React Introduction

React happens to be the most popular JavaScript framework sourced from Stack Overflow Developer Survey (2020). It is the best alternative to two other popular JavaScript frameworks, Angular and Vue. They all allow you to create fast apps in the browser.

According to the official React website:

React is a JavaScript library for building user interfaces

In the definition above, the term user interface is also called a component to React.

A template-like component can be created for reuse in a single web app. It totally adheres to the Don't Repeat Yourself (DRY) principle.

You only have to change the data in the components. These feature in JavaScript makes it easier to write shorter code for complex applications.

What are the components?

A simple definition of components is:

Components are independent and reusable bits of code

They serve the same purpose as JavaScript functions but work in isolation and return HTML (index.html) via a render function.

Components come in three types:

Class components
Function components
Server Components

We will see functional components usage here!

The image below shows part of the official React home page divided into components:

Let's take a look at the latter components (the bottom 3 components). It shows a component reusable in a web app.

How to build components in React?

Let's use Codepen to illustrate.

In normal HTML and CSS, the code source of the bottom three components is shown below:

HTML

<section>
  <div class='component-1 component'>
    <h3>Declarative</h3>
    <p>React makes it painless to create interactive UIs...</p>
    <p>Declarative views make your code more predictable...</p>
  </div>

  <div class='component-2 component'>
    <h3>Component-Based</h3>
    <p>Build encapsulated components that manage their...</p>
    <p>Since component logic is written in JavaScript...</p>
  </div>

  <div class='component-3 component'>
    <h3>Learn Once, Write Anywhere</h3>
    <p>We don't make assumptions about the rest of...</p>
    <p>React can also render on the server using...</p>
  </div>
</section>

CSS

section {
  display: flex;
  flex: 0 1 30%;
  -webkit-flex: 0 1 30%;
  margin: 0 auto;
  width: 97%;
  overflow-x: auto;
}

.component {
  margin-left: 5px;
  margin-right: 5px;
  padding: 10px
}

h3 {
  font-size: 1.25rem;
  font-family: sans-serif;
  margin-bottom: 20px;
  color: #6d6d6d;
  padding-top: 0;
  font-weight: 300;
  line-height: 1.3;
}

p {
  margin-top: 0;
  font-size: 1.0625rem;
  line-height: 1.7;
  max-width: 42rem;
}

a {
  background: #bbeffdad;
  background-color: #bbeffdad;
  color: #1a1a1a;
  border-bottom: 1px solid #00000033;
  text-decoration: none;
}

h3,
p {
  font-family: sans-serif;
}

a:hover {
  background: #bbeffd;
  background-color: #bbeffd;
}

@media (max-width: 779px) {
  .component {
    min-width: 50%;
    margin-left: 0;
    margin-right: 0;
  }
}

@media (max-width: 600px) {
  .component {
    min-width: 80%;
    margin-left: 0;
    margin-right: 0;
  }
}

Template-like Component

In the example shown below, a template-like component,

Adcomponent

is created:

A react component in its most basic form is just a function (functional component). React supports ES6+ syntax, like the fat arrow function.
Components with more than one React element are placed in a parent element. The parent element here is the
```
<div>
```
element

JSX

<div className="component-1 component">
    <h3>{props.adHeading}</h3>
    <p>{props.adParagraph1}</p>
    <p>{props.adParagraph2}</p>
</div>

Place all hierarchy React elements in parenthesis to return them to the DOM (Document Object Model) successfully.

return (
    <div className="component-1 component">
      <h3>{props.adHeading}</h3>
      <p>{props.adParagraph1}</p>
      <p>{props.adParagraph2}</p>
    </div>
  );

The purpose of the template-like functional component is to pass data:

The React functional component element,
```
<Adcomponent ... />
```
allows attributes. The attribute values are passed to the template-like component through the attribute name to change components data.
Select each React functional component element with an id selector. For example,
```
document.querySelector("#ad1")
```
.

In HTML and JSX, the code source of the bottom three components is shown below:

HTML

<section>
  <div class='component-1 component'>
    <div id='ad1'></div>
  </div>

  <div class='component-2 component'>
    <div id='ad2'></div>
  </div>

  <div class='component-3 component'>
    <div id='ad3'></div>
  </div>
</section>

JSX

ReactDOM.render(
  <Adcomponent
    adHeading="Declarative"
    adParagraph1="React makes it painless to create interactive UIs..."
    adParagraph2="Declarative views make your code more predictable..."
  />,
  document.querySelector("#ad1")
);

ReactDOM.render(
  <Adcomponent
    adHeading="Component-Based"
    adParagraph1="Build encapsulated components that manage their..."
    adParagraph2="Since component logic is written in JavaScript..."
  />,
  document.querySelector("#ad2")
);

ReactDOM.render(
  <Adcomponent
    adHeading="Learn Once, Write Anywhere"
    adParagraph1="We don't make assumptions about the rest..."
    adParagraph2="React can also render on the server..."
  />,
  document.querySelector("#ad3")
);

The overall JSX code is shown below:

const Adcomponent = props => {
  return (
    <div className="component-1 component">
      <h3>{props.adHeading}</h3>
      <p>{props.adParagraph1}</p>
      <p>{props.adParagraph2}</p>
    </div>
  );
};

ReactDOM.render(
  <Adcomponent
    adHeading="Declarative"
    adParagraph1="React makes it painless to create interactive UIs..."
    adParagraph2="Declarative views make your code more predictable..."
  />,
  document.querySelector("#ad1")
);

ReactDOM.render(
  <Adcomponent
    adHeading="Component-Based"
    adParagraph1="Build encapsulated components that manage their..."
    adParagraph2="Since component logic is written in JavaScript..."
  />,
  document.querySelector("#ad2")
);

ReactDOM.render(
  <Adcomponent
    adHeading="Learn Once, Write Anywhere"
    adParagraph1="We don't make assumptions about the rest..."
    adParagraph2="React can also render on the server..."
  />,
  document.querySelector("#ad3")
);

We write even lesser code with HTML and JSX files when components are mounted in a root element,

<div id='root'></div>

HTML

<div id='root'></div>

JSX

const Adcomponent = props => {
  return (
    <div className="component-1 component">
      <h3>{props.adHeading}</h3>
      <p>{props.adParagraph1}</p>
      <p>{props.adParagraph2}</p>
    </div>
  );
};

const app = (
  <section>
    <Adcomponent
      adHeading="Declarative" 
      adParagraph1="React makes it painless to create interactive..." 
      adParagraph2="Declarative views make your code more..."
    />
    <Adcomponent
      adHeading="Component-Based" 
      adParagraph1="Build encapsulated components that manage their..." 
      adParagraph2="Since component logic is written in JavaScript..."
    />
    <Adcomponent
      adHeading="Learn Once, Write Anywhere" 
      adParagraph1="We don't make assumptions about the rest..."
      adParagraph2="React can also render on the server..." />
  </section>
);

const rootNode = document.querySelector("#root");
ReactDOM.render(app, rootNode);

The above code snippet shows we can render components once in an application.

TerminologyJSX

JSX stands for JavaScript XML. JSX allows us to write HTML-like in Babel (JavaScript)

It is a custom HTML made available by React, but behind the scene is compiled to JavaScript. JSX is just a syntactical sugar featured in React.

ReactDOM

ReactDOM is a package that provides DOM (Document Object Model) specific methods that can be used at the top level of a web app to enable an efficient way of managing DOM elements of the web page.

ReactDOM provides the developers with API methods, properties, objects, etc. For example, the .render() method on the ReactDOM object.

Props

Props are arguments passed into React components. Props are passed to components via HTML attributes.

It makes changes in a state possible when components or an app is rendered to the screen.

Babel

Babel is a free and open-source JavaScript transcompiler that is mainly used to convert ECMAScript 2015+ code into a backward-compatible version of JavaScript that can be run by older JavaScript engines

It takes care of CSS auto-prefix and browser compatibility (CSS and JavaScript). That is, writes code that works across all browsers.

It is a popular tool for using the newest features of the JavaScript programming language (ECMAScript 2015+).

After the installation of React, via any package manager like Node Package Manager (NPM), there are lots of packages and dependencies you will find in the .json files. For example, webpack comes with other packages.

Webpack

Webpack is a module bundler. Its main purpose is to bundle JavaScript files for usage in a browser, yet it is also capable of transforming, bundling, or packaging just about any resource (asset)

Click the Babel file button then click View Compiled button to see how React compiles JSX (Custom HTML) to JavaScript behind the scene

Happy Coding!!!

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Also published at https://dev.to/bello/what-is-reactjs-g6n