Hooks are a way to use state and other React features without writing a class. They let you use React features, such as useState and useEffect, in functional components. When you call a Hook like useState inside a component, React saves the value that it returns in the component’s state.

For example, the following component uses the useState Hook to store a count:

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import React, { useState } from 'react';

function Example() {
  // Declare a new state variable, which we'll call "count"
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

React Hooks are functions that allow React functional components to have access to state and lifecycle features. They are available starting with React version 16.8.

Example:

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import React, { useState } from 'react';

const ExampleComponent = () => {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

In the above example, the useState() hook is used to provide access to state within the functional component, and let the React app keep track of the count, which is increased each time the button is clicked.

For more information about React Hooks, please see the official documentation: https://reactjs.org/docs/hooks-intro.html

Hooks are a feature introduced in React 16.8 that allows developers to add state to functional components. Hooks allow developers to use logical components, run data fetching and execute side effects. Learn more in the official React documentation: https://reactjs.org/docs/hooks-intro.jsx

Example:

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import React, {useState} from 'react';

function App() { 
  const [state, setState] = useState('Hello');

  return (
    <div>
      <p>{state}</p>
    </div>
  );
}

export default App;

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import React from "react";

class Dropdown extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
            isOpen: false,
            selectedOption: ''
        }
        this.renderOptions = this.renderOptions.bind(this);
        this.handleChange = this.handleChange.bind(this);
        this.toggleOpen = this.toggleOpen.bind(this);
    }

    toggleOpen() {
        this.setState({
            isOpen: !this.state.isOpen
        });
    }

    handleChange(e) {
        this.setState({
            selectedOption: e.target.value
        });
    }

    renderOptions() {
        const options = this.props.options.map( (option) => {
            return (
                <option key={option.value} value={option.value}>
                    {option.title}
                </option>
            );
        });
        return options;
    }

    render() {
        const display = this.state.isOpen? 'block' : 'none';
        return (
            <div className="dropdown">
                <div className="trigger" onClick={this.toggleOpen}>
                    {this.state.selectedOption || this.props.title}
                </div>
                <div className="options-list" style={{display: display}}>
                    <select
                        value={this.state.selectedOption}
                        onChange={this.handleChange}>
                        {this.renderOptions()}
                    </select>
                </div>
            </div>
        );
    }
};

export default Dropdown;

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import React, { useState } from 'react';
import { Link } from 'react-router-dom';

const Login = () => {
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');

  return (
    <div className="login-page">
      <form>
        <h1>Login</h1>
        <label htmlFor="email">Email</label>
        <input
          type="text"
          name="email"
          value={email}
          onChange={(e) => setEmail(e.target.value)}
        ></input>

        <label htmlFor="password">Password</label>
        <input
          type="password"
          name="password"
          value={password}
          onChange={(e) => setPassword(e.target.value)}
        ></input>

        <button type="submit">Login</button>
      </form>
      <p>
        Don't have an account? <Link to="/register">Sign up</Link>
      </p>
    </div>
  );
};

export default Login;

This can be done by passing a function from the parent component to the child component, and having the function call setState on the parent component. This will trigger a re-render on all components with the new data. Here's an example in React:

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// Parent component
function ParentComponent () {
  const [state, setState] = useState({
    name: ''
  });

  // create a function that the child component can call
  const updateName = (name) => {
    setState({
      name
    });
  }

  return (
    <ChildComponent updateName={updateName} />
  )
}

// Child component
function ChildComponent ({ updateName }) {
  const changeName = (e) => {
    updateName(e.target.value);
  };

  return (
    <input type="text" name="name" onChange={changeName} />
  )
}

React states are used to keep track of data in React components and update the UI when the data changes. When state changes, the component re-renders itself. This is done by using the useState hook in React. Here is more information about React states: https://reactjs.org/docs/hooks-state.html

useEffect is a hook in React that is used for performing side effects. It is invoked after the initial render of the component. It is equivalent to the componentDidUpdate method in Class components. useEffect takes a callback that will be triggered after each render of the component, and it has access to props and state. The callback will not be triggered if the props or state did not change. To guarantee that the callback gets invoked each time, regardless of any updates, pass [] (empty array) as the second argument.

Example:

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useEffect(() => {
  // perform side-effects
}, [])

React components are reusable pieces of UI (user interface) code that are written in JavaScript. React components are self-contained pieces of code that manage their own state at runtime and that can be used in other components. They can be used to create complex user interfaces which are easy to manage and maintain. Components are often broken down into smaller pieces called components, making it easier to write and reuse code. React components use a syntax known as JSX, which allows developers to write HTML-like syntax within their JavaScript code. As a result, React components can be very powerful, allowing developers to create user interfaces that are both fast and responsive.

In React, a state management system is used to manage the state of components. It is a technique used to make the data and state of a React application easier to maintain. React provides a few methods that developers can use to manage this state, such as setState(), useState, and useReducer. For more information and examples, checkout the React docs on state and lifecycle.

The useEffect hook can be used in three major cases:

1. To run custom effects when React state or props change such as calls to APIs for data fetching or state updating.
2. To perform side effects such as setting up subscriptions, and cleanup tasks such as canceling those subscriptions.
3. To increase the app’s performance by preventing unnecessary re-renders of components.

For more detailed information visit React's official documentation on useEffect.

React's useEffect hook can be used to avoid unnecessary API calls. To do this, you can check to see if the data from the API call is already present in state or props before making a new API call. If the data is not present, you can set up an effect that calls the API and sets the state.

For example:

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const MyComponent = () => {
  const [data, setData] = useState({});

  useEffect(() => {
    if (!data.name) {
      // Make API call and set data
    }
  }, [data]);

  return <MyComponent />;
};

In the code above, an API call would only be made if data did not already contain a value for name.