31+ React Interview Questions And Answers

React's diffing algorithm is used for updating the DOM. It compares the previous and next virtual DOMs, identifies changes, and updates the real DOM efficiently. This allows for minimal DOM manipulation, improving performance.

PureComponent is similar to Component, but it handles the shouldComponentUpdate method for you. It does a shallow comparison on the state and props data to determine if re-rendering is necessary.

React relies on controlled components for form handling. The form's data control can be centralized in the state of the component. Thus, inputs receive their current values from the state, mutating the state to update inputs.

Context provides a way to pass data through the component tree without having to pass props down manually at every level. It's used for data that can be considered 'global' for a tree of React components.

To optimize performance, make sure to use the production version of React, utilize PureComponent and memo() for preventing unnecessary renders, implement lazy loading with React.lazy() and Suspense, and use shouldComponentUpdate wisely.

React Fiber is the new reconciliation engine or core algorithm in React 16. Its main goal is to enable incremental rendering of the virtual DOM.

React Fragments allow components to return multiple elements without wrapping them in an extra DOM element, such as a div.

An HOC is a function that takes a component and returns a new component. It's a pattern used for reusing component logic. For example, if you had an HOC that provided data fetching capabilities, it could be used with different components to handle data requests.

function withDataFetching(WrappedComponent) {
  return class extends React.Component {
    // Logic to fetch data on mount
    // Pass data as props to the WrappedComponent
  };
}

useRef hook can be used to directly access a DOM element. It's used, for instance, to focus an input element on component mount.

function TextInputWithFocus() {
  const inputEl = useRef(null);

  useEffect(() => {
    inputEl.current.focus();
  }, []);

  return ;
}

Render props are a technique for sharing code between React components using a prop whose value is a function. They allow you to tell a component what it should render.

You can use the useEffect hook to ensure that a component is working with updated state or props. This hook can be configured to rerun whenever certain values change.

This code sets up an effect with React's useEffect hook that logs 'Component mounted' to the console when the component mounts.

useMemo is a hook that memoizes a computed value so that it does not need to be recomputed on every render if the dependencies have not changed. useCallback is a hook that memoizes a callback function to prevent unnecessary renders.

shouldComponentUpdate is a lifecycle method in class components that allows you to decide whether the next component's update should trigger a re-render. Returning false from it prevents the component's update.

A side effect could be a data fetch on component mount. It is handled using the useEffect hook, making sure to properly cleanup on component unmount if necessary.

useEffect(() => {
  const fetchData = async () => {
    const result = await myApi.getData();
    setData(result);
  }

  fetchData();

  return () => {
    // cleanup logic here
  };
}, []);

The children prop is used to pass elements directly into components that represent where those children should be placed in the component tree. It is the content between the opening and closing tags of a component.