50 Vue Interview Questions and Answers

Vue.js is a progressive JavaScript framework used for building user interfaces. It's designed to be incrementally adoptable and can easily scale between a library and a full-featured framework. Vue.js focuses on the view layer and offers a reactive and composable web interface.

Vue CLI (Command Line Interface) is the standard tooling for Vue.js development. It provides a full system for rapid Vue.js development, including project scaffolding, a runtime dependency, and a build setup with webpack. It offers features like hot-reload, lint-on-save, and production-ready builds.

Vue Components are reusable Vue instances with custom elements that encapsulate HTML, CSS, and JavaScript functionality. They help in building large-scale applications by breaking the UI into independent, reusable pieces. Components can be registered globally or locally.

Vue's reactivity system automatically tracks dependencies and updates the DOM when data changes. It uses getter/setter methods to make data properties reactive. When a component initially renders, Vue collects dependencies, and when data changes, it triggers re-rendering of affected components.

A Vue instance is the root of a Vue application created using Vue constructor. It accepts an options object that can include data, methods, lifecycle hooks, and more. When created, it starts the reactivity system and renders the template.

v-model is a two-way data binding directive in Vue that creates a relationship between form inputs and data in the component. It automatically syncs user input with component data, making form handling easier. It's commonly used with input, select, and textarea elements.

Lifecycle hooks are special methods that allow you to run code at specific stages of a Vue component's lifecycle. Common hooks include created(), mounted(), updated(), and destroyed(). They enable performing actions when components are created, mounted to DOM, updated, or destroyed.

Vuex is Vue's official state management pattern and library. It serves as a centralized store for all components in an application, with rules ensuring state can only be mutated in a predictable fashion. It's particularly useful for managing shared state in large applications.

Vue Router is the official routing library for Vue.js. It enables navigation between pages in a Vue application, supports dynamic route matching, nested routes, and route guards. It integrates deeply with Vue.js core to make building Single Page Applications easier.

Vue directives are special attributes with the v- prefix that apply reactive behavior to rendered DOM. Common directives include v-if for conditional rendering, v-for for list rendering, v-bind for attribute binding, and v-on for event handling.

v-if conditionally renders elements by actually adding/removing them from the DOM, while v-show only toggles the display CSS property. v-if has higher toggle costs but better initial render if condition is false, while v-show has higher initial render costs but better toggle performance.

Vue components can communicate through: props (parent to child), events (child to parent using $emit), provide/inject API (for deeply nested components), Vuex (global state management), and event bus (for unrelated components). Each method has specific use cases and trade-offs.

Mixins are a flexible way to distribute reusable functionality for Vue components. They can include any component options (data, methods, lifecycle hooks, etc.). When a component uses a mixin, all mixin options are merged into the component's own options. Useful for code reuse and maintaining DRY principles.

Scoped slots are a feature that allows passing template fragments to child components and accessing child component data in these fragments. They're useful when you want the parent component to have control over a part of the child component's template while accessing child data.

Computed properties are cached based on their reactive dependencies and only re-evaluate when dependencies change, while methods run every time they're called. Computed properties are more efficient for expensive operations that don't need constant re-evaluation.

The Virtual DOM is a lightweight JavaScript representation of the actual DOM. Vue uses it to perform efficient diff computations and minimize actual DOM manipulations. When state changes, Vue creates a new Virtual DOM tree, compares it with the old one, and updates only what's necessary.

Watchers (watch option) are used to respond to data changes and perform asynchronous operations. Unlike computed properties, watchers are more suitable for operations that need to be performed when specific data changes, like API calls, or complex data changes that need side effects.

Dynamic components allow switching between multiple components using the element with 'is' attribute. They're useful for building tabs, wizards, or any interface where components need to be swapped dynamically. Can be optimized using 'keep-alive' for better performance.

Vue apps can be optimized by: using key attribute properly, implementing lazy loading for routes, using keep-alive for frequently toggled components, avoiding unnecessary component re-renders, code splitting, proper use of v-show vs v-if, and utilizing production builds.

Filters are used to format text output in Vue 2. They can be used in mustache interpolations and v-bind expressions. In Vue 3, filters are removed, and it's recommended to use computed properties or methods instead for text formatting needs.

The provide/inject API in Vue enables passing data deeply through component hierarchy without prop drilling. Parent components 'provide' data, and any child component can 'inject' it. While powerful, it makes component coupling less obvious and should be used carefully.

Custom directives are used when you need direct DOM manipulation. They have hooks like bind, inserted, update, and unbind (Vue 2) or mounted, updated, unmounted (Vue 3). Useful for reusable DOM manipulations that can't be easily achieved with standard directives.

Render functions are JavaScript alternatives to templates, offering more programmatic power. They use createElement (h) function to create Virtual DOM nodes. They're useful for complex dynamic rendering logic that's hard to express in templates.

Form validation can be handled through: custom validation logic with computed properties and watchers, third-party libraries like Vuelidate or VeeValidate, or using HTML5 validation attributes with v-model. Each approach has different complexity and feature trade-offs.

The Composition API is a new feature in Vue 3 that provides a way to organize component logic by feature rather than by options type. It offers better TypeScript support, code reuse capabilities, and more flexible composition of component logic.

Authentication in Vue typically involves: setting up route guards, managing auth state (often in Vuex), handling JWT tokens, implementing login/logout logic, and protecting routes. Can use navigation guards in Vue Router to check auth status before route changes.

Mixins merge component options while composables (Vue 3) use the Composition API to share logic. Composables are more flexible, have better type inference, and avoid naming conflicts. They represent the modern approach to code reuse in Vue.

Vue provides errorHandler config for global error handling, errorCaptured hook for component-level errors, and try-catch in async methods. These can be used to implement error boundaries, log errors, and provide fallback UI for better user experience.

Vue components can be tested using: Unit tests with Vue Test Utils for isolated component testing, Integration tests for component interactions, E2E tests with tools like Cypress. Testing usually involves mounting components, simulating user interactions, and asserting results.

Slots are content distribution APIs in Vue. Types include: default slots for basic content projection, named slots for multiple distribution points, and scoped slots for passing data back to parent scope. They enable flexible and reusable component compositions.

Render functions provide programmatic way to create HTML with JavaScript instead of templates. They offer more flexibility and power by using createElement (h) function. JSX can be used with render functions for a React-like syntax. While templates are easier to read and maintain, render functions are better for complex dynamic rendering logic and programmatic DOM manipulation.

Advanced state management can involve combining Vuex with other patterns like: Module federation for micro-frontends, custom reactive stores using Vue 3's reactivity API, state machines for complex state transitions, and event sourcing patterns. Each approach needs careful consideration of scaling, maintenance, and debugging implications.

Vue uses Proxy (Vue 3) or Object.defineProperty (Vue 2) to intercept property access/modifications. It maintains a dependency tracking system where each reactive property has a dependency collector. When properties are accessed during rendering, dependencies are collected; when modified, affected components are re-rendered efficiently.

Micro-frontend implementation with Vue can use: Module Federation with Webpack 5, custom event-based communication between apps, shared dependencies management, runtime integration via Web Components, and smart routing strategies. Need to consider load time, shared state, and consistency in styling/UX.

Large-scale optimization involves: Code splitting with dynamic imports, Virtual Scrolling for large lists, Worker threads for computation, Service Worker for caching, Tree-shaking unused code, Lazy loading routes and components, Implementing proper memoization, and Strategic component decomposition for better render performance.

Advanced composition patterns include: Higher-Order Components (HOCs), Renderless Components for logic reuse, Compound Components for complex UIs, Controlled Components for form handling, and Provider Components for dependency injection. Each pattern solves specific composition challenges.

Advanced reactivity uses include: Creating custom reactive stores, implementing computed properties with side effects, handling deep reactivity for complex objects, managing external state sources, and building reactive utilities. The composition API enables more granular reactivity control.

Advanced routing includes: Meta fields for route metadata, Navigation Guards for complex auth flows, Dynamic Route Matching patterns, Lazy Loading with custom loading states, Route Level Code Splitting, and handling of deep linking in mobile apps. These enable sophisticated navigation control.

Advanced testing involves: Component integration testing, E2E testing with Cypress/Playwright, Testing complex user interactions, Mocking complex dependencies, Testing async operations, Performance testing, and Snapshot testing for UI regression. Need to balance coverage with maintenance.

Complex forms require: Dynamic form generation, Multi-step form workflows, Form state management patterns, Custom validation logic, File upload handling, Rich text editing, and Auto-save functionality. Often involves combining multiple techniques and possibly external form libraries.

State persistence strategies include: Local Storage integration with Vuex, IndexedDB for larger datasets, Custom persistence layers, State hydration patterns, Offline-first architecture, and Sync mechanisms with backend. Need to consider security and performance implications.

Advanced animations involve: Page transitions, List animations, Route-based animations, State-driven animations, WebGL integrations, and complex choreographed sequences. Often requires combining Vue's transition system with external animation libraries.

Complex API handling includes: Request cancellation patterns, Retry logic, Cache strategies, Real-time updates with WebSocket, GraphQL integration, Error boundary implementation, and API state management. Need to consider error handling and performance optimization.

Advanced error handling involves: Global error boundaries, Component-level error handling, Custom error tracking, Recovery mechanisms, Graceful degradation, and User feedback systems. Must consider both development and production scenarios.

Code splitting strategies include: Route-based splitting, Component-based splitting, Module federation, Dynamic imports with custom loading states, Shared chunks optimization, and Vendor chunk management. Balance between initial load time and overall performance.

Security implementations include: XSS prevention, CSRF protection, Content Security Policy setup, Secure state management, Authentication patterns, and Sanitization strategies. Must consider both frontend and API security aspects.

Dependency management involves: Proper module architecture, Circular dependency prevention, Dynamic dependency injection, Lazy loading patterns, and Dependency optimization techniques. Need to balance modularity with performance.

State sync patterns include: Real-time updates, Offline-first architecture, Optimistic updates, Conflict resolution strategies, and Multi-tab synchronization. Must consider network conditions and data consistency.

Performance monitoring includes: Custom performance metrics, Runtime performance tracking, Memory leak detection, Bundle size optimization, and Real user monitoring. Need to establish performance budgets and monitoring strategies.

Advanced communication patterns include: Event bus alternatives, Cross-component state management, Pub/sub patterns, Portal-based communication, and Web Worker integration. Must consider scalability and maintenance implications.