JavaScript Basics

Quick reference for JavaScript syntax, methods, and common patterns.

let variable = "value";
const constant = "immutable";
var oldWay = "avoid";

typeof "string"     // "string"
typeof 42           // "number"
typeof true         // "boolean"
typeof undefined    // "undefined"
typeof null         // "object" (bug!)
typeof {}           // "object"
typeof []           // "object"
typeof function(){} // "function"

// Function declaration
function greet(name) {
  return `Hello, ${name}!`;
}

// Arrow function
const greetArrow = (name) => `Hello, ${name}!`;

// Function expression
const greetExpr = function(name) {
  return `Hello, ${name}!`;
};

const arr = [1, 2, 3, 4, 5];

// Map
arr.map(x => x * 2);        // [2, 4, 6, 8, 10]

// Filter
arr.filter(x => x > 2);     // [3, 4, 5]

// Reduce
arr.reduce((sum, x) => sum + x, 0); // 15

// Find
arr.find(x => x > 3);       // 4

// Includes
arr.includes(3);            // true

const obj = { name: "John", age: 30 };

// Object.keys()
Object.keys(obj);           // ["name", "age"]

// Object.values()
Object.values(obj);         // ["John", 30]

// Object.entries()
Object.entries(obj);        // [["name", "John"], ["age", 30]]

// Destructuring
const { name, age } = obj;  // name = "John", age = 30

// Promise
new Promise((resolve, reject) => {
  // async operation
  resolve("success");
});

// Async/Await
async function fetchData() {
  try {
    const response = await fetch('/api/data');
    const data = await response.json();
    return data;
  } catch (error) {
    console.error('Error:', error);
  }
}

// Template literals
const message = `Hello ${name}!`;

// Spread operator
const newArr = [...arr, 6, 7];
const newObj = { ...obj, city: "NYC" };

// Rest parameters
function sum(...numbers) {
  return numbers.reduce((a, b) => a + b, 0);
}

// Optional chaining
const value = obj?.prop?.subProp;

// Nullish coalescing
const result = value ?? "default";