JavaScript

Variables and Scoping

var legacy = 'function-scoped, avoid'; // avoid: hoisted, function-scoped
let count = 0; // block-scoped, reassignable
const name = 'Ada'; // block-scoped, cannot be reassigned

• Prefer const by default, let when a value needs to change, and avoid var.
• const prevents reassignment, not mutation — const arr = [] still allows arr.push(1).

Functions and Arrow Functions

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

const add2 = (a, b) => a + b; // implicit return
const greet = (name) => {
  return `Hello, ${name}!`;
};

• Arrow functions don’t bind their own this — they inherit it from the enclosing scope, which is why they’re preferred for callbacks.

Arrays and Objects

const numbers = [1, 2, 3];
const user = { name: 'Ada', age: 36 };

numbers.push(4); // add to end
numbers.map((n) => n * 2); // [2, 4, 6, 8]
numbers.filter((n) => n > 1); // [2, 3]
Object.keys(user); // ['name', 'age']
Object.entries(user); // [['name', 'Ada'], ['age', 36]]

Destructuring

const { name, age } = user;
const [first, second] = numbers;

function greetUser({ name }) {
  return `Hi, ${name}`;
}

const { name: userName = 'Anonymous' } = {}; // default value

Promises and Async/Await

function fetchUser(id) {
  return fetch(`/api/users/${id}`).then((res) => res.json());
}

async function loadUser(id) {
  try {
    const res = await fetch(`/api/users/${id}`);
    return await res.json();
  } catch (error) {
    console.error('Failed to load user', error);
  }
}

// Run requests in parallel:
const [a, b] = await Promise.all([fetchUser(1), fetchUser(2)]);

Modules (import/export)

// math.js
export const PI = 3.14159;
export function square(x) {
  return x * x;
}
export default function add(a, b) {
  return a + b;
}

// app.js
import add, { PI, square } from './math.js';

Common Array and String Methods

[1, 2, 3].reduce((sum, n) => sum + n, 0); // 6
[1, 2, 3].find((n) => n > 1); // 2
[1, 2, 3].includes(2); // true
[1, [2, [3]]].flat(Infinity); // [1, 2, 3]

'Hello'.toUpperCase(); // 'HELLO'
'  hi  '.trim(); // 'hi'
'a,b,c'.split(','); // ['a', 'b', 'c']
`Hello, ${name}!`; // template literal

References

• MDN JavaScript Guide
• MDN JavaScript Reference