React JS Interview Questions
1 .
List some of the cases when you should use Refs.
Following are the cases when refs should be used:
 
* When you need to manage focus, select text or media playback
* To trigger imperative animations
* Integrate with third-party DOM libraries
2 .
What were the major problems with MVC framework?
* DOM manipulation was very expensive
* Applications were slow and inefficient
* There was huge memory wastage
* Because of circular dependencies, a complicated model was created around models and views
3 .
How are Actions defined in Redux?
Actions in React must have a type property that indicates the type of ACTION being performed. They must be defined as a String constant and you can add more properties to it as well. In Redux, actions are created using the functions called Action Creators. Below is an example of Action and Action Creator:
function addTodo(text) {
       return {
                type: ADD_TODO,    
                 text    
    }
}
4 .
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.
5 .
Why is switch keyword used in React Router v4?
Although a <div> is used to encapsulate multiple routes inside the Router. The ‘switch’ keyword is used when you want to display only a single route to be rendered amongst the several defined routes. The <switch> tag when in use matches the typed URL with the defined routes in sequential order. When the first match is found, it renders the specified route. Thereby bypassing the remaining routes.
6 .
How do you tell React to build in Production mode and what will that do?
Ordinarily, you'd utilize Webpack's DefinePlugin strategy to set NODE_ENV to production. This will strip out things like prototype approval and additional notices. Over that, it's likewise a smart thought to minify your code in light of the fact that React utilizes Uglify's dead-code end to strip out advancement just code and remarks, which will radically diminish the measure of your package.
7 .
What do you understand with the term polling?
The server needs to be monitored to for updates with respect to time. The primary aim in most of the cases is to check whether novel comments are there or not. This process is basically considered as pooling. It checks for updates approximately every 5 seconds. It is possible to change this time period easily. Pooling help keeping an eye on the users and always make sure that no negative information is present on the servers. Actually, it can create issues related to several things and thus pooling is considered.
8 .
What do you mean by virtual DOM?
For all the available DOM objects in ReactJS, there is a parallel virtual DOM object. It is nothing but can be considered as the lighter version of the true copy and is powerful in eliminating the complex code. It is also used as a Blue Print for performing several basic experiments. Many developers also use it while practicing this technology.
9 .
Compare MVC with Flux?
MVC approaches are presently considered as outdated. Although they are capable to handle data concerns, controllers as well as UI, many developers found that it doesn’t properly work when applications size increases. However, they are capable to handle some of the key issues such as eliminating the lack of data integrity as well as managing the data flow which is not properly defined. On the other side, Flux works perfectly with all the sizes irrespective of their size.
10 .
How error boundaries handled in React v15?
React v15 provided very basic support for error boundaries using unstable_handleError method. It has been renamed to componentDidCatch in React v16.