Hello World 👋
Hooks are special types of functions in React that you can call inside React functional components. They let you store data, add interactivity, and perform some actions, otherwise known as side-effects.
The most common hooks are:
- useState
- useEffect
- useRef
- useContext
- useReducer
In the previous article (React Hooks: Managing State With useState Hook), we learned about useState hook. We will be using the useState hook in this article, so if you haven't read the previous one yet, please go and read that before going through this. In this article, we will learn about useEffect hook and then later use it to build a custom and reusable localStorage hook.
useEffect
useEffect is a built-in function in React. It takes a callback function as an argument and does not return anything.
For example,
useEffect(() => {
})
Note:
- React runs the callback present in
useEffect after every render and rerender of the component.
Creating a reusable LocalStorage Hook
Simple useEffect
Let's take a simple counter example as shown below.
function Counter() {
const [count, setCount] = useState(0);
const incrementCount = () => {
setCount(count + 1);
};
return <button onClick={incrementCount}>{count}</button>;
}
Try to increment the counter in the above sandbox and reload the sandbox page. You will see that as soon as you reload the page, the counter is reset to 0. Let's say that we don't want that. We want the counter to stay at the same value even after you reload the sandbox page. One way to do this is to store the value of the counter in local storage and sync the counter state from there when you reload.
Let's see how we can achieve that using useEffect.
useEffect(() => {
localStorage.setItem('count', count)
})
What this does is, every time the component rerenders, it updates the value of the count key in local storage.
function Counter() {
const [count, setCount] = useState(0);
const incrementCount = () => {
setCount(count + 1);
};
useEffect(() => {
localStorage.setItem('count', count)
})
return <button onClick={incrementCount}>{count}</button>;
}
As you increase the count, you will see that the count in localStorage is getting increased. But as soon as you realod the page, the count is being reset to 0 again, even in localStorage. This is because we are not getting the initial value of count from localStorage.
Let's change the component to get the initial value from localstorage.
function Counter() {
const [count, setCount] = useState(() => localStorage.getItem('count') || 0);
const incrementCount = () => {
setCount(count + 1);
};
useEffect(() => {
localStorage.setItem('count', count)
})
return <button onClick={incrementCount}>{count}</button>;
}
Note:
Here we are doing a lazy initialization of the state.
Try to increment the count and reload the sandbox. You will see that the counter no longer resets to 0. But, we are facing a new problem.
To reproduce the problem,
- Increment the count a few times.
- Reload the page.
- Now increment the count again by clicking the count button.
- You will see that instead of incrementing the count by 1, one is getting concatenated to the existing count.
This is happening because of how localStorage stores the values. It stores everything in the form of a string. So even when we try to store the number in localStorage, it converts it into a string and then stores it. So, when we fetch the value from localStorage, we are getting a string instead of a number. That is why incrementing the count is not behaving as it should.
Let's try to fix this.
function convertNumberToString(num) {
return `${num}`
}
function convertStringToNumber(str) {
return Number(str)
}
function getInitialValue() {
const localStorageValue = localStorage.getItem('count')
return convertStringToNumber(localStorageValue) || 0
}
function Counter() {
const [count, setCount] = useState(() => getInitialValue());
const incrementCount = () => {
setCount(count + 1);
};
useEffect(() => {
localStorage.setItem('count', convertNumberToString(count))
})
return <button onClick={incrementCount}>{count}</button>;
}
Now, everything seems to work. But, we can optimize this even further.
Dependency Array
Let's try to add a console log in the useEffect and see when it is being run.
useEffect(() => {
console.log('useEffect callback is getting executed')
localStorage.setItem('count', convertNumberToString(count))
})
You will see that the useEffect callback is getting executed every time the component re-renders. Try to click on "UPDATE SOME OTHER STATE" button. You will see that even though the count doesn't change, the useEffect is getting called. This is the expected behavior. But we want to set the value in localStorage only when the value of count changes.
React gives us a way to achieve this.
useEffect takes an array as the second argument. It is called dependency array. You can specify all the dependencies that your useEffect depends on, in that array. And that useEffect callback will only run when any of those dependencies change.
For example, we want the useEffect in our example to run only when count changes. You can achieve this as follows.
useEffect(() => {
console.log('useEffect callback is getting executed')
localStorage.setItem('count', convertNumberToString(count))
}, [count])
Now, when you try to click on "UPDATE SOME OTHER STATE", the component rerenders, but the useEffect callback will not get executed.
Let's put everything together.
import React, { useState, useEffect } from "react";
function convertNumberToString(num) {
return `${num}`;
}
function convertStringToNumber(str) {
return Number(str);
}
function getInitialValue() {
const localStorageValue = localStorage.getItem("count");
return convertStringToNumber(localStorageValue) || 0;
}
function Counter() {
const [count, setCount] = useState(() => getInitialValue());
const incrementCount = () => {
setCount(count + 1);
};
useEffect(() => {
localStorage.setItem("count", convertNumberToString(count));
}, [count]);
return (
<button className="btn" onClick={incrementCount}>
{count}
</button>
);
}
export default Counter;
Creating a reusable hook
The useLocalStorageState hook example shown here is based on the example from Kent C. Dodds's EpicReact.Dev React Hooks workshop.
Since we may need the same logic of storing state in localStorage at many places, we can create a custom hook that does it, and then we can use it wherever we want to store the state in localStorage.
function convertNumberToString(num) {
return `${num}`;
}
function convertStringToNumber(str) {
return Number(str);
}
function getInitialValue() {
const localStorageValue = localStorage.getItem("count");
return convertStringToNumber(localStorageValue) || 0;
}
function useLocalStorageState() {
const [count, setCount] = useState(() => getInitialValue());
const incrementCount = () => {
setCount(count + 1);
};
useEffect(() => {
localStorage.setItem("count", convertNumberToString(count));
}, [count]);
return [count, setCount]
}
This is what we have until now. Let's refactor this a bit to generalize things.
function getInitialValue(key, defaultValue, convertFromString) {
const localStorageValue = localStorage.getItem(key);
return convertFromString(localStorageValue) || defaultValue;
}
function useLocalStorageState(
key,
defaultValue = "",
{ convertToString = JSON.stringify, convertFromString = JSON.parse } = {}
) {
const [state, setState] = useState(() =>
getInitialValue(key, defaultValue, convertFromString)
);
useEffect(() => {
localStorage.setItem(key, convertToString(state));
}, [key, state, convertToString]);
return [state, setState];
}
What did we do here?
- We changed the variable
count and setCount to state and setState
- We are asking the user to provide the
key as an argument. We will store the state in this key in localStorage.
- We are asking the user to also pass the initial default value as an argument. Previously in our example, it was 0.
- We are asking the user to optionally pass the
convertToString and convertFromString functions as arguments.
- If the user doesn't provide them, we are defaulting them to
JSON.stringify and JSON.parse.
- We updated the dependency array of
useEffect and added all of its dependents.
- Finally, we are returning
state and useState in the form of an array, similar to how the inbuilt useState hook returns an array.
Let's change our example to use this custom hook.
function Counter() {
const [count, setCount] = useLocalStorageHook('count', 0);
const incrementCount = () => {
setCount(count + 1);
};
return (
<button className="btn" onClick={incrementCount}>
{count}
</button>
);
}
We can go a bit further and allow the user to also pass a function as the initial value, similar to how useState works.
function getInitialValue(key, defaultValue, convertFromString) {
const localStorageValue = localStorage.getItem(key);
if(localStorageValue) {
return convertFromString(localStorageValue)
}
return typeof defaultValue === 'function' ? defaultValue() : defaultValue
}
Sometimes, the convertFromString function can throw an error when the value against the given key already exists in the local storage. In that case, we can remove the corresponding key-value pair from local storage before adding it with new values.
function getInitialValue(key, defaultValue, convertFromString) {
const localStorageValue = localStorage.getItem(key);
if(localStorageValue) {
try {
return convertFromString(localStorageValue)
} catch {
localStorage.removeItem(key)
}
}
return typeof defaultValue === 'function' ? defaultValue() : defaultValue
}
Let's put everything together.
function getInitialValue(key, defaultValue, convertFromString) {
const localStorageValue = localStorage.getItem(key);
if(localStorageValue) {
try {
return convertFromString(localStorageValue)
} catch {
localStorage.removeItem(key)
}
}
return typeof defaultValue === 'function' ? defaultValue() : defaultValue
}
function useLocalStorageState(
key,
defaultValue = "",
{ convertToString = JSON.stringify, convertFromString = JSON.parse } = {}
) {
const [state, setState] = useState(() =>
getInitialValue(key, defaultValue, convertFromString)
);
useEffect(() => {
localStorage.setItem(key, convertToString(state));
}, [key, state, convertToString]);
return [state, setState];
}
That's it. You can use this hook whenever you want to store the state in localStorage and keep it in sync with the actual state. The API is also very similar to how you use useState
const [state, setState] = useLocalStorageState('state', {})
What have you learned?
- useEffect hook
- It runs every time the component renders and re-renders when no dependency array is passed.
- You can pass a dependency array as a second argument.
- Callback in
useEffect only runs when any of the value in the dependency array changes.
- If you pass an empty array as a dependency array, then the callback will only run after the component is first rendered.
- We also learned how to create a reusable localStorage hook using
useState and useEffect.
What's Next?
In the next article, we will see the flow of hooks. We will see exactly at what time different hooks will be run in the component lifecycle especially useState and useEffect.
Until Next Time 👋
References:
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