Functional programming is a programming paradigm that emphasises the use of pure mathematical functions to write computer programs. Programs written in functional programming are often described as “data transformations”, rather than “scheduler instructions”.

The main features of functional programming include the use of pure immutable functions, non-modification of the global state, and the use of programming patterns such as recursion and lambda expressions.

Using these concepts, functional programming makes it easier to understand and maintain code by using immutable, pure and composable functions to transform data. Thus, it allows to write programs that are more reliable, easier to understand and maintain.

Taking JavaScript as an example, you can use functions such as map, filter and reduce to transform arrays of data in a declarative way.

const numbers = [1, 2, 3, 4, 5];
const doubledNumbers = numbers.map(x => x * 2);
console.log(doubledNumbers); // [2, 4, 6, 8, 10]

You can also use lambda expressions to create anonymous functions:

const add = (a, b) => a + b;
const multiply = (a, b) => a * b;

const compose = (f, g) => (...args) => f(g(...args));
const addAndMultiply = compose(add, multiply);
console.log(addAndMultiply(2, 3, 4)); // 14

Using the concepts of currying and function composition, you can create reusable functions that can be combined to create new functions:

function add(a) {
    return function(b) {
        return a + b;
    }
}

const add10 = add(10);
console.log(add10(5)); // 15

Functional programming is often used in conjunction with other programming paradigms, such as object-oriented programming or declarative programming, to create robust and scalable programs. It is also particularly suited to processing data, using predefined functions to process data, combining functions to create new functionality, and using immutable and pure functions to write more reliable and scalable programs.