Lets understand React


What is React? React.js creates user interfaces for single-page web apps by dividing the UI into composable elements. React.Js is a UI library, that was originally developed as an in-house library for Facebook, and then it was later open-sourced in 2013. It's famous and mostly used for a multitude of reasons Why Use React.js?

  • Easy of Learning: Since it requires a minimal understanding of JavaScript and HTML, the learning curve is fairly low. In some cases, we will even be using the ‘vanilla DOM JavaScript API’(you’ll see what I mean later), which means that the programmer doesn’t need to learn anything new before getting started.

  • Reusablity: If you’ve developed simple websites in HTML, there might’ve been times where you wanted to group a bunch of HTML elements and then save them into some sort of ‘variable’ so that it could be re-used later on. For a developer, this is a lifesaver. React has the ability to implement such a facility.

Now that we understand basic tenet of React, it’s finally time to do practical:


Getting Started: Installing React First, go to the terminal and then type, npx create-react-app app-name This instruction creates a template of a React application with the name of app-name . Note: npx is not a typo. It is a package runner command included within npm . Now, you need to navigate to the directory of this application and then view it in your browser like so: cd app-name npm start If the above operation is successful, your browser will open up to a URL of localhost:3000 . This is the output in the browser Output in the browserNow let’s create a simple Hello World! application with this project.


Hello World! — With React Within your project directory, go to src/index.js and replace the contents of the file index.jsas follows: Code for index.js

  • Lines 1 and 2: Importing the react and react-dom libraries. The react module is used to write HTML code within JavaScript(also known as JSX) and react-app is used to execute the render function that will display the contents onto the page.

  • Line 4: Render an h1 element in a container which has an id of root. The explanation of this method is done in the next part.

The output of the code is as follows: The output of the codeNote: You might be wondering that HTML tags within JavaScript are incorrect syntax. You’re absolutely right, but the React Library uses its own language that is similar to JavaScript but can use JSX within the language.


The ReactDOM.render method The ReactDOM.render method can be explained as follows: Explanation of the render methodAs the arguments suggest, the

  • The first argument(also known as element) specifies which element do you want to display, for instance h1 , p and so on and so forth.

  • The second argument(also known as container) specifies where you want to display the element. In normal convention, it’s a div element with id of 'root' .We use JavaScript DOM API to identify this such element.

Question: Where can I find this div element that has the id of 'root’ ? Look into the code of the HTML file, public/index.html . Notice at line 31 : Line 31 in index.htmlAll of the elements we specify in the element argument will be shown within the div element that has the specified id of root . Thus, the compiler tells the browser that: the element will be displayed within container . But there are several things you should know when using this method: Things to know about ReactDOM.render() The ReactDOM.render method cannot contain two-parent elements in the first argument. The example below is wrong and will give you an error: Wrong codeInstead, you can do this: Correct codeWe can specify one single parent element to display 2 child h1 elements on the page. We can even display a small unordered list as follows: Displaying a list in ReactIn practical cases though, it’s not feasible to insert 40+ lines of HTML in the render method. This is where Functional Components come into place.


Functional Components Let’s take this piece of HTML code: Can this code be made modular?As a React developer, it’s not viable to put so many elements within the render method. It is good practice to implement modularity. You can put the elements you want to render within a separate function. Core Idea function ComponentName() { return( <component> </component> ) }ReactDOM.render(<ComponentName/> , document.getElementById('root')) component is the HTML element you want to render, like h1 or p . Basic Example As an example, let’s rewrite the function of the Hello World! app. Basic Functional Components exampleNote: Always remember to use return on your elements, otherwise, it will throw an error To render it, Use the function within the render methodAs you can notice, we can write our function as a JSX tag. Keep in mind that you should

  • use self-closing tags.

  • Your function should start with a capital letter.

The code gives the following output Code outputHere also, we cannot have multiple parent elements. Like before, we will use one parent element with two child elements. Multiple elements in a functional componentExercise As a little task, I want you to render an h1 element with your name in it, and then write a list that contains 3 facts about you. Perform this activity using functional components. Solution: We cannot have two parents elements. Let’s use a div and then insert an h1 element followed by a list(ul) . And then it will be rendered. The output of the code is as follows: The output of the codeIn real-life projects, it is common practice to put your functional components within separate files as it makes your program more modular.


Move Functional Components Into Separate Modules Let’s say your functional component is too complex. As a developer, you want to move your component into another file so that it can be used in your main file. Thus, this makes your program modular. Furthermore, modularity also makes your code look neater. Core Idea First, create a functional component in a separate file. Then, we will export it as follows: export default ComponentName ComponentName is the name of your functional component. Basic Example Let’s rewrite the above exercise. Create a new file, called MyApp.js and write your code as follows: Functional Component in a separate file

  • Line 1 : Import the react module to use JSX tags within our code

  • Lines 2–12 : Our functional component

  • Line 13 : Export our MyApp function so that it could be used within our index.js file.

Now we can use it in our index.js file like so: using components located in a different file