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Vue's Options API constrains the reusability of logic across multiple components. Patterns involving mixins and higher-order components (HOCs) have been established to create reusable blocks of code and consolidate repeated state and functionality. However, each pattern has a drawback: With Vue 3's Composition API , component logic can be extracted, shared and reused amongst multiple components. Exposing Vue's core capabilities (reactivity and lifecycle hooks) as standalone, globally available functions allows developers to create custom hooks that are decoupled from any UI, but yet, can be consumed by any component. This can greatly improve the maintainability and flexibility of a large application composed of hundreds of components. Let's walkthrough an example demo to demonstrate the simplicity of writing reusable code with the Composition API.

As the most starred open source, JavaScript library/framework repository on GitHub, Vue.js has become a top three front-end technology alongside React.js and Angular in terms of popularity, usage, ecosystem activity and developer satisfaction. Compared to React.js and Angular, Vue.js is incrementally adoptable and provides a declarative API that resonates with AngularJS developers. Evan You , the author of Vue.js, explained the original concept of Vue.js as extracting the best parts of AngularJS, such as directives (i.e., v-if and v-show ), and building a lightweight, flexible alternative. Building large Vue.js applications requires composing components together with state management (i.e., Vuex ) and routing (i.e., Vue Router ) libraries. In September 2020, the Vue.js team officially released Vue 3, which welcomed a number of improvements and new features: Particularly, the Composition API of Vue 3 has been met with controversy due to its resemblance to React hooks and its philosophical divergence from the Options API , which emphasizes separation of concerns by defining component logic within specific options ( data , props , computed , methods , etc.). For those who prefer the Options API, unlike other major version upgrades that introduce incompatible changes, the Composition API will not break existing Vue components using Options API (" purely additive "). Although it offers similar logic composition capabilities as React hooks, the Composition API leverages Vue's reactivity system to circumvent several of React hooks' issues.

Tests make sure that changes in code don't break its functionality. For testing an app, we need a test runner. Create React App provides us a test runner called jest . Let's create a new app and inspect what it contains. The src directory contains all the app code and a file called App.test.tsx . This file contains a test that makes sure that the App component renders a link:

Chances are you, if you've ever looked at a React component, you've seen JavaScript that looks like this: What is that stuff that looks like HTML or XML? That's called JSX and it stands for JavaScript eXtension. It might seem weird to have XML tags in your JavaScript, but it turns out to be super useful. By the end of this blog post, you'll have a solid understanding of what JSX is, and how we'll use it to build powerful React apps. All of our React components have a render function that specifies what the HTML output of our React component will be. JSX is a "language extension" that allows us to write JavaScript that looks like HTML.

Let's talk a bit about props and state , two core concepts in React that we'll use in every React application. The short of it is, props are like the "arguments" (or properties ) that you pass into your components and state stores "component-local data". Let's look at an example to make it concrete: Let's say that we have a

component that includes a title. The code might look something like this: