57. Functional Programming in Javascript

Functional programming is a programming paradigm that treats computation as an evaluation of mathematical functions and avoids state change and mutable data. In JavaScript, this is made easier by the fact that functions are first-class objects, meaning they can be passed as arguments to other functions, returned as values ​​from other functions, and assigned to variables.

Functional programming in JavaScript generally involves the use of pure functions, which are functions that give the same result for the same arguments and have no side effects. This means that they do not modify any state or variable outside the function. This makes its behavior predictable and easy to test.

For example, consider the following pure function:

function sum(a, b) {
    return a + b;
}

This function will always return the same result for the same arguments and will not modify any external state.

Another important concept in functional programming is immutability, which is the idea that once a variable is created, its value cannot be changed. Instead, any modification to a variable will result in a new variable. This is useful to avoid side effects and make the code more predictable.

let a = 1;
let b = a + 1; // b is now 2, but a is still 1

High-order functions are another pillar of functional programming. These are functions that operate on other functions, either taking them as arguments or returning them as a result. In JavaScript, high-order functions are commonly used with array methods such as map, filter, and reduce.

let numbers = [1, 2, 3, 4, 5];

let doubled = numbers.map(function(n) {
    return n * 2;
}); // [2, 4, 6, 8, 10]

let pairs = numbers.filter(function(n) {
    return n % 2 === 0;
}); // [2, 4]

let sum = numbers.reduce(function(total, n) {
    return total + n;
}, 0); // 15

Using higher order functions like these can make code more concise and readable.

Finally, function composition is another common technique in functional programming. This involves creating complex functions by combining simpler functions. In JavaScript, this can be done using the compose function, which can be implemented as:

function compose(f, g) {
    return function(x) {
        return f(g(x));
    };
}

With this function, you can create new functions by combining other functions. For example, you could create a function to double and then add 1 to a number:

let foldEAdd1 = compose(function(n) { return n + 1; }, function(n) { return n * 2; });

foldAndAdd1(4); // 9

In summary, functional programming is a powerful paradigm that can make code more predictable, easier to test, and easier to understand. While it may be a little different than what you're used to if you come from an imperative or object-oriented background, it's worth learning and using in your JavaScript code.