JavaScript의 함수형 프로그래밍 살펴보기

What is Functional Programming?

Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions. It avoids changing state and mutable data. The fundamental idea is to build programs using pure functions, avoid side effects, and work with immutable data structures.

The main characteristics of functional programming include:

• Pure functions: Functions that, given the same input, will always produce the same output and have no side effects.
• Immutability: Data cannot be changed once created. Instead, when you need to modify data, you create a new copy with the necessary changes.
• First-class functions: Functions are treated as first-class citizens, meaning they can be passed as arguments, returned from other functions, and assigned to variables.
• Higher-order functions: Functions that either take other functions as arguments or return them as results.
• Declarative code: The focus is on what to do rather than how to do it, making the code more readable and concise.

Core Concepts of Functional Programming in JavaScript

Let’s explore some of the most important concepts that define FP in JavaScript.

1. Pure Functions

A pure function is one that does not cause side effects, meaning it doesn’t modify any external state. It depends solely on its input parameters, and given the same input, it will always return the same output.

Example:

// Pure function example
function add(a, b) {
  return a + b;
}

add(2, 3); // Always returns 5

A pure function has several advantages:

• Testability: Since pure functions always return the same output for the same input, they are easy to test.
• Predictability: They behave consistently and are easier to debug.

2. Immutability

Immutability means once a variable or object is created, it cannot be modified. Instead, if you need to change something, you create a new instance.

Example:

const person = { name: "Alice", age: 25 };

// Attempting to "change" person will return a new object
const updatedPerson = { ...person, age: 26 };

console.log(updatedPerson); // { name: 'Alice', age: 26 }
console.log(person); // { name: 'Alice', age: 25 }

By keeping data immutable, you reduce the risk of unintended side effects, especially in complex applications.

3. First-Class Functions

In JavaScript, functions are first-class citizens. This means that functions can be assigned to variables, passed as arguments to other functions, and returned from functions. This property is key to functional programming.

Example:

const greet = function(name) {
  return `Hello, ${name}!`;
};

console.log(greet("Bob")); // "Hello, Bob!"

4. Higher-Order Functions

Higher-order functions are those that take other functions as arguments or return them. They are a cornerstone of functional programming and allow for greater flexibility and code reuse.

Example:

// Higher-order function
function map(arr, fn) {
  const result = [];
  for (let i = 0; i < arr.length; i++) {
    result.push(fn(arr[i]));
  }
  return result;
}

const numbers = [1, 2, 3, 4];
const squared = map(numbers, (x) => x * x);

console.log(squared); // [1, 4, 9, 16]

JavaScript’s Array.prototype.map, filter, and reduce are built-in examples of higher-order functions that help in functional programming.

5. Function Composition

Function composition is the process of combining multiple functions into a single function. This allows us to create a pipeline of operations, where the output of one function becomes the input to the next.

Example:

const multiplyByTwo = (x) => x * 2;
const addFive = (x) => x + 5;

const multiplyAndAdd = (x) => addFive(multiplyByTwo(x));

console.log(multiplyAndAdd(5)); // 15

Function composition is a powerful technique for building reusable, maintainable code.

6. Currying

Currying is the technique of converting a function that takes multiple arguments into a sequence of functions that each take a single argument. It’s particularly useful for creating reusable and partially-applied functions.

Example:

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

const addFive = add(5);
console.log(addFive(3)); // 8

This technique allows you to create specialized functions without needing to rewrite the logic.

7. Recursion

Recursion is another functional programming technique where a function calls itself to solve a smaller instance of the same problem. This is often used as an alternative to loops in FP, as loops involve mutable state (which functional programming tries to avoid).

Example:

function factorial(n) {
  if (n === 0) return 1;
  return n * factorial(n - 1);
}

console.log(factorial(5)); // 120

Recursion enables you to write cleaner, more readable code for tasks that can be broken down into smaller sub-problems.

8. Avoiding Side Effects

Side effects occur when a function modifies some external state (like changing a global variable or interacting with the DOM). In functional programming, the goal is to minimize side effects, keeping functions predictable and self-contained.

Example of Side Effect:

let count = 0;

function increment() {
  count += 1;  // Modifies external state
}

increment();
console.log(count);  // 1

In functional programming, we avoid this kind of behavior by returning new data instead of modifying existing state.

FP Alternative:

function increment(value) {
  return value + 1;  // Returns a new value instead of modifying external state
}

let count = 0;
count = increment(count);
console.log(count);  // 1

Advantages of Functional Programming

Adopting functional programming in JavaScript offers numerous benefits:

• Improved readability: The declarative nature of FP makes code easier to read and understand. You focus on describing the "what" rather than the "how."
• Reusability and modularity: Pure functions and function composition promote reusable, modular code.
• Predictability: Pure functions and immutability reduce the number of bugs and make the code more predictable.
• Easier testing: Testing pure functions is straightforward since there are no side effects or dependencies on external state.
• Concurrency and parallelism: FP allows easier implementation of concurrent and parallel processes because there are no shared mutable states, making it easier to avoid race conditions and deadlocks.

Functional Programming Libraries in JavaScript

While JavaScript has first-class support for functional programming, libraries can enhance your ability to write functional code. Some popular libraries include:

1. Lodash (FP module): Lodash provides utility functions for common programming tasks, and its FP module allows you to work in a more functional style.

Example:

   const _ = require('lodash/fp');
   const add = (a, b) => a + b;
   const curriedAdd = _.curry(add);
   console.log(curriedAdd(1)(2)); // 3

1. Ramda: Ramda is a library specifically designed for functional programming in JavaScript. It promotes immutability and function composition.

Example:

   const R = require('ramda');
   const multiply = R.multiply(2);
   const add = R.add(3);
   const multiplyAndAdd = R.pipe(multiply, add);

   console.log(multiplyAndAdd(5)); // 13

1. Immutable.js: This library provides persistent immutable data structures that help you follow FP principles.

Example:

   const { Map } = require('immutable');

   const person = Map({ name: 'Alice', age: 25 });
   const updatedPerson = person.set('age', 26);

   console.log(updatedPerson.toJS()); // { name: 'Alice', age: 26 }
   console.log(person.toJS()); // { name: 'Alice', age: 25 }

Conclusion

Functional programming offers a powerful paradigm for writing clean, predictable, and maintainable JavaScript code. By focusing on pure functions, immutability, and avoiding side effects, developers can build more reliable software. While not every problem requires a functional approach, integrating FP principles can significantly enhance your JavaScript projects, leading to better code organization, testability, and modularity.

As you continue working with JavaScript, try incorporating functional programming techniques where appropriate. The benefits of FP will become evident as your codebase grows and becomes more complex.

Happy coding!

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