19 Best Bootstrap Interview Questions & Answers

If you are looking for Bootstrap interview questions and answers, look no further. We've compiled the absolute best Bootstrap interview questions to help you prepare for your upcoming front-end engineering interview. As you'll be interviewing with the technology department, it's imperative that you spend time thinking through how you might answer questions to programming quizzes.

Bootstrap is a front-end framework that was created by Mark Otto while working at Twitter. It is part of the original "Twitter Blueprint", which was designed to help designers and engineers more easily work together in a consistent fashion. Mark realized that the value of this superseded Twitter and open-sourced the framework he developed. It gained in popularity very quickly and is still one of the most widely used front-end frameworks on the market.

Why Is Bootstrap Important?

Front-end frameworks like Bootstrap are used at nearly every company that has an online presence. As such, technical interviews for front-end developers are almost certain to involve questions about Bootstrap’s usage and advantages. Bootstrap provides a consistent way for organizations to layout their web-pages, design page elements, and its usage helps an organization onboard developers quickly who are already familiar with its features. For this reason, the feature set, strategies, and advantages to using Bootstrap are essential knowledge for any technical role. We’ve compiled these questions to assess a candidate’s technical experience with Bootstrap, as well as their practical understanding of why Bootstrap is an important and useful tool.

19 Bootstrap Interview Questions & Answers

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1. What is Bootstrap?

Bootstrap is a front-end framework that was developed by Twitter. Its primary purpose is to make the styling process for an application consistent and fast. Additionally, Bootstrap provides javascript-powered elements like dropdown menus, accordions, tooltips, and more in order to provide a consistent experience for users and a faster development cycle for engineers using the tool.

2. In a practical sense, what is a front-end styling framework like Bootstrap used for?

Bootstrap offers convenient CSS styles to make styling HTML markup faster for developers. Common use cases of stylesheets are pre-written in Bootstrap, so developers do not have to implement their own custom versions. With Bootstrap, elements like buttons, headers, alert-boxes, modals, labels, etc are all handled with a consistent and understandable CSS class syntax.

3. What are some examples of common stylesheet use cases that are pre-written in Bootstrap to save time?

One of the common use cases that is covered by Bootstrap are grids, for example. Most web pages or web application have their content cut into columns. Rarely does everything on a webpage stretch across the entire screen. While using Bootstrap, rather than writing your own CSS to define a grid structure, you already have CSS classes available.

Buttons are another common use case of CSS that is prewritten in Bootstrap. With the .btn-default CSS class, for example, an aesthetic blue button with a drop-shadow will be rendered without needing to write any CSS.

Other types of css classes are for dropdown menus, hero banners, slideshows, labels, accordions, and more.

4. What is the set of CSS classes used in Bootstrap to define grid columns?

The grid structure defined by Bootstrap is broken into twelfths. Meaning, you can create a grid on your page that has 12 columns, or a number of columns that can be divided into 12.

The CSS classes to create a column are .col-md-1, .col-md-2, … , col-md-11, col-md-12.

5. If your HTML markup had 3 < div > elements, what Bootstrap CSS class would you give to each element in order to make a 3 column layout?

You would add the class .col-md-4, (or .col-sm-4 or another variant) to each < div > element. This works because, as noted previously, Bootstrap grid structure is divided into twelfths. Because 12 divided by 4 is equal to 3, .col-md-4 will create a three column layout.

6. What is a practical benefit to an organization using a Front-end framework like Bootstrap?

The practical advantage has several facets.

The styles and appearance of your app is guaranteed consistency, as long as the developers on your team use the framework. Button and label sizes, grid column widths, headers, colors, etc will be consistent and give a cohesive feel to the app.

Less code bloat. Without the usage of a Front-end framework for your styles, as your organization grows and developers separate into teams, these teams will naturally begin to re-implement the same sorts of logic for their own needs. The result of this is a bloated set of stylesheets which are redundant in their style descriptions. Bootstrap removes this risk by providing a consistent set of styles which do not need to be rewritten.

Clear and fast upgrades + modification paths to styling are made possible because there is one “source of truth” for your application’s look and feel. If your organization decides a button color should be changed, there is one place to modify that style.

Easier onboarding for developers. Front-end frameworks like Bootstrap are extremely common so many developers are already knowledgeable in their features. This means a new developer is entering an environment where he/she feels comfortable writing code immediately, as opposed to an environment where he/she must learn a proprietary set of styles for the application they’ll be working on.

Documentation! Front end frameworks are well documented, and developers within your organization who need to learn more about how bootstrap works have clear sets of guides, community feedback, and examples to work with and understand the technology. Often, with a homegrown set of stylesheets, this sort of documentation falls behind, leaving developers within your organization facing a much steeper learning curve.

7. Is Bootstrap a good tool for mobile development?

Yes, Bootstrap is a tool designed for mobile development because it has built-in style sheets to support responsive design and mobile-friendly layouts. These features will not need to be re-implemented by your developers working with bootstrap. For example, using the standard column classes (e.g. col-md-3) will not only divide your page into a column structure on a desktop browser, but the columns will automatically fall below one another for mobile-friendly design on smaller devices.

8. What are the various button styles available in Bootstrap?

Bootstrap has seven styles aimed at coloring or modifying the size and emphasis of buttons within your app:

.btn-default
.btn-primary
.btn-success
.btn-info
.btn-warning
.btn-danger
.btn-link

9. Describe Bootstrap alert elements.

Alert elements in bootstrap are rectangular text containers with colorful/stylized borders to represent the importance or context of an alert. For example, alerts are available in colors like green, blue, or red to show positive, neutral, or warning alerts in your application. The CSS styles used to implement these alerts are:

.alert-success, .alert-info, .alert-warning, .alert-danger.

10. What is the meaning of “normalize” in the context of Bootstrap.

Bootstrap normalize ensures cross browser consistency when using Bootstrap styles. Depending on which browser is used to open a web page, default styling for that page may be different. For example, Firefox might give different default H1 sizes or different default page margins than Chrome. Without Bootstrap normalize, these discrepancies run the risk of your site or web application displaying differently in each browser despite the use of Bootstrap. In the cases described above, Normalize will ensure, for example, that H1 tags and default page margins are identical in whichever browser is used to access your page.

11. Does Bootstrap require any dependencies?

Bootstrap CSS styles can be used without any dependencies installed; however, this will limit your usage of bootstrap to only its CSS style functionality. In addition to CSS styles, Bootstrap can provide Javascript-powered elements like dropdown menus, accordions, carousels, and more. In order to use Bootstrap’s Javascript functionality, jQuery is a dependency.

12. What are glyphicons, explain their use in Bootstrap.

Glyphicons are a set of icons that can be added to your pages using Bootstrap CSS classes. There is a large library of glyphicon graphics that can be used, each of which has its own classname. The typical format of glyphicon classnames is as follows: “glyphicon glyphicon-search” (This specific example, if added as a class to a DOM element on your page, will display a magnifying glass icon.)

13. Explain the concept of a Jumbotron element in Bootstrap.

A Jumbotron element is a wrapper element which increases the default font sizes, paddings, and margins of the elements like H1 or H2 which are placed inside of it. It is intended to be used for landing page content which requires a larger emphasis than the rest of your site or application.

14. Explain the concept of a Modal element in Bootstrap.

A modal element is a rectangular container that can be used to “pop up” alerts to the user, and give them button options to confirm or cancel choices. A modal element renders with a faded background to occlude the rest of your webpage so that the modal’s content is front and center for your users. Modals are a great way to catch users’ attention with a prompt before taking an important or irreversible action in your application.

15. What are the contextual classes in the Bootstrap’s progress bar? Explain how you might use them.

The contextual classes in Bootstrap have the names of success, info, warning, and danger. By appending these classes to the progress class, (e.g. .progress-success), the color of the Bootstrap progress bar will appear as green, blue, yellow, or red, respectively.

These can be useful, intuitive indicators to a user about the progress of a given operation has been successful or not. For example, if an image upload fails and its progress is being tracked with a Bootstrap progress bar, appending .progress-danger is a simple way to represent this failed state to the user.

16. Explain the usage of responsive utility classes in Bootstrap. Give an example of one such class.

Responsive utility classes are a way to make mobile development easier. For example, perhaps as a developer you have an element that should display on your page only if the user is viewing your page on a mobile device. In this case, you can use a responsive utility class to indicate that the element should disappear above some given screen resolution. An example of one such utility class is .visible-xs; applying this class to an element will make it so the element is visible only on mobile-phone sized screens.

17. What is the proper usage of the elements .container, .row, and .col in Bootstrap?

Container elements are intended to wrap your entire page, while row elements are intended to wrap sets of columns. For example, one example of correct usage of these elements is as follows:

18. What would be the output of the HTML and Bootstrap CSS usage in question 18?

This HTML & CSS would render a blank page with the words “Hello” and “World”, separated into two identical width columns. This is because the .col-md-6 CSS class describes a 50% width column. (Remember, the Bootstrap grid system is divided into twelfths).

19. Assume for your project that you only want to use a subset of Bootstrap’s features. What might be your line of reasoning, and what would be the most efficient way to accomplish this?

Bootstrap has an enormous number of useful features, yet they might not all apply to your specific project. For example, if you are using an advanced front-end Javascript framework like React or Angular, it might be preferable to use that framework to render Javascript elements like modals and dropdowns which are also offered in Bootstrap. Doing so can reduce the file size of your application, removing bloat and unused code from your application’s source.

The most practical way to accomplish this is by using Bootstrap’s customize feature. Using customize, you can select the features pertinent to your project, and download a condensed version of Bootstrap that excludes the features you do not need.

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