In short, React uses AJAX. They are not comparable in the way you're asking.

Keep reading for a crash coarse in what React is, what AJAX is, and how they are used to make modern web applications.

This is probably a more simple explanation than you're looking for, but for anyone else who may be confused...

AJAX and Airplanes

Think about an Airplane. The most important part of an airplane is that it flies. But an airplane also has wheels. And the wheels serve a very important purpose, because without them the airplane would never fly or land, and despite all the awesome stuff a plane could do in the air, it wouldn't matter without wheels.

This is the same relationship that React has with AJAX. React is the airplane, and AJAX are the wheels. But, ya know, other things have wheels too. Tractors, cars, even some boats have wheels, and they're all very important, and crippled without wheels. So too is AJAX to other web technologies, but when you're talking about airplanes, its wheels are usually the farthest thing from your mind.

So React is to AJAX, what an Airplane is to Wheels.

But let's talk about AJAX. What is it? Why is it so important? How it is used in websites today. Then I'll show how it's used by React. Then show you what React does that's so impressive, it makes you forget about AJAX - Like an Airplane to its wheels.

Remember Websites in the 90's?

When you clicked anything, a new page would have to load to show the effect of your click - even if it was nothing. Here's an awesome example. Go to that page and click around... See how clicks whisk you away to a completely different page? That is the Internet before AJAX.

Now, take a look at this very page: next to each answer is an Up Arrow... Go ahead and click one of them... Notice the page doesn't reload, but you are given feedback: the arrow turns Orange. This may seem insignificant, but it represents big advancements in web technology: AJAX.

AJAX allows that to happen! And this is no big deal now, it's so intrinsic to the web experience now, it's difficult to imagine an Internet without it.

AJAX and a Clock Face

A good analogy of AJAX, and how it changed the web is a simple wrist watch, or a wall clock... Imagine the minute, hour and second hands moving around the clock's face to show time. Now, suppose every movement of the second hand caused the entire clock to be destroyed and rebuilt?

All that effort of destroying and rebuilding just to show a tiny change. Well, that would be an outrageous waste of resources, and that was the Internet of the 90's. Thankfully, we have AJAX now. Just as a clock seamlessly displays the time, AJAX allows webpages to show changes in data immediately, without the page needing to be refreshed; you click an up arrow, and it turns orange. No page reload needed!

Everything nowadays uses AJAX, and as a developer it's becoming easier to write an app without even knowing what AJAX is. This is because it has become assimilated into the JavaScript technologies we use, like React - which uses fetch, among other commands, to make AJAX easier and more readable.

And AJAX works just like webpages:

1. A client (A Web Browser like Firefox) requests data from a server (Like the Stack Overflow (SO) Server).
2. The server processes the request.
3. The server sends a response back to the client.
4. and the code of the webpage, based on the response, decides where it will go (HTML) and how it will look (CSS).

All that is hidden from the user so it just seems like the webpage is reacting to their actions.

Single-Page Applications

Since we're not rebuilding the entire page with every click, you can keep stuff stored in the browser. This can be used throughout your entire visit and future visits to the site.

The first time you visit Stack Overflow, all of the CSS, JS, and HTML is loaded. These three languages define the style (CSS), behavior (JS), and structure (HTML) of the data sent back and forth from the server. And guess how that data is sent! AJAX.

This is how most of the web works now. Google, Facebook, Amazon, Youtube, Reddit, every site built with WordPress and WIX, even Stack Overflow - they all use this basic paradigm for delivering their sites to users efficiently. The difference comes in how the Single-page application is built and managed...

React.js

React is a javascript library for building and maintaining Single-Page Applications.

But that's not even that big of a deal. The big deal about React is how it allows you to build applications...

Basically, you build things separately, then put them together: Components come together to form an Application. So take a look at this totally plausible but fake code for all the answers on this page:

answerArray.map(a => <Answer answerData={a}></Answer>)

This is one line that shows most of the information on this page. That is a big deal. The developers at Stack Overflow created their own component, called "Answer" and its only job is to show an Answer. You run that in a loop, and bam, all the intricacies of all the answers are abstracted, hidden in the Answer Component, which is completely separate from other components.

Now take a look at this:

<App>  <Header />  <LeftSidebar />  <Question>    { answerArray.map(a => <Answer answerData={a} />  )}  </Question>  <RightSidebar />  <Footer/></App>

This is the whole Stack Overflow site.

Each tag (Header, Question, Answer, etc.) is a component. These components are completely separate and have self-contained code, but here they are used together to build the more complex application.

Composition

An important concept of React is composition, and we just defined it above. "Composition allows you to build more complex functionality by combining small and focused functions" (flaviocopes). Our Application is composed of smaller components.

It's also important to note that each component contains its own functionality. That means if the user clicks a button and a warning appears, the button and the code that makes the warning appear are in the same component.

Functional Programming

Surprise, we already defined this too. Functional programming, for our purposes, means 1. objects; and 2. how they behave; are in the same place. Like the button example above. Click a button, get a warning. And that's all in the same file.

This is different than pre-React development where all the buttons would be in one file, and all effects of the buttons would be in another. And this isn't necessarily a wrong way to do things, but for web development, it is easier to think in terms of self-contained building blocks, rather than widely dispersed tools that don't work by themselves.

Why you shouldn't care about Moustache and Handlebars

These two technologies have been cannibalized by React. Similar to how React uses AJAX but makes it easier, Moustache and Handlebars are already inside React, and you're using them all the time without even knowing it. And to me, that's ok. There are arguments to the contrary, and knowledge is never a bad thing, so investigate further if you want, but this is already long enough, so that's all I'll say about that.

Instead, I will tell you about 3 technologies you should care about.

What you should care about instead...

Node

The main point of Node.js is that it executes JavaScript outside a browser. Big whoop, right? Well, it turns out this is one of the most influential advancements for web developers ever. In fact, downloading Node is often done before downloading React.

Node is important for 2 huge reasons:

1. It lets you download other stuff
2. It lets you process JavaScript before sending it to a browser

I could write pages and pages about Node, but your takeaway from this should be "Node is important, I should be on the lookout for more knowledge about Node and how it relates to React and web development."

NPM

NPM does not stand for "Node Package Manager", but it should, because that's exactly what it does. React, SASS, Angular, Vue, pretty much everything mentioned here you will probably use npm to install and keep updated.

Webpack

Webpack is a "module bundler". It takes all your js and css files and writes them to one file so you only have to worry about writing one <script> tag.

Each React component will have at least one js file associated with it. Each component should have its own file too. Keeping track of all those files is very demanding. Webpack does it for you, it just makes life easier, so learn about it early and don't shy away from it.

This is something so inherent to React Apps that most of the time it will just be working and you won't even know it. For instance, create-react-app installs it automatically, and does not require you to do anything - same with Babel...

Babel

Translates all your code to ECMA5 so it can be read by most browsers and most versions of those browsers.

Again, this can be installed with npm, or if you just want to play around with React and not get too bogged down with the minutia like this, you can run create-react-app, and this will just work with automatic settings and will be out of your hair while you learn.

They make stuff easier

NPM, Webpack, Babel, and many other Node packages are only there to make your life easier. Building web apps require a lot of maintenance - or small, non-programming annoyances that typically you don't even need to think about.

Try not to be intimidated by new packages because wielding their power can mean countless hours devoted to more interesting things.

Conclusion

Hopefully, this post has helped you learn the difference between React, AJAX, and the ongoing nature of web application development. React and AJAX are not comparable, but React uses AJAX, or rather you - the developer - use AJAX in React to get data without the page needing to reload.

AJAX and other technologies were monumental to the advancement of web applications, but because of how absolutely essential they were to applications, they were assimilated into new technologies so much so that you don't even have to know about them to reap their benefits.

My goal was to correct some misconceptions on your path of learning; explain the "why" of the current state of web dev; and introduce technologies you didn't mention but should know about: Node, npm, Babel.

If you want to continue learning, I highly recommend doing a tutorial in React. I have done some at platform.ui.dev/, and enjoy their approach to learning and their payment structure (I haven't been paid to say this). Good luck out there, and I hope this was helpful.

Ajax is used to refresh a web page without having to reload it : it sends a request to the server, but typically the response is processed by the javascript that displays dynamically a new element on the browser without having to reload the entire page.

React is a javascript library that dynamically update the page with inferface components. The components are calculated either by javascript interactions or by an ajax request that go through the server. So ReactJS can also use Ajax requests to update the page.

Mustache and Handlebars are a bit different from ReactJS as the main goal is to transform a template in a component that will be displayed in a page. It can also use Ajax to get data (for getting templates or json datas).

Ajax

We are using Ajax to send http requests. And we can't re-render a particular area of the page(DOM) by using Ajax alone. We need jQuery to re-render the page after an ajax call came up with the response. Actually comparing jQuery + HTML and React.js is far better than comparing ajax and React.js.

React.js

The role of the react.js is dividing page(DOM) into small pieces (Components). ex:- Profile image area, Main Navigation, Sidebar, Textfield, Button. etc. from Big pieces to small pieces. Most importantly we can bind functionalities into these components. Example:- Let's assume users need a popup to upload a profile image by clicking on above "Profile image area". We can write a function to open a popup. And also we can write another function to upload profile image to the database. In this way we can use ajax inside the React.js

Please follow this tutorial.