid direct manipulation when not necessary
  
      // useEffect(() => {
      //   document.getElementById("name").addEventListener("change", handleInput);
      //   return () => {
      //     document.getElementById("name").removeEventListener("change", handleInput);
      //   }
      // });
  
      return (
        <>
          {/* <input id="name" /> */}
  
          <input onChange={handleInput} value={input} />
          <p>{ input }</p>
        </>
      );
    }
  

• You do not need an effect to reset the state based on a condition most of the time. You have learned about keys in React. Just like using a key on a list’s item, adding one to a component, based on the state on which it should be reset creates a unique version of that component for each change in the value of the state.

• If you are having issues with managing your state and want to use an effect to update the state of a parent or some other non-child component, consider lifting the state. As we know, in React, the state flows in one direction, generally down the DOM. So the parents know of the data before passing it to the children. If multiple children are required to make use of a single state, it should be moved up to the parent that has all of the components that need it, instead of using escape hatches like an effect.

Assignment

Knowledge check

The following questions are an opportunity to reflect on key topics in this lesson. If you can’t answer a question, click on it to review the material, but keep in mind you are not expected to memorize or master this knowledge.

• What is an effect?
• What constitutes a useEffect hook?
• What is the one question we can ask to know when to use an effect?
• What do we mean by lifting up the state?

Additional resources

This section contains helpful links to related content. It isn’t required, so consider it supplemental.

• This article from Epic React goes over some common mistakes while using effects.
• This article, again from Epic React, goes into how dependency arrays are supposed to be used.