What Are Controlled And Uncontrolled Components In React.js?

In React.js, controlled components are those form elements whose value is controlled by React. This means that the value of the input field is stored in the component's state and is updated and reflected based on the changes made by the user. This allows React to have full control over the input element.

On the other hand, uncontrolled components are those form elements whose value is handled by the DOM itself. This means that the value of the input field is not stored in the component's state and its behavior is managed by the browser. In this case, React does not have direct control over the input element.

In general, controlled components are preferred in React.js as they provide a more predictable and controlled behavior, which can help in managing the overall state of the application more effectively.

How to update state in controlled components in React.js?

To update state in controlled components in React.js, you can use the setState method provided by React. Here's an example of how to update state in a controlled component:

1. Define your initial state in the constructor:

1
2
3
4
5
6

constructor(props) {
  super(props);
  this.state = {
    inputValue: ''
  };
}

1. Create a method that handles the change of the input field and updates the state:

1
2
3
4
5

handleInputChange = (event) => {
  this.setState({
    inputValue: event.target.value
  });
}

1. Bind the handleInputChange method in the constructor or using arrow function syntax:

1
2
3
4
5
6
7
8

constructor(props) {
  super(props);
  this.state = {
    inputValue: ''
  };

  this.handleInputChange = this.handleInputChange.bind(this);
}

or

1
2
3
4
5

handleInputChange = (event) => {
  this.setState({
    inputValue: event.target.value
  });
}

1. In your input field, set the value and onChange props to the state value and the handleInputChange method respectively:

1
2
3
4
5

<input 
  type="text"
  value={this.state.inputValue}
  onChange={this.handleInputChange}
/>

Now, whenever the input field value changes, the handleInputChange method will be called, updating the state with the new value. React will automatically re-render the component with the updated state.

How to manage forms using controlled components in React.js?

You can manage forms using controlled components in React.js by following these steps:

1. Create a form component: Create a new component that will represent your form. This component will contain input fields that will accept user input.
2. Set up state: In the form component, set up state variables to hold the values of the form inputs. These state variables will be updated with the user’s input and will be used to control the form inputs.
3. Update state with user input: Add event handlers to the form inputs that will update the state variables when the user types in a value.
4. Submit form: Add a submit button to your form that will trigger a submit event. When the form is submitted, you can access the values of the form inputs from the state variables and perform any necessary actions, such as making an API call or updating a database.
5. Handle form validation: You can implement form validation by checking the values of the form inputs in the submit event handler. If any of the inputs are invalid, you can prevent the form from being submitted and display error messages to the user.

By using controlled components in React.js, you can easily manage the state of your form inputs and create dynamic forms that respond to user input.

How to handle user input in controlled components in React.js?

In React.js, controlled components are input elements whose value is controlled by React state. This allows you to handle user input through onChange event handlers and update the state accordingly.

Here's how you can handle user input in controlled components in React.js:

1. Initialize a state object in your component constructor to hold the value of the input element:

1
2
3
4
5
6

constructor(props) {
  super(props);
  this.state = {
    inputValue: ''
  };
}

1. Render the input element and set its value attribute to the state value:

1
2
3
4
5

<input
  type="text"
  value={this.state.inputValue}
  onChange={this.handleInputChange}
/>

1. Create a method to handle the onChange event and update the state with the new input value:

1
2
3
4
5

handleInputChange = (event) => {
  this.setState({
    inputValue: event.target.value
  });
}

1. Now, whenever the user types into the input element, the handleInputChange method will update the state with the new value and re-render the component with the updated value.

By following these steps, you can easily handle user input in controlled components in React.js. This approach is recommended for managing form inputs and other interactive elements in your React applications.