Functional programming is a programming paradigm that emphasizes the use of functions as the core building blocks of software. Unlike imperative programming, which focuses on giving explicit commands, functional programming revolves around creating and composing pure functions. A pure function is a function that, given the same input, will always return the same output and has no side effects. This predictability makes code easier to understand, debug, and test. One of the main principles of functional programming is immutability. This means that once a value is assigned, it cannot be changed. Instead of modifying variables, you create new ones. This leads to fewer bugs related to unexpected changes in state. Functions in functional programming are first-class citizens, meaning you can pass them as arguments, return them from other functions, and store them in data structures. Higher-order functions, which take other functions as parameters or return them, are common in functional programming. Languages like Haskell, Scala, and to some extent JavaScript, promote functional programming practices. A key concept in functional programming is recursion, where functions call themselves to solve problems. Instead of using loops, recursive functions handle repetitive tasks. Functional programming also discourages the use of side effects, like modifying global variables or interacting with external systems, as these can introduce unpredictability. The paradigm encourages declarative code, where you describe *what* needs to be done, not *how*. Libraries like React.js embrace functional concepts, making them widely used in modern development. Functional programming can be more difficult to grasp initially but offers long-term benefits in code maintainability and scalability.