JavaScript — Part 2

Promise Basics

Promise Combinators 🔥

Promise Chaining

Promisify Callbacks

Core Syntax

Parallel vs Sequential ⭐

Async Iteration & Generators

AbortController ⭐

Execution Order (Memorize This)

Task Queue Types

setTimeout / setInterval

setTimeout(fn, 0) doesn't run instantly. It's still a macrotask — runs after all microtasks.

Class Syntax

Inheritance ⭐

Mixins Pattern

Static blocks (ES2022)

How Prototype Chain Works

Constructor Functions (old way)

Prototype Methods

Export

Import

import.meta

Map — key/value pairs

Set — unique values

WeakMap & WeakSet

Symbol Basics

Well-Known Symbols ⭐

Proxy — Intercept Object Operations

Reflect API

Proxy Use Cases

Selecting Elements

Modify DOM

Attributes & Classes & Style

Dimensions & Position

addEventListener

Event Object

Event Delegation ⭐

Custom Events

localStorage & sessionStorage

Fetch API ⭐

Observer APIs ⭐

Other Useful Browser APIs

Module Pattern

Observer Pattern

Singleton & Factory

Pipe / Compose

Classic JS Traps

typeof null === "object" — Historical bug. Check null with: x === null.

NaN !== NaN — NaN is the only value not equal to itself. Use Number.isNaN(x), not x === NaN.

0.1 + 0.2 !== 0.3 — Floating point. Use Math.abs(a - b) < Number.EPSILON for comparison.

Array.sort() is alphabetical by default — [10,9,2].sort() → [10,2,9]. Always pass a comparator: .sort((a,b) => a-b).

var in loops with closures — Use let or capture with IIFE. Classic setTimeout(() => console.log(i)) in var-loops prints max value.

arguments object doesn't exist in arrow functions — Use rest params (...args) instead.

== coercion surprises — [] == false is true. [] == ![] is true. Always use ===.

Object reference equality — {} === {} is false. Objects are equal only if same reference. Use JSON.stringify or deep-equal for comparison.

Async function always returns Promise — Even if you return 42, callers get Promise<42>. You must await it.

Promise executor runs synchronously — The (resolve, reject) callback inside new Promise() runs immediately, before .then(). Only the resolution is async.

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