React JS FAQ

What do you understand by Virtual DOM? Explain its working.

A virtual DOM is a lightweight JavaScript object which originally is just the copy of the real DOM. It is a node tree that lists the elements, their attributes and content as Objects and their properties. React’s render function creates a node tree out of the React components. It then updates this tree in response to the mutations in the data model which is caused by various actions done by the user or by the system.

This Virtual DOM works in three simple steps.

  • Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.
  • Then the difference between the previous DOM representation and the new one is calculated.
  • Once the calculations are done, the real DOM will be updated with only the things that have actually changed.

What are the different phases of React component’s lifecycle?

There are three different phases of React component’s lifecycle:

  1. Initial Rendering Phase: This is the phase when the component is about to start its life journey and make its way to the DOM.
  2. Updating Phase: Once the component gets added to the DOM, it can potentially update and re-render only when a prop or state change occurs. That happens only in this phase.
  3. Unmounting Phase: This is the final phase of a component’s life cycle in which the component is destroyed and removed from the DOM.

Explain the lifecycle methods of React components in detail.

Some of the most important lifecycle methods are:

  1. componentWillMount() – Executed just before rendering takes place both on the client as well as server-side.
  2. componentDidMount() – Executed on the client side only after the first render.
  3. componentWillReceiveProps() – Invoked as soon as the props are received from the parent class and before another render is called.
  4. shouldComponentUpdate() – Returns true or false value based on certain conditions. If you want your component to update, return true else return false. By default, it returns false.
  5. componentWillUpdate() – Called just before rendering takes place in the DOM.
  6. componentDidUpdate() – Called immediately after rendering takes place.
  7. componentWillUnmount() – Called after the component is unmounted from the DOM. It is used to clear up the memory spaces.

What are synthetic events in React?

Synthetic events are the objects which act as a cross-browser wrapper around the browser’s native event. They combine the behavior of different browsers into one API. This is done to make sure that the events show consistent properties across different browsers.


What are Pure Components?

Pure components are the simplest and fastest components which can be written. They can replace any component which only has a render(). These components enhance the simplicity of the code and performance of the application.


What are the three principles that Redux follows?

  1. Single source of truth: The state of the entire application is stored in an object/ state tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.
  2. State is read-only: The only way to change the state is to trigger an action. An action is a plain JS object describing the change. Just like state is the minimal representation of data, the action is the minimal representation of the change to that data.
  3. Changes are made with pure functions: In order to specify how the state tree is transformed by actions, you need pure functions. Pure functions are those whose return value depend solely on the values of their arguments.

List down the components of Redux.

Redux is composed of the following components:

  1. Action – It’s an object that describes what happened.
  2. Reducer – It is a place to determine how the state will change.
  3. Store – State/ Object tree of the entire application is saved in the Store.
  4. View – Simply displays the data provided by the Store.

Explain the role of Reducer.

Reducers are pure functions which specify how the application’s state changes in response to an ACTION. Reducers work by taking in the previous state and action, and then it returns a new state. It determines what sort of update needs to be done based on the type of the action, and then returns new values. It returns the previous state as it is, if no work needs to be done.


What is the significance of Store in Redux?

A store is a JavaScript object which can hold the application’s state and provide a few helper methods to access the state, dispatch actions and register listeners. The entire state/ object tree of an application is saved in a single store. As a result of this, Redux is very simple and predictable. We can pass middleware to the store to handle processing of data as well as to keep a log of various actions that change the state of stores. All the actions return a new state via reducers.


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