Vue.js is a progressive JavaScript framework for building user interfaces. Unlike other monolithic frameworks, Vue is designed from the ground up to be incrementally adoptable.

Key Features:

• Reactive Data Binding: Automatic synchronization between data and DOM

• Component System: Reusable, self-contained components with props and events

• Virtual DOM: Efficient rendering and performance optimization

• Directives: Special attributes (v-bind, v-model, v-for) for template functionality

• Computed Properties: Cached derived data that updates reactively

• Watchers: React to data changes with custom logic

• Lifecycle Hooks: Methods that execute at specific component stages

• Single-File Components: .vue files with template, script, and style in one file

• Vue Router: Official router for single-page applications

• Vuex: State management pattern + library

• Vue DevTools: Browser extension for debugging Vue applications

Vue is particularly known for its gentle learning curve, excellent documentation, and flexibility.

The Vue instance is the root of every Vue application, created with the Vue constructor.

Lifecycle Hooks (Vue 2):

1. beforeCreate: Called immediately after instance initialization, before data observation

2. created: After instance creation - data is reactive, computed properties are set up

3. beforeMount: Before initial render and when the template is compiled

4. mounted: After the instance has been mounted and the DOM is rendered

5. beforeUpdate: When data changes, before the DOM is re-rendered and patched

6. updated: After data changes and the DOM has been re-rendered

7. beforeDestroy: Before instance is destroyed - instance is still fully functional

8. destroyed: After instance is destroyed - all directives unbound, event listeners removed

Vue 3 Composition API Lifecycle:

• setup(): Replaces beforeCreate and created

• onBeforeMount, onMounted, onBeforeUpdate, onUpdated, onBeforeUnmount, onUnmounted

Example:

new Vue({

el: '#app',

data: { message: 'Hello Vue!' },

beforeCreate() { console.log('beforeCreate') },

created() { console.log('created') },

mounted() { console.log('mounted - DOM ready') }

})

v-if vs v-show:

v-if:

• Conditionally renders the element in the DOM

• Adds/removes element completely from DOM

• Supports v-else and v-else-if

• Higher toggle cost but better initial load performance

• Use when: The condition changes infrequently, or you need conditional rendering with else blocks

v-show:

• Always renders the element in the DOM

• Uses CSS display property to show/hide (display: none)

• Does not support else blocks

• Higher initial render cost but better toggle performance

• Use when: You need frequent toggling, or the content is expensive to re-render

Example:

<!-- v-if - element removed from DOM when false -->

<div v-if="isVisible">This may not be in DOM</div>

<div v-else>Else content</div>

<!-- v-show - always in DOM, just hidden -->

<div v-show="isVisible">Always in DOM, just hidden</div>

Performance Considerations:

• Use v-if when the condition is unlikely to change at runtime

• Use v-show for elements that need to be toggled frequently

• v-if has higher toggle costs, v-show has higher initial render costs