A Functional JavaScript Developer uses functional programming principles in JavaScript to develop software applications. They primarily focus on creating pure functions, avoiding shared state, mutable data, and side-effects. They use concepts like first-class and higher-order functions, currying, recursion, and functional composition. They write code that is easier to test and debug, improving the overall quality and maintainability of the software. They use JavaScript libraries such as Lodash or Ramda that support functional programming. They also often work with frameworks like React or Vue.js, which utilize functional programming concepts.