ReactJS Training in Vadodara

The useEffect hook is one of the most important hooks in React. It allows you to perform side effects in functional components, such as fetching data from an API, setting up subscriptions, manually changing the DOM, and setting timers. useEffect runs after the component renders and can be controlled using a dependency array to limit when it should run. It effectively replaces class component lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount, offering more flexibility in a functional programming style.

useState is a hook that lets you add state management to functional components. Before hooks, only class components could have a state. With useState, developers can define state variables directly inside a function and manage them efficiently. It returns an array containing the current state value and a function to update that state. When the state changes, React automatically re-renders the component to reflect the new value, ensuring a reactive and interactive user interface.

The useRef hook returns a mutable object with a .current property that can hold any value across the lifetime of a component. Unlike state, changing a ref does not trigger a re-render. It is commonly used for accessing DOM elements directly (like focusing an input field) or preserving values between renders, such as storing the previous value of a variable, managing timers, or caching expensive computations. It acts like an instance variable in class components.

useContext is a hook that allows components to access values from a context without having to pass props through multiple levels (a process known as props drilling). It consumes the value from the nearest matching <Context.Provider> in the component tree. This is particularly helpful when dealing with global data like themes, authentication status, or user information, allowing for more maintainable and readable code in deeply nested component structures.

useMemo is a performance optimization hook that memoizes the result of an expensive computation and returns the cached result unless the dependencies have changed. It is helpful when a component performs a heavy calculation during render or re-render. By wrapping such a computation inside useMemo, React avoids recalculating the value on every render, boosting performance especially in large or complex applications.

useCallback is similar to useMemo, but it is used specifically to memoize functions. It returns a memoized version of a callback function that only changes if one of its dependencies changes. This is useful when passing callback functions to child components that are memoized, as it prevents unnecessary re-renders caused by function re-creation on every render.

The dependency array in the useEffect hook tells React when to re-run the effect. If the array is empty ([]), the effect runs only once after the initial render (similar to componentDidMount). If specific variables are included, the effect will run only when those variables change. Omitting the dependency array means the effect runs after every render, which can lead to performance issues or infinite loops if not handled carefully.

No, React hooks can only be used in functional components. They were introduced to add capabilities like state and side effects to function-based components, which were previously only available in class components. If you're using class components, you must rely on lifecycle methods like componentDidMount or this.state instead of hooks.

A custom hook is a reusable JavaScript function that starts with the prefix use and allows developers to extract and share logic between components. Custom hooks let you combine multiple built-in hooks (like useState, useEffect, useRef, etc.) into a single function that can be reused across different components. This promotes code modularity, simplifies complex components, and helps in maintaining cleaner, DRY (Don’t Repeat Yourself) codebases.

Redux is based on three core principles:

• Single Source of Truth: All application state is stored in a single object.
• State is Read-Only: The state can only be changed by dispatching actions.
• Changes are Made Using Pure Functions: Reducers specify how the state changes based on actions, ensuring predictable behavior.

connect() is a higher-order function from the react-redux library that connects React components to the Redux store. It allows components to read state and dispatch actions as props, enabling separation of UI and business logic.

• useSelector is a hook used to extract state from the Redux store.
• useDispatch returns a reference to the dispatch function, allowing components to send actions to the store.
  Together, they enable functional components to interact with Redux.

 Middleware is a function that sits between action dispatching and reducer execution. It intercepts actions to perform asynchronous tasks, logging, or conditional processing. Common middlewares include redux-thunk (for async logic) and redux-saga (for managing side effects).

 What is context in React?

 React Context provides a way to share data across components without explicitly passing props at every level. It is useful for global data like themes, user authentication, or language settings. It enhances modularity and simplifies deep component trees.

React Context provides a way to share data across components without explicitly passing props at every level. It is useful for global data like themes, user authentication, or language settings. It enhances modularity and simplifies deep component trees.

You create a context using React.createContext(). Then, you wrap your components inside a <Provider> to supply the context value. Any nested component can consume the context using the useContext() hook or <Context.Consumer> for class components.

useRef() creates a mutable reference that persists across renders. It is often used to directly interact with DOM elements or to store values without triggering a re-render. For example, it can hold a reference to an input field or store a previous state value.

Memoization is a performance optimization technique that caches the result of expensive computations. In React, it prevents unnecessary re-renders using tools like React.memo, useMemo, and useCallback. These help in avoiding recalculations or function re-creations unless dependencies change.

• useEffect is used in functional components and can mimic the behavior of componentDidMount, componentDidUpdate, and componentWillUnmount by managing side effects.
• useEffect(() => {}, []) is similar to componentDidMount().
  It provides more flexibility through dependency arrays.