REPL

REPL stands for Read-Eval-Print Loop. It's an interactive programming environment that takes single user inputs (i.e., single expressions), evaluates them, and returns the result to the user. It's essentially a console window where you can enter your JavaScript or React code line by line and see the immediate output.

A REPL environment for React typically includes the entire React library, and allows programmers to execute their React code as though it's using a web browser, but without having to set up a full development environment.

Here's a simple example of JavaScript code that could be run in a REPL:

```javascript
// Declare a variable
var name = 'REPL';

// Create a function
function sayHello(name) {
return 'Hello, ' + name;
}

// Call the function
sayHello(name);
```

In this example, the REPL reads each line of code, evaluates it, prints the result, and then loops back to wait for more code to be input. In the case of the function call (`sayHello(name);`), it would print 'Hello, REPL', and then wait for further commands.

As an added note, an example of a REPL for React JS would be https://repl.it/languages/reactjs. Here, you could write and test your React code directly.