Tutorial

How To Convert React Class Components to Functional Components with React Hooks

JavaScriptReact

Introduction

The latest alpha release of React introduced a new concept called Hooks. Hooks were introduced to React to solve common problems. However, they primarily serve as an alternative for classes. With Hooks, you can create functional components that use state and lifecycle methods.

Hooks are currently available in React v16.7.0-alpha. There are no plans to remove classes. Hooks provide another way to write React.

Given that Hooks are still new, many developers are looking to apply the concept in their existing React applications or new applications. In this post, you’ll explore five ways to convert React class components to functional components using React Hooks.

Prerequisites

To complete this tutorial, you’ll need:

No local development is required, but CodeSandbox examples are provided for further experimentation.

Step 1 — Understanding a Class without State or Lifecycle Methods

Let’s start with a React class that has neither state nor lifecycle components:

ExampleClassComponent.js
import React, { Component } from 'react';

class App extends Component {
  alertName = () => {
    alert('John Doe');
  };

  render() {
    return (
      <div>
        <h3>This is a Class Component</h3>
        <button onClick={this.alertName}>
          Alert
        </button>
      </div>
    );
  }
};

export default App;

Here you have a typical React class, which lacks state or a lifecycle method. It alerts a name when a button is clicked.

The functional equivalent of this class will look like this:

ExampleFunctionalComponent.js
import React from 'react';

function App() {
  const alertName = () => {
    alert('John Doe');
  };

  return (
    <div>
      <h3>This is a Functional Component</h3>
      <button onClick={alertName}>
        Alert
      </button>
    </div>
  );
};

export default App;

Like the first example, this functional class behaves in a typical way.

However, this example does not use Hooks or anything new yet. In these examples, you have no need for state or lifecycle.

Let’s take a look at class-based components with state and learn how to convert them to functional components using Hooks.

Step 2 — Adding Hooks to Classes with State

Let’s consider a situation where you have a global name variable that you can update within the app from a text input field.

In React, you handle cases like this by defining the name variable in a state object and calling setState() when we have a new value to update the name variable with:

ExampleClassComponentWithState.js
import React, { Component } from 'react';

class App extends Component {
  state = {
    name: ''
  }

  alertName = () => {
    alert(this.state.name);
  };

  handleNameInput = e => {
    this.setState({ name: e.target.value });
  };

  render() {
    return (
      <div>
        <h3>This is a Class Component</h3>
        <input
          type="text"
          onChange={this.handleNameInput}
          value={this.state.name}
          placeholder="Your Name"
        />
        <button onClick={this.alertName}>
          Alert
        </button>
      </div>
    );
  }
}

export default App;

When a user types a name in the input field and clicks the Alert button, it pops up an alert with the name defined in state.

You can convert this entire class into a functional React component using Hooks:

ExampleFunctionalComponentWithState.js
import React, { useState } from 'react';

function App() {
  const [name, setName] = useState('John Doe');

  const alertName = () => {
    alert(name);
  };

  const handleNameInput = e => {
    setName(e.target.value);
  };

  return (
    <div>
      <h3>This is a Functional Component</h3>
      <input
        type="text"
        onChange={handleNameInput}
        value={name}
        placeholder="Your Name"
      />
      <button onClick={alertName}>
        Alert
      </button>
    </div>
  );
};

export default App;

Here, you have introduced the useState Hook. It allows you to make use of state in React functional components. With the useState() Hook, you can use state in this functional component. It uses a similar syntax with a destructuring assignment for arrays.

Consider this line:

const [name, setName] = useState('John Doe')

Here, name is the equivalent of this.state in a normal class component, and setName is the equivalent of this.setState.

The initial value of the state in the useState() Hook comes from an argument. In other words, the useState() argument is the initial value of the state. In your case, you set it to 'John Doe'. This means that the initial state of the name in state is 'John Doe'.

This code is an example of how you can convert a class-based React component with state to a functional component using Hooks.

Let’s explore other scenarios, including classes with multiple state properties.

Step 3 — Adding Hooks to Classes with Multiple State Properties

You have looked at how you might convert one state property with useState, but the same approach won’t quite work when you have multiple state properties. If, for example, you had two or more input fields for userName, firstName, and lastName, then you would have a class-based component with three state properties:

ExampleClassComponentWithMultipleStateProperties.js
import React, { Component } from 'react';

class App extends Component {
  state = {
    userName: '',
    firstName: '',
    lastName: ''
  };

  logName = () => {
    console.log(this.state.userName);
    console.log(this.state.firstName);
    console.log(this.state.lastName);
  };

  handleUserNameInput = e => {
    this.setState({ userName: e.target.value });
  };
  handleFirstNameInput = e => {
    this.setState({ firstName: e.target.value });
  };
  handleLastNameInput = e => {
    this.setState({ lastName: e.target.value });
  };

  render() {
    return (
      <div>
        <h3>This is a Class Component</h3>
        <input
          type="text"
          onChange={this.handleUserNameInput}
          value={this.state.userName}
          placeholder="Your Username"
        />
        <input
          type="text"
          onChange={this.handleFirstNameInput}
          value={this.state.firstName}
          placeholder="Your First Name"
        />
        <input
          type="text"
          onChange={this.handleLastNameInput}
          value={this.state.lastName}
          placeholder="Your Last Name"
        />
        <button
          className="btn btn-large right"
          onClick={this.logName}
        >
          Log Names
        </button>
      </div>
    );
  }
}

export default App;

To convert this class to a functional component with Hooks, you will have to take a somewhat unconventional route. Using the useState() Hook, the above example can be written as:

ExampleFunctionalComponentWithMultipleStateProperties.js
import React, { useState } from 'react';

function App() {
  const [userName, setUsername] = useState('');
  const [firstName, setFirstname] = useState('');
  const [lastName, setLastname] = useState('');

  const logName = () => {
    console.log(userName);
    console.log(firstName);
    console.log(lastName);
  };

  const handleUserNameInput = e => {
    setUsername(e.target.value);
  };
  const handleFirstNameInput = e => {
    setFirstname(e.target.value);
  };
  const handleLastNameInput = e => {
    setLastname(e.target.value);
  };

  return (
    <div>
      <h3>This is a Functional Component</h3>
      <input
        type="text"
        onChange={handleUserNameInput}
        value={userName}
        placeholder="Your Username"
      />
      <input
        type="text"
        onChange={handleFirstNameInput}
        value={firstName}
        placeholder="Your First Name"
      />
      <input
        type="text"
        onChange={handleLastNameInput}
        value={lastName}
        placeholder="Your Last Name"
      />
      <button
        className="btn btn-large right"
        onClick={logName}
      >
        Log Names
      </button>
    </div>
  );
};

export default App;

Here’s a CodeSandbox for this example.

This demonstrates how you can convert a class-based component with multiple state properties to a functional component using the useState() Hook.

Step 4 — Adding Hooks to a Class with State and componentDidMount

Let’s consider a class with state and componentDidMount. To demonstrate, you will look at a scenario where you set an initial state for the three input fields and have them all update to a different set of values after five seconds.

To achieve this, you will declare an initial state value for the input fields and implement a componentDidMount() lifecycle method that will run after the initial render to update the state values:

ExampleClassComponentWithStateAndComponentDidMount.js
import React, { Component } from 'react';

class App extends Component {
  state = {
    // initial state
    userName: 'johndoe',
    firstName: 'John',
    lastName: 'Doe'
  }

  componentDidMount() {
    setInterval(() => {
      this.setState({
        // update state
        userName: 'janedoe',
        firstName: 'Jane',
        lastName: 'Doe'
      });
    }, 5000);
  }

  logName = () => {
    console.log(this.state.userName);
    console.log(this.state.firstName);
    console.log(this.state.lastName);
  };

  handleUserNameInput = e => {
    this.setState({ userName: e.target.value });
  };
  handleFirstNameInput = e => {
    this.setState({ firstName: e.target.value });
  };
  handleLastNameInput = e => {
    this.setState({ lastName: e.target.value });
  };

  render() {
    return (
      <div>
        <h3>This is a Class Component</h3>
        <input
          type="text"
          onChange={this.handleUserNameInput}
          value={this.state.userName}
          placeholder="Your Username"
        />
        <input
          type="text"
          onChange={this.handleFirstNameInput}
          value={this.state.firstName}
          placeholder="Your First Name"
        />
        <input
          type="text"
          onChange={this.handleLastNameInput}
          value={this.state.lastName}
          placeholder="Your Last Name"
        />
        <button
          className="btn btn-large right"
          onClick={this.logName}
        >
          Log Names
        </button>
      </div>
    );
  }
}

export default App;

When the app runs, the input fields will have the initial values you’ve defined in the state object. These values will then update to the values you’ve defined inside the componentDidMount() method after five seconds.

Next, you will convert this class to a functional component using the React useState and useEffect Hooks:

ExampleFunctionalComponentWithStateAndComponentDidMount.js
import React, { useState, useEffect } from 'react';

function App() {
  const [userName, setUsername] = useState('johndoe');
  const [firstName, setFirstname] = useState('John');
  const [lastName, setLastname] = useState('Doe');

  useEffect(() => {
    setInterval(() => {
      setUsername('janedoe');
      setFirstname('Jane');
      setLastname('Doe');
    }, 5000);
  });

  const logName = () => {
    console.log(userName);
    console.log(firstName);
    console.log(lastName);
  };

  const handleUserNameInput = e => {
    setUsername({ userName: e.target.value });
  };
  const handleFirstNameInput = e => {
    setFirstname({ firstName: e.target.value });
  };
  const handleLastNameInput = e => {
    setLastname({ lastName: e.target.value });
  };

  return (
    <div>
      <h3>This is a Functional Component</h3>
      <input
        type="text"
        onChange={handleUserNameInput}
        value={userName}
        placeholder="Your Username"
      />
      <input
        type="text"
        onChange={handleFirstNameInput}
        value={firstName}
        placeholder="Your First Name"
      />
      <input
        type="text"
        onChange={handleLastNameInput}
        value={lastName}
        placeholder="Your Last Name"
      />
      <button
        className="btn btn-large right"
        onClick={logName}
      >
        Log Names
      </button>
    </div>
  );
};

export default App;

Here’s a CodeSandbox for this example.

In terms of functionality, this component does exactly the same thing as the previous example. The only difference is that instead of using the conventional state object and componentDidMount() lifecycle method as you did in the class component, you used the useState and useEffect Hooks.

Step 5 — Adding Hooks to a Class with State, componentDidMount, and componentDidUpdate

Next, let’s look at a React class with state and two lifecycle methods: componentDidMount and componentDidUpdate. Most of the solutions up to this point have used the useState Hook. In this example, you will focus on the useEffect Hook.

To best demonstrate how this works, let’s modify your code to dynamically update the <h3> header on the page.

Currently, the header says This is a Class Component. Now, you will define a componentDidMount() method to update the header to say Welcome to React Hooks after three seconds:

ExampleClassComponentWithStateAndTwoLifecycleMethods.js
import React, { Component } from 'react';

class App extends Component {
  state = {
    header: 'Welcome to React Hooks'
  }

  componentDidMount() {
    const header = document.querySelectorAll('#header')[0];
    setTimeout(() => {
      header.innerHTML = this.state.header;
    }, 3000);
  }

  render() {
    return (
      <div>
        <h3 id="header">This is a Class Component</h3>
      </div>
    );
  }
}

export default App;

When the app runs, it will start with the initial header This is a Class Component and change it to Welcome to React Hooks after three seconds. This is the classic componentDidMount() behavior since it runs after the render function is executed successfully.

Let’s add functionality to dynamically update the header from another input field so that the header gets updated with the new text while you type.

To accomplish this, you will need to implement the componentDidUpdate() lifecycle method:

ExampleClassComponent.js
import React, { Component } from 'react';

class App extends Component {
  state = {
    header: 'Welcome to React Hooks'
  }

  componentDidMount() {
    const header = document.querySelectorAll('#header')[0];
    setTimeout(() => {
      header.innerHTML = this.state.header;
    }, 3000);
  }

  componentDidUpdate() {
    const node = document.querySelectorAll('#header')[0];
    node.innerHTML = this.state.header;
  }

  handleHeaderInput = e => {
    this.setState({ header: e.target.value });
  };

  render() {
    return (
      <div>
        <h3 id="header">This is a Class Component</h3>
        <input
          type="text"
          onChange={this.handleHeaderInput}
          value={this.state.header}
        />
      </div>
    );
  }
}

export default App;

Here, you have state, componentDidMount(), and componentDidUpdate(). When you run the app, the componentDidMount() function will update the header to Welcome to React Hooks after three seconds. When you start typing in the header text input field, the <h3> text will update with the input text as defined in the componentDidUpdate() method.

Next, you will convert this class to a functional component with the useEffect() Hook:

ExampleFunctionalComponentWithStateAndTwoLifecycleMethods.js
import React, { useState, useEffect } from 'react';

function App() {
  const [header, setHeader] = useState('Welcome to React Hooks');

  useEffect(() => {
    const newheader = document.querySelectorAll('#header')[0];
    setTimeout(() => {
      newheader.innerHTML = header;
    }, 3000);
  });

  const handleHeaderInput = e => {
    setHeader(e.target.value);
  };

  return (
    <div>
      <h3 id="header">This is a Functional Component</h3>
      <input
        type="text"
        onChange={handleHeaderInput}
        value={header}
      />
    </div>
  );
};

export default App;

Check out this example on CodeSandbox.

You achieved the same functionality with this component as you did previously by using the useEffect() Hook. You optimized the code as well, since you did not have to write separate code for the componentDidMount() and componentDidUpdate() functions. With the useEffect() Hook, you get the functionality of both. This is because useEffect() runs both after the initial render and after every subsequent update by default.

Step 6 — Converting PureComponent to React memo

React PureComponent works in a similar manner to Component. The major difference between them is that React.Component doesn’t implement the shouldComponentUpdate() lifecycle method while React.PureComponent does.

If you have an application where the render() function renders the same result given the same props and state, you can use React.PureComponent for a performance boost in some cases.

React.memo() works in a similar way. When your function component renders the same result given the same props, you can wrap it in a call to React.memo() to enhance performance. Using PureComponent and React.memo() gives React applications a considerable increase in performance as it reduces the number of render operations in the app.

To understand what they both do, you will first look at code where a component renders every two seconds, whether or not there’s a change in value or state:

ExampleClassComponent.js
import React, { Component } from 'react';

function Unstable(props) {
  // monitor how many times this component is rendered
  console.log('Rendered Unstable component');
  return (
    <div>
      <p>{props.value}</p>
    </div>
  );
};

class App extends Component {
  state = {
    value: 1
  };

  componentDidMount() {
    setInterval(() => {
      this.setState(() => {
        return { value: 1 };
      });
    }, 2000);
  }

  render() {
    return (
      <div>
        <Unstable value={this.state.value} />
      </div>
    );
  }
}
export default App;

When you run the app and check the logs, you will notice that it renders the component every two seconds, without any change in state or props. This is a situation that you can improve with both PureComponent and React.memo().

Console log output for multiple render operations

Most of the time, you only want to re-render a component when there’s been a change in state or props. Using the example above, you can improve it with PureComponent so that the component only re-renders when there’s a change in state or props.

You can accomplish this by importing PureComponent and extending it:

ExamplePureComponent.js
import React, { PureComponent } from 'react';

function Unstable(props) {
  console.log('Rendered Unstable component');
  return (
    <div>
      <p>{props.value}</p>
    </div>
  );
};

class App extends PureComponent {
  state = {
    value: 1
  };

  componentDidMount() {
    setInterval(() => {
      this.setState(() => {
        return { value: 1 };
      });
    }, 2000);
  }

  render() {
    return (
      <div>
        <Unstable value={this.state.value} />
      </div>
    );
  }
}

export default App;

Now, if you run the app again, you only get the initial render. Nothing else happens after that. This is because you have class App extends PureComponent {} instead of class App extends Component {}.

Console log output for a single render operation

This solves the problem of components being re-rendered without respect to the current state. However, if you implement a state change within your setState method, you would run into another issue.

For example, consider the following changes to setState():

Currently, value set to 1:

componentDidMount() {
  setInterval(() => {
    this.setState(() => {
      return { value: 1 };
    });
  }, 2000);
}

Let’s consider a situation where value is set to Math.random():

componentDidMount() {
  setInterval(() => {
    this.setState(() => {
      return { value: Math.round(Math.random()) };
    });
  }, 2000);
}

In this scenario, the first example component would re-render each time the value updates to the next random number. However, PureComponent makes it possible to re-render components only when there has been a change in state or props.

Now you can explore how to use React.memo() to achieve the same fix. To accomplish this, wrap the component with React.memo():

ExampleReactMemo.js
import React, { Component } from 'react';

const Unstable = React.memo(function Unstable (props) {
  console.log('Rendered Unstable component');
  return (
    <div>
      <p>{props.value}</p>
    </div>
  );
});

class App extends Component {
  state = {
    val: 1
  };

  componentDidMount() {
    setInterval(() => {
      this.setState(() => {
        return { value: 1 };
      });
    }, 2000);
  }

  render() {
    return (
      <div>
        <Unstable val={this.state.val} />
      </div>
    );
  }
}

export default App;

Here’s the CodeSandbox for this example.

This achieves the same result as using PureComponent. The component only renders after the initial render and does not re-render again until there is a change in state or props.

Conclusion

In this tutorial, you have explored a few approaches to covert an existing class-based component to a functional component using React Hooks.

You have also looked at a special case of converting a React PureComponent class to React.memo().

To use Hooks in your applications, be sure to update your version of React to the supported version:

"react": "^16.7.0-alpha",
"react-dom": "^16.7.0-alpha",

You now have a foundation to experiment further with React Hooks.

Learn more about Getting Started with React Hooks and Build a React To-Do App with React Hooks.

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