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A transpiler, also known as a source-to-source compiler, is a tool that takes source code written in one programming language as its input and produces the equivalent source code in another programming language.

In the context of React JS, a transpiler is used to convert JSX and ES6 (the latest version of JavaScript that React is written in) into the older ES5 JavaScript syntax that browsers can understand.

Babel is the most commonly used transpiler in a React development environment. It takes the cutting-edge JavaScript features and React's JSX syntax that we write, and transforms them into an older version of JavaScript which most browsers can understand.

Here is an illustrative example:

Original React JS code (JSX and ES6 syntax):
class MyComponent extends React.Component {
render() {
return <div>Hello World!</div>;

After transpilation by Babel into ES5 syntax:
var MyComponent = React.createClass({
render: function render() {
return React.createElement("div", null, "Hello World!");

This transpiled code is what actually sent to the browser and gets executed. The beauty of transpilation is that as a developer, you get to write code with the latest and most efficient syntax while not having to worry about support for older browsers, since the transpiler takes on the responsibility of converting your code for maximum compatibility.

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