Node.js Documentation
console.log(str);
console.error(str);
// Example:
console.log("Hello, world!");
// Printing without a new line:
process.stdout.write("Hello, world. ");
process.stdout.write("How are you?");
// Declare and initialize variables
let myVarName = 5; // Block-scoped
const myConstant = 10; // Constant variable
var oldVar = 20; // Function-scoped
// Assign to an existing variable
myVarName = 10;
// Print a variable
console.log(myVarName);
console.log("The value is: " + myVarName);
// Functions with parameters
function printText(input) {
console.log(input);
}
// Arrow function syntax
const addTwo = (number) => {
return number + 2;
};
// Inline arrow function
const multiplyByTwo = (number) => number * 2;
// Using the 'readline' module for input
const readline = require("readline");
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout
});
rl.question("What is your name? ", (name) => {
console.log("Hello, " + name);
rl.close();
});
// Comparison operators return booleans
x == y // equality (loose)
x === y // strict equality
x != y // not equal
x !== y // strict not equal
x > y // greater than
x >= y // greater than or equal
x < y // less than
x <= y // less than or equal
// In an if statement
if (x === y) {
console.log("x and y are strictly equal");
}
if (x > 5) {
console.log("x is greater than 5");
}
// Math operators
let sum = x + y;
let product = x * y;
// Increment and decrement
x++;
x--;
// Shortcuts
x += y;
x -= y;
x *= y;
x /= y;
// Math methods
const abs = Math.abs(-10); // 10
const sqrt = Math.sqrt(16); // 4
const rounded = Math.round(3.14); // 3
const floored = Math.floor(5.7); // 5
const ceiling = Math.ceil(5.1); // 6
// Random numbers
let random = Math.random(); // 0 <= random < 1
let randomInt = Math.floor(Math.random() * 10) + 1; // Random 1-10
// For loop
for (let i = 0; i < 5; i++) {
console.log("Iteration:", i);
}
// While loop
let count = 5;
while (count > 0) {
console.log(count);
count--;
}
// Do-while loop
let num = 0;
do {
console.log(num);
num++;
} while (num < 5);
// Timers in Node.js
setTimeout(() => {
console.log("This runs after 2 seconds");
}, 2000);
let interval = setInterval(() => {
console.log("Repeating every 1 second");
}, 1000);
// Stop the timer
setTimeout(() => {
clearInterval(interval);
console.log("Timer stopped");
}, 5000);
// Event Emitter in Node.js
const EventEmitter = require("events");
const emitter = new EventEmitter();
// Register an event
emitter.on("myEvent", (arg) => {
console.log("Event triggered with:", arg);
});
// Emit the event
emitter.emit("myEvent", "some data");
// Exporting a module
// File: myModule.js
exports.sayHello = (name) => {
console.log("Hello, " + name);
};
// Importing a module
// File: app.js
const myModule = require("./myModule");
myModule.sayHello("John");
const fs = require("fs");
// Writing to a file
fs.writeFile("example.txt", "Hello, world!", (err) => {
if (err) throw err;
console.log("File created!");
});
// Reading from a file
fs.readFile("example.txt", "utf8", (err, data) => {
if (err) throw err;
console.log("File content:", data);
});
// Callback example
fs.readFile("example.txt", "utf8", (err, data) => {
if (err) return console.error(err);
console.log("File content:", data);
});
// Promises
const readFile = (filename) => {
return new Promise((resolve, reject) => {
fs.readFile(filename, "utf8", (err, data) => {
if (err) reject(err);
else resolve(data);
});
});
};
readFile("example.txt")
.then((data) => console.log("Content:", data))
.catch((err) => console.error(err));
// Async/Await
const main = async () => {
try {
const data = await readFile("example.txt");
console.log("Content:", data);
} catch (err) {
console.error(err);
}
};
main();
const http = require("http");
const server = http.createServer((req, res) => {
res.writeHead(200, { "Content-Type": "text/plain" });
res.end("Hello, Node.js HTTP Server!");
});
server.listen(3000, () => {
console.log("Server is running on http://localhost:3000");
});
// Create an array
const fruits = ["apple", "banana", "cherry"];
// Access an element
console.log(fruits[0]); // "apple"
// Add an element
fruits.push("date"); // Adds to the end
// Remove an element
fruits.pop(); // Removes the last element
// Loop through an array
for (let fruit of fruits) {
console.log(fruit);
}
// Using an Object
const user = {
name: "Alice",
age: 25
};
// Access a property
console.log(user.name); // "Alice"
// Add a property
user.email = "alice@example.com";
// Delete a property
delete user.age;
// Create a Set
const mySet = new Set();
// Add elements
mySet.add(1);
mySet.add(2);
// Check if an element exists
console.log(mySet.has(1)); // true
// Loop through a Set
for (let value of mySet) {
console.log(value);
}
const jsonString = '{"name": "John", "age": 30}';
// Parse JSON string into an object
const jsonObject = JSON.parse(jsonString);
console.log(jsonObject.name); // John
// Convert object to JSON string
const jsonOutput = JSON.stringify(jsonObject);
console.log(jsonOutput);
