JavaScript Cheatsheet

// 1. Variable Declaration
const constantVar = "Cannot be reassigned";
let blockScopedVar = "Can be reassigned";
var functionScopedVar = "Legacy, avoid if possible"; // Avoid

// 2. Data Types
const aString = `Hello with template literal ${blockScopedVar}`; // String
const aNumber = 42; // Number
const aBoolean = true; // Boolean
const aNull = null; // Null
const anUndefined = undefined; // Undefined
const aSymbol = Symbol("unique"); // Symbol
const aBigInt = 9007199254740991n; // BigInt

// 3. Operators
console.log(2 + 2); // Addition
console.log("hello" + " world"); // String concatenation
console.log(aNumber === 42); // Strict equality (no type coercion)
console.log(aNumber !== "42"); // Strict inequality
console.log(true && false); // Logical AND
console.log(true || false); // Logical OR

// Arrays
const fruits = ["apple", "banana", "cherry"];
fruits.push("orange");
console.log(`First fruit: ${fruits[0]}`);
fruits.forEach(fruit => console.log(fruit));

// Array methods: map, filter, reduce
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2); // [2, 4, 6, 8, 10]
const evens = numbers.filter(n => n % 2 === 0); // [2, 4]
const sum = numbers.reduce((acc, n) => acc + n, 0); // 15

// Objects
const person = {
  firstName: "John",
  lastName: "Doe",
  age: 30,
  fullName: function() { return `${this.firstName} ${this.lastName}`; }
};
console.log(person.firstName); // Dot notation
console.log(person["age"]); // Bracket notation

// Sets (unique values)
const mySet = new Set([1, 1, 2, 3]); // {1, 2, 3}
mySet.add(4);

// Maps (key-value pairs, any type can be a key)
const myMap = new Map();
myMap.set("name", "Alice");
console.log(myMap.get("name"));

const value = 10;
if (value > 5) {
  console.log("Greater than 5");
} else {
  console.log("Not greater than 5");
}

// For...of loop (for iterable objects like arrays)
const colors = ["red", "green", "blue"];
for (const color of colors) {
  console.log(color);
}

// For...in loop (for object properties)
const user = { name: "Bob", role: "Admin" };
for (const key in user) {
  console.log(`${key}: ${user[key]}`);
}

// Arrow Function
const add = (a, b) => a + b;

// Function with default parameters
const greet = (name = "Guest") => `Hello, ${name}!`;
console.log(greet()); // "Hello, Guest!"
console.log(greet("Alice")); // "Hello, Alice!"

// Rest and Spread operators
const sumNumbers = (...numbers) => numbers.reduce((acc, n) => acc + n, 0);
console.log(sumNumbers(1, 2, 3)); // 6

const arr1 = [1, 2];
const arr2 = [...arr1, 3, 4]; // Spread operator
console.log(arr2); // [1, 2, 3, 4]

class Animal {
  constructor(name) {
    this.name = name;
  }

  speak() {
    console.log(`${this.name} makes a noise.`);
  }
}

// Inheritance
class Dog extends Animal {
  constructor(name, breed) {
    super(name); // Call parent constructor
    this.breed = breed;
  }

  speak() {
    console.log(`${this.name} barks.`);
  }
}

const myDog = new Dog("Buddy", "Golden Retriever");
myDog.speak(); // "Buddy barks."

// Promises
const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("Promise resolved!");
  }, 1000);
});

myPromise
  .then(result => console.log(result))
  .catch(error => console.error(error));

// Async/Await (syntactic sugar for Promises)
const fetchData = async () => {
  try {
    // Assuming fetch is in a browser/Node environment
    // const response = await fetch('https://api.example.com/data');
    // const data = await response.json();
    const data = await myPromise; // Using the promise from above
    console.log("Async/Await:", data);
  } catch (error) {
    console.error("Error fetching data:", error);
  }
};

fetchData();

// --- In file: math.js ---
// export const PI = 3.14;
// export default function add(a, b) {
//   return a + b;
// }

// --- In file: main.js ---
// import add, { PI } from './math.js';
// console.log(PI);
// console.log(add(5, 2));

console.log("ES Modules use 'import' and 'export' keywords.");
console.log("This example is commented out as it requires separate files.");

# 1. Initialize a new project (creates package.json)
npm init -y

# 2. Install a package (e.g., lodash)
npm install lodash

# 3. Install a development dependency (e.g., jest)
npm install --save-dev jest

# package.json will now list lodash as a dependency
# and jest as a devDependency.

# 4. Run scripts defined in package.json
# "scripts": { "test": "jest" }
npm test