Key Concepts of Functional Programming

1. Pure Functions: Pure functions are deterministic; they always return the same output for a given input and don’t cause side effects (such as modifying a global variable). This characteristic makes them predictable and easy to test in isolation.
2. Immutability: In FP, data is immutable, meaning once a data structure is created, it cannot be changed. Instead of modifying existing data, functions return new copies with modifications. This immutability helps avoid bugs caused by unintended data mutations, which is common in multi-threaded applications.
3. First-Class and Higher-Order Functions: Functions are first-class citizens in FP, meaning they can be assigned to variables, passed as arguments, or returned from other functions. Higher-order functions take other functions as arguments or return them, enabling powerful abstractions like mapping, filtering, and reducing.
4. Function Composition: Function composition is combining smaller functions to build more complex functions. This modularity allows for constructing complex behavior from simple building blocks, improving readability and reusability.
5. Declarative Programming: FP is often more declarative than imperative programming, meaning that it focuses on what to accomplish rather than how to accomplish it. This approach can make code more concise and expressive.