Functional Programming in JavaScript is a programming paradigm that involves building software by composing pure functions, avoiding shared state, mutable data, and side-effects. It leverages JavaScript's inherent support for functions as first-class citizens, enabling powerful patterns like higher-order functions, where functions are passed as arguments or returned as results. Developers utilize techniques such as function composition and built-in methods like `.map()`, `.filter()`, and `.reduce()` to handle data immutably, leading to code that is more predictable, declarative, and easier to test and debug.