Functions in Node.js

Functions in Node.js

Functions are everywhere in Node.js. They are the building blocks of everything — from simple scripts to complex APIs.

Function Declarations

// Function declaration
function add(a, b) {
  return a + b;
}

// Function expression
const multiply = function(a, b) {
  return a * b;
};

// Arrow function (ES6)
const divide = (a, b) => a / b;

// Arrow function with body
const greet = (name) => {
  const message = `Hello, ${name}!`;
  return message;
};

Default Parameters

function createUser(name, role = "user", active = true) {
  return { name, role, active };
}

createUser("Alice");           // { name: "Alice", role: "user", active: true }
createUser("Bob", "admin");    // { name: "Bob", role: "admin", active: true }

Rest Parameters

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

sum(1, 2, 3);       // 6
sum(1, 2, 3, 4, 5); // 15

Closures

Functions remember the variables from where they were created:

function createCounter() {
  let count = 0;
  return {
    increment: () => ++count,
    decrement: () => --count,
    getCount: () => count
  };
}

const counter = createCounter();
counter.increment();  // 1
counter.increment();  // 2
counter.getCount();   // 2

This pattern is incredibly common in Node.js. Closures let you create private state without classes.

Higher-Order Functions

Functions that take other functions as arguments:

function repeat(n, action) {
  for (let i = 0; i < n; i++) {
    action(i);
  }
}

repeat(3, (i) => console.log(`Iteration ${i}`));

// Array methods use higher-order functions
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2);
const evens = numbers.filter(n => n % 2 === 0);
const total = numbers.reduce((sum, n) => sum + n, 0);