JSX is a XML-like syntax extension to ECMAScript without any defined semantics. Many more details about it can be found here:
JSX is popular in the React community which "recommend[s] using JSX because it is a concise and familiar syntax for defining tree structures with attributes. It's more familiar for casual developers such as designers." Many more details about that can be found here:
Many NetBeans IDE users have been asking for JSX support in NetBeans IDE, since they are using React:
As you can see from the above, the NetBeans team has added parsing and lexing support for JSX over the past few days.
Let's take a look at the result, using this repo as an example project to try out JSX support in NetBeans IDE:
Here's what I see in a very (very very) recent NetBeans IDE build (click to enlarge the image):
Contrast the above to the same file opened in the current release of NetBeans IDE, i.e., NetBeans IDE 8.1, to see the difference:
Great news, isn't it? :-) At this point, don't expect extensive support for JSX or for React. However, as you can see above, at least you can now edit your files without there being error markings everywhere, which made NetBeans IDE impossible to work with when working with JSX.