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Components in React - Explained

Components in React - Explained

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How does React provide reusability of code? This is done using components. Let's see how React does it and what these components are.

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So far, you have learnt that components are just the JavaScript way of writing independent, reusable, and dynamic code.

Let’s look at the types of components in the next video.

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In the last video, you learned that there are two types of components in React — functional components and class components. As the names suggest, functional components are written in the form of functions whereas class components are written in the form of classes.

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Let’s look at components in context of our Phone Directory application.

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Going back to the Phone Directory application, let’s look at creating the header as a component, which can be utilised as a reusable entity across different pages in your application.

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That was smooth progress towards building the first component in React. Good job!

Following is an example to recall how you wrote Header as a functional as well as class component inside the Phone Directory application.

Header:

Functional Component:

import React from 'react';

const Header = function() {
   return (
       <div className="header">
         Phone Directory
       </div>
   )
}

Class Component:

import React, { Component } from 'react';

class Header extends Component {
   render() {
       return (
           <div className="header">
               Phone Directory
           </div>
       )
   }
}

Notes:

A component’s name must start with a capital letter in order to distinguish between the predefined HTML elements and the custom elements (components) created by users. An example can be:
```
MyComponent
```
A class component must extend from a base class named Component. For this, you also need to include Component as a named import from the ‘react’ library. A class component must always have a render() function.
```
import React, {Component} from 'react';

class MyComponent extends Component {
   render() {
      // code here
   }
}
```
A component must always return something.
```
import React, {Component} from 'react';

class MyComponent extends Component {
   render() {
      return (
         // code here
      )
   }
}
```
This returned value is the content that is actually rendered into the DOM; it replaces the name of the component. In case you miss the return statement in a functional component, you will get an error saying, “Nothing was returned from render. This usually means a return statement is missing.”. On the other hand, if you miss a return statement inside the render() function in a class component, you get an error saying, “Your render method should have return statement”.
If you do not wish to return anything, you can return null from the component, as written below:
```
return null;
```
In any case, the return statement needs to be mandatorily written, no matter whether you wish to return anything or not.
In order to reference a component written in a separate file, you need to first export the component from the file where it has been defined and then import the component in the required file where it needs to be used.
A component can have a file extension as .js or .jsx.
In case of components where the file extension is .js or .jsx, the extension is not required to be explicitly mentioned while writing the import statement.
For example, you can skip mentioning the file extension while writing the import statement when the Header file has the extension ‘.js’ or ‘.jsx’.
```
import Header from './Header';
```
However, for all other files, the extension should also be mentioned along with the file name while writing the import statement. Let’s say there’s a logo file with .svg extension, then you would need to mention the file extension when importing the logo file, as written below:
```
import logo from './logo.svg';
```

In React, you ought to write reusable code snippets; this is one of the recommended coding guidelines in software development. Thus, using components saves you the trouble of rewriting redundant code at multiple places. This is why React is all about components.

You can commit your code at this stage with “Created ‘Header’ Component” as the commit message. You can view the diff here.

Additional Resources: