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Describe how functional components are created in Vue.js and what they can be used for.

Functional components in Vue.js are components that do not have instances or state. They are used for faster template rendering and more efficient application performance. Functional components are optimal when we only need to display input data and do not need component life cycle methods such as `created`, `mounted`, etc.

Creating a functional component is simple. We will use the `functional` directive in the template to mark the component as functional. Functional components always need to have a single root element.

Here is an example of a functional component:

<template functional>
    <div>
        <h1>{{ props.title }}</h1>
        <p>{{ props.content }}</p>
    </div>
</template>
<script>
export default {
    props: ['title', 'content']
}
</script>

Notice that the `props` variable is available directly inside the template. We do not use the `this` keyword because functional components do not have instances.

Functional components are useful when you need a simple component that acts like a pure function. They are also beneficial for optimizing performance, as they render faster compared to standard Vue.js components.

How does Vue.js handle data reactivity and how does it affect application performance?

Vue.js supports data reactivity through so-called observers. When we create a Vue instance and pass a data object to it, Vue.js goes through each of the data fields and converts them into "getters" and "setters" using Object.defineProperty.

While the getter does not modify the value, the setter is able to detect when a variable's value changes. When the setter detects changes, it notifies the observers, who then update the view.

// Simple data object example
var vm = new Vue({
  data: {
    message: 'Hello world'
  }
})

In the above code, the "message" field is transformed to take advantage of Vue.js's reactivity. When the getter detects any change in "message", the observer is notified and automatically updates the view.

In terms of performance, reactivity in Vue.js is sufficiently optimized to ensure smooth view update while minimizing the number of unnecessary operations. Vue uses asynchronous queueing to collect changes and update the view in one aggregate "tick" operation. This prevents multiple refreshes of the same piece of code and improves performance.

What is Vue CLI and what are its main functions?

Vue CLI is a Vue.js command-line tool that enables rapid creation and configuration of new projects. It offers a range of features that significantly streamline and accelerate the Vue.js app development process.

The key features of Vue CLI include:

Project creation and configuration - Vue CLI allows for generating new Vue.js projects with a pre-configured development environment. It includes a default configuration optimal for most projects, but also allows for individual setting adjustments.

Hot-reloading - Hot-reloading is a feature that automatically refreshes the page after changes in the code have been made. This significantly improves the efficiency of the developer's work.

Linting and testing - Vue CLI includes configurations for ESLint and various unit and integration tests.

Support for production builds - This tool can prepare the code for a production version which is optimized for performance.

Facilitation of multilingual application development - Thanks to Vue I18n support, Vue CLI facilitates the creation of multilingual applications.

Support for preprocessors - Vue CLI supports preprocessors such as Babel, TypeScript, ESLint, PostCSS, PWA, Unit Testing & End-to-end Testing.

A simple project initialization with Vue CLI goes as follows:

# Installation
npm install -g @vue/cli
# OR
yarn global add @vue/cli

# Creating a new project
vue create my-project

What are the differences between computed properties and watchers in Vue.js?

**Computed properties** and **watchers** enable certain actions to be performed when the value of a property in a Vue.js component changes. Despite some similarities, there are, however, significant differences between them.

**Computed properties** are functions that are used to perform certain operations on a given set of Vue.js variables, which return a result.
When we use **computed properties**, Vue.js remembers the dependencies between our data and the computational function. When any dependency changes, the function is called again.
**Computed properties** are also lazy, which means that they are computed only when they are used somewhere in the code.

computed: {
  fullName: function () {
    return this.firstName + ' ' + this.lastName
  }
}

**Watchers**, on the other hand, are more general. They allow observing any component properties and reacting to their changes.
Watchers only execute when the observed property changes. They are especially useful when we need to track the changes of a single property.

watch: {
  firstName: function (val) {
    this.fullName = val + ' ' + this.lastName
  },
  lastName: function (val) {
    this.fullName = this.firstName + ' ' + val
  }
}

In summary, **computed properties** are better when we want to transform input data in a consistent way, while **watchers** are applicable when we want to perform asynchronous operations in response to changing input data.

Vue.js Flashcards

Sample Vue.js flashcards from our app

Describe how functional components are created in Vue.js and what they can be used for.

How does Vue.js handle data reactivity and how does it affect application performance?

What is Vue CLI and what are its main functions?

What are the differences between computed properties and watchers in Vue.js?

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