Javascript Technical Interview Questions

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React’s popularity has grown significantly in popularity in recent years.

According to the 2019 Stack Overflow Developer Survey, which studied development tools and practices used by over 90,000 programmers, React. js was the second most popular web framework among respondents.

React has become so popular because it gives developers the ability to quickly build scalable applications, a value that many companies around the world have recognized. As a result, React interview questions are common in web development job interviews.

In this guide, we explore the 15 most common React.js interview questions in job interviews and give you tips on how to answer them like an expert. We’ll focus on theoretical questions designed to test your knowledge of the React ecosystem.

# 1: what is React JS

In an interview this might seem like a simple question to ask; you may even think it’s a gimmick. However, it is a very common question in interviewing people. ’hiring techniques precisely because it’s so simple.

Here are a few points you should cover in your answer:

  • React is a library Front-end JavaScript for building web apps.
  • React is declarative, which means you can design simple views for each state of a We app. b.
  • React uses components, which allow you to create reusable elements on a web page.

# 2: What is the difference between accessories and condition ?

Accessories and condition are two essential concepts you should be familiar with in order to pass data through components .

State refers to the default data value in a React component. The state of a component can change over time and can be changed manually by a developer. Accessories, on the other hand, describe how a React component is configured. The accessories do not change.

# 3: what are React components?

Components are a core feature of React - they are the building blocks of all React-based applications. You need to know this and be able to describe what a component is.

Components allow developers to break up a user interface into smaller parts that can be reused. React renders each component on a web page only when needed, allowing applications to display faster.

# 4: When are keys used in React ? < / h2>

Keys are used in React to identify unique elements in the virtual DOM Using keys React can optimize rendering because the framework can recycle existing elements loaded into the DOM.

Suppose we have two instances of a component on a th web page. Without keys, React would render both elements in the DOM and not rearrange the El DOM elements. But, when using keys, React rearranges the DOM elements, which provides a number of performance benefits to an application.

# 5: What are the most common approaches to styling a React component?

There are several ways to model a React component, and you should be able to describe how they work. Here are the main ways React components are styled:

  • CSS classes. React - allows you to use traditional CSS class names to define styles for a component
  • Inline CSS: Inline CSS allows you to define styles in a particular element of a web page.
  • JavaScript Modules: JavaScript modules such as styled-jsx and stylized components are also used to style components.
  • Preprocessors: Preprocessors like Sass and Less are another option for styling a component.

# 6: When you need to use a class component instead of a functional component

There are two component types in React: class and functional.

Functional components are defined by a component with immutable properties (accessories). Class components, on the other hand, are more complex components that allow you to manage the state of a component.

You must use class components on a functional component when you need to manage the state of a component. component or use component lifecycle methods.

# 7: What Happens During the Lifecycle of a React Component ?

The lifecycle of a React component are the events that occur from the creation of a component until its destruction.

Three main parts make up the lifecycle of a React component: initialization, state update, and destruction.

For example, when creating a component, you can use methods such as componentDidMount () to add event listeners to the component. This happens after the initialization of a component. Then you can use componentWillUnmount () to outrun an event listener when the component is about to be removed from the web page.

# 8: What is a virtual DOM ? How does it work ?

When an HTML document is rendered by a web browser, a representative tree called the Document Object Model (DOM). This provides the browser with all the information it needs to properly display a web page.

By default, a web page will use HTML to update its DOM and make changes to the page (like updated content that appears on a website). React, on the other hand, creates a virtual DOM, which is a copy of the "real" web page. DOM.

This Virtual DOM can be used to change the appearance of a web page based on an action taken by a user. Then, once a change is made in the virtual DOM, the virtual DOM will be compared to the "real" DOM ‚Äã‚Äãand the "real" DOM ‚Äã‚Äãwill be updated with the relevant changes.

The virtual DOM offers a number of performance benefits for a React application, because the entire " real " DOM ; DOM does not need to be reloaded every time a change is made to a site.

# 9: What is JSX ?

JSX is a tool that allows you to embed unprocessed HTML templates into JavaScript code.

JSX cannot be read by browser, so you need to use tools like webpack and Babel to translate it into browser readable JavaScript.

JSX is used because it makes it easier to work with HTML and JavaScript on a website. Developers don’t need to use JSX, but it’s a great way to reduce the complexity of an application.

# 10: these are stateless components

Stateless components are a type of reusable component whose rendering is based only on the properties passed to the component.

While normal components use both props and state, stateless components only use props. This means that a component will display based on the properties initially provided and will not change over time.

# 11: What tools can you use to make a React app more accessible ?

There are two main types of tools you can use to identify accessibility issues in an application. The first is a static analysis tool, such as ESLint, which allows you to analyze your code at the component level.

The second type of tool you can use is a browser accessibility tool. such as Google Lighthouse, which runs app-level accessibility testing. These plugins allow you to uncover bigger accessibility issues because they simulate how a user will interact with a given web application.

# 12: What is the difference between React and React Native?

Both frameworks have the word "React" in their name, but they serve different purposes.

React JS is a front-end JavaScript library used to create transparent user interfaces. On the other hand, React Native is an open source framework that allows developers touse React to build mobile apps for iOS and Android. React Native is based on React, but it’s a separate framework.

# 13: Qu ’ is Redux ? What is it used for ?

Redux is a library that complements React and allows developers to build easy-to-test web applications. Redux can be used with React or any other JS UI library and comes with pre-packaged debugging tools.

The Redux library is often used because it adds more structure to the code, making it easier to maintain the code. code base. Redux can help with performance monitoring such as events and state changes. This data can give you more information about how your application works and how it can be made more efficient.

# 14: Why should you use React over other UI frameworks like Ember ?

During a job interview, the employer wants to know if you understand not only how React is used, but why it is used in the first place.

Here are some of the reasons you might mention:

# 15: Explain the purpose of two commonly used lifecycle methods in React.

React offers a wide range of methods that run at different points in a component’s lifecycle. Here is a list of some of these methods and descriptions of how they work:

There are other lifecycle methods that React offers and you are free to describe the ones you know if you are asked this question.


An effective way The way to prepare for a technical job interview is to practice the answers to the most common questions.

Exercising the questions discussed in this article should help you prepare for your next technical interview and increase your chances of success.

You can also ask a friend or colleague to question you. This is effective because the other person can give you their opinion on your answers.

Keep in mind that in a job interview for a web developer role, you will likely be asked for more than React. They may ask you about JavaScript or another framework the company uses and lists in the job description. However, if you’re about to interview for a role that requires a React skill, the questions we’ve covered in this article will give you a good idea of ‚Äã‚Äãwhat to expect.

Now all you need to do to complete your interview is practice !

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Javascript Technical Interview Questions __del__: Questions


How can I make a time delay in Python?

5 answers

I would like to know how to put a time delay in a Python script.


Answer #1

import time
time.sleep(5)   # Delays for 5 seconds. You can also use a float value.

Here is another example where something is run approximately once a minute:

import time
while True:
    print("This prints once a minute.")
    time.sleep(60) # Delay for 1 minute (60 seconds).


Answer #2

You can use the sleep() function in the time module. It can take a float argument for sub-second resolution.

from time import sleep
sleep(0.1) # Time in seconds

Javascript Technical Interview Questions __del__: Questions


How to delete a file or folder in Python?

5 answers

How do I delete a file or folder in Python?


Answer #1

Path objects from the Python 3.4+ pathlib module also expose these instance methods:


Removing white space around a saved image in matplotlib

2 answers

I need to take an image and save it after some process. The figure looks fine when I display it, but after saving the figure, I got some white space around the saved image. I have tried the "tight" option for savefig method, did not work either. The code:

  import matplotlib.image as mpimg
  import matplotlib.pyplot as plt

  fig = plt.figure(1)
  img = mpimg.imread(path)

  extent = ax.get_window_extent().transformed(fig.dpi_scale_trans.inverted())
  plt.savefig("1.png", bbox_inches=extent)


I am trying to draw a basic graph by using NetworkX on a figure and save it. I realized that without a graph it works, but when added a graph I get white space around the saved image;

import matplotlib.image as mpimg
import matplotlib.pyplot as plt
import networkx as nx

G = nx.Graph()
pos = {1:[100,120], 2:[200,300], 3:[50,75]}

fig = plt.figure(1)
img = mpimg.imread("image.jpg")

nx.draw(G, pos=pos)

extent = ax.get_window_extent().transformed(fig.dpi_scale_trans.inverted())
plt.savefig("1.png", bbox_inches = extent)


Answer #1

You can remove the white space padding by setting bbox_inches="tight" in savefig:


You"ll have to put the argument to bbox_inches as a string, perhaps this is why it didn"t work earlier for you.

Possible duplicates:

Matplotlib plots: removing axis, legends and white spaces

How to set the margins for a matplotlib figure?

Reduce left and right margins in matplotlib plot


Answer #2

I cannot claim I know exactly why or how my “solution” works, but this is what I had to do when I wanted to plot the outline of a couple of aerofoil sections — without white margins — to a PDF file. (Note that I used matplotlib inside an IPython notebook, with the -pylab flag.)

plt.subplots_adjust(top = 1, bottom = 0, right = 1, left = 0, 
            hspace = 0, wspace = 0)
plt.savefig("filename.pdf", bbox_inches = "tight",
    pad_inches = 0)

I have tried to deactivate different parts of this, but this always lead to a white margin somewhere. You may even have modify this to keep fat lines near the limits of the figure from being shaved by the lack of margins.


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