VueJS #1 Fundamental Topics

Vue is a JavaScript framework for building user interfaces. It builds on top of standard HTML, CSS and JavaScript, and provides a declarative and component-based programming model that helps you efficiently develop user interfaces, be it simple or complex.

What is Vue?

Vue is a JavaScript framework for building interfaces. It buids on top of 3 standards:

HTML
CSS
JavaScript

and provides a declarative, component-based programming model that helps you efficiently develop user interfaces of any complexity.

Here is a minimal example:

import { createApp } from 'vue'

createApp({
  data() {
    return {
      count: 0
    }
  }
}).mount('#app')

<div id="app">
  <button @click="count++">
    Count is: {{ count }}
  </button>
</div>

The above example demonstrates the two core features of Vue:

Declarative Rendering: extends standard HTML with a template syntax that allows us to declaratively describe HTML output based on JavaScript state.
Reactivity: automatically tracks JavaScript state changes and efficiently updates the DOM when changes happen.

Prerequisites

The rest of the documentation assumes basic familiarity with HTML, CSS, and JavaScript. If you are totally new to frontend development, it might not be the best idea to jump right into a framework as your first step - grasp the basics and then come back! You can check your knowledge level with these overviews for JavaScript, HTML and CSS if needed. Prior experience with other frameworks helps, but is not required.

The Progressive Framework

Vue is designed to be flexible and incrementally adoptable. Depending on your use case, Vue can be used in different ways:

Enhancing static HTML without a build step
Embedding as Web Components on any page
Single-Page Application (SPA)
Fullstack / Server-Side Rendering (SSR)
Jamstack / Static Site Generation (SSG)
Targeting desktop, mobile, WebGL, and even the terminal

This is why we call Vue “The Progressive Framework”: it’s a framework that can grow with you and adapt to your needs.

Single-File Components

In most build-tool-enabled Vue projects, we author Vue components using an HTML-like file format called Single-File Component (also known as *.vue files, abbreviated as SFC). A Vue SFC, as the name suggests, encapsulates the component’s logic (JavaScript), template (HTML), and styles (CSS) in a single file. Here’s the previous example, written in SFC format:

<script>
export default {
  data() {
    return {
      count: 0
    }
  }
}
</script>

<template>
  <button @click="count++">Count is: {{ count }}</button>
</template>

<style scoped>
button {
  font-weight: bold;
}
</style>

API Styles

Vue components can be authored in two different API styles: Options API and Composition API.

Options API

With Options API, we define a component’s logic using an object of options such as data, methods, and mounted. Properties defined by options are exposed on this inside functions, which points to the component instance:

<script>
export default {
  // Properties returned from data() become reactive state
  // and will be exposed on `this`.
  data() {
    return {
      count: 0
    }
  },

  // Methods are functions that mutate state and trigger updates.
  // They can be bound as event handlers in templates.
  methods: {
    increment() {
      this.count++
    }
  },

  // Lifecycle hooks are called at different stages
  // of a component's lifecycle.
  // This function will be called when the component is mounted.
  mounted() {
    console.log(`The initial count is ${this.count}.`)
  }
}
</script>

<template>
  <button @click="increment">Count is: {{ count }}</button>
</template>

Composition API

With Composition API, we define a component’s logic using imported API functions. In SFCs, Composition API is typically used with <script setup>. The setup attribute is a hint that makes Vue perform compile-time transforms that allow us to use Composition API with less boilerplate. For example, imports and top-level variables / functions declared in <script setup> are directly usable in the template.

Here is the same component, with the exact same template, but using Composition API and <script setup> instead:

<script setup>
import { ref, onMounted } from 'vue'

// reactive state
const count = ref(0)

// functions that mutate state and trigger updates
function increment() {
  count.value++
}

// lifecycle hooks
onMounted(() => {
  console.log(`The initial count is ${count.value}.`)
})
</script>

<template>
  <button @click="increment">Count is: {{ count }}</button>
</template>

Which to Choose?

The Options API: is centered around the concept of a “component instance” (this as seen in the example), which typically aligns better with a class-based mental model for users coming from OOP language backgrounds. It is also more beginner-friendly by abstracting away the reactivity details and enforcing code organization via option groups.

The Composition API: is centered around declaring reactive state variables directly in a function scope and composing state from multiple functions together to handle complexity. It is more free-form and requires an understanding of how reactivity works in Vue to be used effectively. In return, its flexibility enables more powerful patterns for organizing and reusing logic.

Both API styles are fully capable of covering common use cases. They are different interfaces powered by the exact same underlying system. In fact, the Options API is implemented on top of the Composition API! The fundamental concepts and knowledge about Vue are shared across the two styles.

If you are new to Vue, here’s our general recommendation:

For learning purposes, go with the style that looks easier to understand to you. Again, most of the core concepts are shared between the two styles. You can always pick up the other style later.
For production use:
- Go with Options API if you are not using build tools, or plan to use Vue primarily in low-complexity scenarios, e.g. progressive enhancement.
- Go with Composition API + Single-File Components if you plan to build full applications with Vue.

You don’t have to commit to only one style during the learning phase. The rest of the documentation will provide code samples in both styles where applicable, and you can toggle between them at any time using the API Preference switches at the top of the left sidebar.