A higher-order component is a pattern that can be created with the React components. We can pass the reusable logic as props to the functional components to implement abstraction, reusability, and uniformity in the code.
The principles of OOPS are embedded deep in the programming and in one way or another using them helps to make the code more readable, and less verbose and promotes reusability.
A higher-order component (HOC) is a pattern that emerges in React given its compositional nature, where we can pass one component to another component as an argument, and this parent component (Higher-order) will have certain logic that it will pass down to this received component as props.
For example, if you worked with Redux, you will notice how we use the connect() HOC to compose the components with the states and the actions.
const ConnectedUsersList = connect(usersSelector, usersActions)(UsersList);
Here the connect() method receives two arguments and returns another function that accepts the UsersList as arguments forming a composition of components.
Now in the UsersList, you will receive the states and the actions as props.
This can be written more discretely as,
// create a connection first const connectRedux = connect(usersSelector, usersActions); // then connect the component const ConnectedUsersList = connectRedux(UsersList);
This helps to abstract all the logic at the single place in the connect() method making it a single source of truth and all the components passing through it will receive similar props for the same inputs making it a pure higher-order component function.
This pattern is really helpful when you want to abstract the common logic in a single place.
For example, let’s say you have a product list website with different categories.
We can create a product component that will receive the list of the products as props and will display it.
const ProductsList = ({ data }) => {
return data.map((e) => (
<div key={e.id} className="product-item">
<h2>Item: {e.title}</h2>
</div>
));
};
Assuming that we have a uniform response for all the API’s for every category, we can create a higher-order component that will take the API URL and the products list component input and will fetch the data and pass it to the products list to display.
This higher-order component will maintain the loading and the error state logic abstracting everything making the product list independent to just show the list without worrying about anything.
To create an HOC we follow a naming convention starting the function name with with keyword like withFetchData().
This is a functional component that will take an argument and return another function from it forming a closure.
const withFetchData = (Element, url) => {
return (props) => {
//your logic goes here
}
}
Now we can add all our logic inside it to make the API calls.
const withFetchData = (Element, url) => {
return (props) => {
const [isLoading, setIsLoading] = useState(false);
const [error, setError] = useState(false);
const [data, setData] = useState([]);
useEffect(() => {
const makeApiCall = async () => {
setIsLoading(true);
try {
let res = await fetch(url);
if (res.ok) {
res = await res.json();
setData(res);
} else {
setError(true);
}
} catch (e) {
setError(true);
} finally {
setIsLoading(false);
}
};
makeApiCall();
}, []);
if (isLoading) {
return <div>....Loading</div>;
}
if (error) {
return <div>Something went wrong!...</div>;
}
return <Element data={data} {...props} />;
};
};
Note – Here we have added additional check to throw error if we did not receive 200 in the response status code as fetch does not throws error on 4XX.
As this is a function that returns another function, to invoke the HOC we have to store the returned value in the variable and then use it is as a component if you are not exporting the HOC.
import { withFetchData } from ‘./hoc’;
export default function App() {
const Todos = withFetchData(
ProductsList,
"https://jsonplaceholder.typicode.com/todos"
);
return (
<div className="App">
<Todos />
</div>
);
}
Otherwise you can export the HOC and then can use it as a component.
// todos.jsx export default withFetchData( ProductsList, "https://jsonplaceholder.typicode.com/todos" );
import TodosProductList from 'todos'
export default function App() {
return (
<div className="App">
<TodosProductList />
</div>
);
};
With this we can define the reusable logic multiple times.
// products.jsx
const Todos = withFetchData(
ProductsList,
"https://jsonplaceholder.typicode.com/todos"
);
const Photos = withFetchData(
ProductsList,
"https://jsonplaceholder.typicode.com/photos"
);
const Albums = withFetchData(
ProductsList,
"https://jsonplaceholder.typicode.com/albums"
);
export {
Todos,
Photos,
Albums
};
And then we can use it.
import {Todos, Photos, Albums} from 'products'
export default function App() {
return (
<div className="App">
<Todos />
<Photos />
<Albums />
</div>
);
};
The good thing about the HOC’s are that we can compose multiple HOCs together.
For example, let’s say we have another HOC that helps to style the product list as a card.
We will create another HOC function withStyles() that will handle all the styles logic.
// hoc.js
const withStyles = (Element) => {
return (props) => {
const cardStyle = { flex: "1 33%" };
return (
<div
style={{
display: "flex",
justifyContent: "center",
alignItems: "center",
flexWrap: "wrap"
}}
>
<Element style={cardStyle} {...props} />
</div>
);
};
};
This will display the items in a 3 column layout, we have to pass the style prop to the product items and then compose the HOC’s to make it work.
import { withFetchData, withStyles } from "./hoc";
const ProductsList = ({ data, style }) => {
return data.map((e) => (
<div key={e.id} className="product-item" style={style}>
<h2>Item: {e.title}</h2>
</div>
));
};
const Todos = withFetchData(
withStyles(ProductsList),
"https://jsonplaceholder.typicode.com/todos"
);
const Photos = withFetchData(
withStyles(ProductsList),
"https://jsonplaceholder.typicode.com/photos"
);
const Albums = withFetchData(
withStyles(ProductsList),
"https://jsonplaceholder.typicode.com/albums"
);
export { Todos, Photos, Albums };
Advantages of using Higher-order components in React
- Reusability – Same logic can be reused for multiple components.
- Uniformity – Components can be used for a single purpose making them easy to manage.
- Abstraction – Single source of truth, helps to trace errors.
Disadvantages of using Higher-order components in React
- Too much nesting – Composing the components with the multiple HOC’s increases the nesting.
- Performance issues – Props drill down makes unnecessary re-renders even though the middle components may not be utilizing the props.
To mitigate the issues of higher-order components we can use the hooks, which also helps to abstract logic, but it prevents the nesting of components.
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