React uses a syntax extension of JavaScript called JSX that allows you to write HTML directly within JavaScript

because JSX is a syntactic extension of JavaScript, you can actually write JavaScript directly within JSX

include the code you want to be treated as JavaScript within curly braces: { ‘this is treated as JavaScript code’ }

JSX code must be compiled into JavaScript

under the hood the challenges are calling ReactDOM.render (JSX, document.getElementById(‘root’))

One important thing to know about nested JSX is that it must return a single element.

For instance, several JSX elements written as siblings with no parent wrapper element will not transpile.

With React, we can render this JSX directly to the HTML DOM using React’s rendering API known as ReactDOM.

ReactDOM offers a simple method to render React elements to the DOM which looks like this:

• the first argument is the React element or component that you want to render,
• and the second argument is the DOM node that you want to render the component to.

ReactDOM.render() must be called after the JSX element declarations, just like how you must declare variables before using them.

key difference in JSX is that you can no longer use the word class to define HTML classes.

• — -> This is because class is a reserved word in JavaScript. Instead, JSX uses className

the naming convention for all HTML attributes and event references in JSX become camelCase

a click event in JSX is onClick, instead of onclick. Likewise, onchange becomes onChange. While this is a subtle difference, it is an important one to keep in mind moving forward.

• The constant JSX should return a div element.
• The div should have a class of myDiv.

In HTML, almost all tags have both an opening and closing tag: <div></div>; the closing tag always has a forward slash before the tag name that you are closing.

there are special instances in HTML called “self-closing tags”, or tags that don’t require both an opening and closing tag before another tag can start.

For example the line-break tag can be written as <br> or as <br />, but should never be written as <br></br>, since it doesn't contain any content.

In JSX, the rules are a little different. Any JSX element can be written with a self-closing tag, and every element must be closed. The line-break tag, for example, must always be written as <br /> in order to be valid JSX that can be transpiled. A <div>, on the other hand, can be written as <div />or<div></div>. The difference is that in the first syntax version there is no way to include anything in the <div />.

There are two ways to create a React component. The first way is to use a JavaScript function.

Defining a component in this way creates a stateless functional component.

think of a stateless component as one that can receive data and render it, but does not manage or track changes to that data.

• React requires your function name to begin with a capital letter.

Here’s an example of a stateless functional component that assigns an HTML class in JSX:

Because a JSX component represents HTML, you could put several components together to create a more complex HTML page.

The other way to define a React component is with the ES6 class syntax. In the following example, Kitten extends React.Component:

This creates an ES6 class Kitten which extends the React.Component class.

So the Kitten class now has access to many useful React features, such as local state and lifecycle hooks.

Also notice the Kitten class has a constructor defined within it that calls super()

It uses super() to call the constructor of the parent class, in this case React.Component

The constructor is a special method used during the initialization of objects that are created with the class keyword. It is best practice to call a component’s constructor with super, and pass props to both.

This makes sure the component is initialized properly. For now, know that it is standard for this code to be included.

Imagine you are building an App and have created three components, a Navbar, Dashboard, and Footer.

To compose these components together, you could create an App parent component which renders each of these three components as children. To render a component as a child in a React component, you include the component name written as a custom HTML tag in the JSX.

• For example, in the render method you could write:

When React encounters a custom HTML tag that references another component (a component name wrapped in < /> like in this example), it renders the markup for that component in the location of the tag. This should illustrate the parent/child relationship between the App component and the Navbar, Dashboard, and Footer.

In the code editor, there is a simple functional component called ChildComponent and a class component called ParentComponent. Compose the two together by rendering the ChildComponent within the ParentComponent. Make sure to close the ChildComponent tag with a forward slash.

• Note:ChildComponent is defined with an ES6 arrow function because this is a very common practice when using React.
• However, know that this is just a function.