VueJs Interview Questions & Answers

Basics of Vue.js:

1. What is Vue.js?

   • Answer: Vue.js is a progressive JavaScript framework used for building user interfaces. It is designed to be incrementally adoptable and can be easily integrated into other projects.

2. What are the key features of Vue.js?

   • Answer: Vue.js features a reactive data-binding system, a component-based architecture, directives for DOM manipulation, a virtual DOM, and a simple and flexible API.

3. Explain the concept of reactive data binding in Vue.js.

   • Answer: Reactive data binding in Vue.js means that changes in the application state are automatically reflected in the user interface, and vice versa. This is achieved through the use of the virtual DOM and a reactivity system.

4. How do you create a Vue.js instance?

   • Answer: You create a Vue.js instance using the Vue constructor. For example:

     var vm = new Vue({
       // options
     });
     

5. What is the purpose of the el option in a Vue.js instance?

   • Answer: The el option specifies the element to which the Vue instance is attached. It is often used to mount the instance on a specific DOM element.

Components:

6. What is a Vue component?

   • Answer: A Vue component is a reusable and self-contained unit of code that encapsulates a specific piece of functionality, including its template, script, and styles.

7. How do you create a Vue component?

   • Answer: Components can be created by using the Vue.component method or by defining a component with the components option in a parent component.

8. What is the lifecycle of a Vue component?

   • Answer: The lifecycle of a Vue component includes various hooks such as beforeCreate, created, beforeMount, mounted, beforeUpdate, updated, beforeDestroy, and destroyed.

9. Explain the concept of props in Vue.js.

   • Answer: Props are custom attributes that can be passed to a child component from a parent component. They allow data to flow from the parent to the child.

10. How do you emit an event from a child component to a parent component in Vue.js?

    • Answer: You can use the $emit method to emit a custom event from a child component, and then listen for that event in the parent component using the v-on directive.

Directives:

11. What are directives in Vue.js?

    • Answer: Directives are special tokens in the markup that tell the library to do something to a DOM element. Examples include v-if, v-for, and v-bind.

12. Explain the purpose of the v-model directive.

    • Answer: The v-model directive is used for two-way data binding on form elements. It creates a binding between the element and a variable in the component's data.

13. What is the purpose of the v-for directive in Vue.js?

    • Answer: The v-for directive is used for rendering a list by iterating over an array or an object. It helps in dynamically generating content based on the data.

14. Explain the v-if and v-show directives in Vue.js.

    • Answer: v-if is a conditional directive that renders or removes an element based on a condition. v-show also toggles visibility based on a condition but does not remove the element from the DOM.

15. How do you bind attributes in Vue.js?

    • Answer: Use the v-bind directive or its shorthand : to bind attributes dynamically. For example:

      <a v-bind:href="url">Link</a>
      <!-- Shorthand -->
      <a :href="url">Link</a>
      

Vue Router:

16. What is Vue Router?

    • Answer: Vue Router is the official routing library for Vue.js. It provides a way to navigate between different components in a Vue.js application.

17. How do you set up routing in Vue.js using Vue Router?

    • Answer: First, install Vue Router using npm or yarn. Then, configure routes using the VueRouter instance and use the router-view component to display the components based on the route.

18. What is dynamic routing in Vue Router?

    • Answer: Dynamic routing involves passing parameters to routes, allowing components to respond to changes in the URL and display different content based on those parameters.

19. How do you navigate between routes in Vue.js?

    • Answer: You can use the router-link component for declarative navigation or programmatically navigate using the $router object, which is available in components.

20. What is the purpose of route guards in Vue Router?

    • Answer: Route guards in Vue Router are used to control navigation by executing functions before or after a route is entered or exited. They provide a way to protect routes based on certain conditions.

State Management with Vuex:

21. What is Vuex?

    • Answer: Vuex is the official state management library for Vue.js. It enables the centralization of state management in large Vue.js applications.

22. Explain the concept of state in Vuex.

    • Answer: State in Vuex represents the application's state or data. It is stored in a single, centralized store and can be accessed by any component in the application.

23. What are mutations in Vuex?

    • Answer: Mutations in Vuex are functions that are responsible for modifying the state. They are synchronous and must be committed to change the state.

24. How do actions differ from mutations in Vuex?

    • Answer: Actions in Vuex are similar to mutations but are asynchronous and can contain business logic. They are dispatched, and then mutations are committed to change the state.

25. Explain the role of getters in Vuex.

    • Answer: Getters in Vuex are used to compute derived state or perform calculations on the state. They are similar to computed properties in Vue components.

Forms and Validation:

26. How do you handle forms in Vue.js?

    • Answer: Forms in Vue.js can be handled by using the v-model directive for two-way data binding. You can also use the @submit event to handle form submissions.

27. What is the purpose of the v-model directive in form elements?

    • Answer: The v-model directive provides two-way data binding on form elements, allowing the value of the element to be bound to a variable in the component's data.

28. How do you perform form validation in Vue.js?

    • Answer: Form validation can be done by using conditional rendering based on the state of form fields, or by using third

-party libraries like Vuelidate or VeeValidate.

29. Explain the concept of custom input components in Vue.js.

    • Answer: Custom input components are components created to encapsulate the behavior of a specific form input. They often use the v-model directive to enable two-way data binding.

30. How do you prevent the default behavior of a form submission in Vue.js?

    • Answer: Use the .prevent modifier on the @submit event to prevent the default behavior of form submission. For example:

      <form @submit.prevent="handleSubmit">
        <!-- form fields -->
      </form>
      

Filters and Mixins:

31. What are filters in Vue.js?

    • Answer: Filters in Vue.js are used to format data in the template. They are similar to pipes in Angular or filters in AngularJS.

32. How do you create a custom filter in Vue.js?

    • Answer: You can create a custom filter by defining a global filter using Vue.filter or by using local filters within a component.

33. What is the purpose of mixins in Vue.js?

    • Answer: Mixins in Vue.js are a way to reuse and share component options. They allow you to encapsulate common functionality and include it in multiple components.

34. How do you use a mixin in a Vue component?

    • Answer: Use the mixins option in a component to include a mixin. For example:

      const myMixin = {
        // mixin options
      };
      
      new Vue({
        mixins: [myMixin],
        // component options
      });
      

35. Explain the concept of scoped slots in Vue.js.

    • Answer: Scoped slots in Vue.js allow a child component to pass data to a parent component. They provide a way for a parent component to customize the content rendered by a child component.

Vue CLI:

36. What is Vue CLI?

    • Answer: Vue CLI is a command-line interface for Vue.js that provides a standardized project structure, build tools, and development server. It simplifies the process of setting up, developing, and building Vue.js applications.

37. How do you create a new Vue.js project using Vue CLI?

    • Answer: Use the following commands to create a new Vue.js project with Vue CLI:

      vue create my-project
      

38. What is the purpose of the vue.config.js file in a Vue.js project?

    • Answer: The vue.config.js file is used to configure the Vue CLI service. It allows developers to customize the build process, configure webpack, and set various options for their Vue.js project.

39. Explain the concept of Vue CLI plugins.

    • Answer: Vue CLI plugins are extensions that can be added to a Vue CLI project to enhance its functionality. They can add features, configure webpack, or set up additional tools.

40. How do you serve a Vue.js project in development mode using Vue CLI?

    • Answer: Use the following command to serve a Vue.js project in development mode:

      npm run serve
      

Unit Testing in Vue.js:

41. What testing tools are commonly used for unit testing in Vue.js?

    • Answer: Popular testing tools for unit testing in Vue.js include Jest, Mocha, Chai, and Vue Test Utils.

42. How do you write unit tests for Vue components?

    • Answer: Unit tests for Vue components are typically written using testing libraries such as Jest or Mocha along with Vue Test Utils. You can create test files with a .spec.js or .test.js extension.

43. Explain the concept of snapshot testing in Jest for Vue components.

    • Answer: Snapshot testing in Jest involves capturing a snapshot of the rendered output of a component and comparing it with a stored snapshot. It helps detect unexpected changes in the UI.

44. What is the purpose of the shallowMount method in Vue Test Utils?

    • Answer: The shallowMount method is used to create a shallow wrapper for a Vue component during testing. It only renders the component itself, not its child components.

45. How do you mock dependencies in Vue.js unit tests?

    • Answer: You can use tools like Jest to mock dependencies by using the jest.mock function or by providing custom implementations for specific modules.

Vue 3:

46. What are the major features introduced in Vue 3?

    • Answer: Vue 3 introduces the Composition API, a new, function-based API that provides more flexibility in organizing and reusing component logic. It also features improved performance and a better TypeScript integration.

47. What is the Composition API in Vue 3?

    • Answer: The Composition API is a set of functions that allows developers to organize component logic in a more modular and reusable way. It provides a better alternative to the Options API.

48. Explain the concept of the ref function in Vue 3.

    • Answer: The ref function in Vue 3 is used to create a reactive reference to a value. It is commonly used for creating and managing reactive data in the Composition API.