JavaScript

The spread operator (…) takes an iterable — an array, a string, a Set, a Map — or an object, and expands its contents into individual elements. It’s the opposite of the rest parameter, which collects values into an array. Same syntax, different purpose. Key point: Spread expands; rest collects. Both use …, and the…

A Promise is JavaScript’s way of representing a value that isn’t available yet but will be — or won’t be — at some point in the future. It’s the foundation of modern asynchronous code, and it solves the “callback hell” problem that plagued JavaScript before ES6. Key point: A Promise is an object with three…

async and await are the syntax sugar on top of Promises that makes asynchronous code read like synchronous code. Instead of chaining .then() calls, you write await in front of a Promise — and the function pauses until the Promise settles, without blocking the rest of the program. Key point: async marks a function as…

ES Modules (ESM) are the official, standardized way to split JavaScript code into reusable files. They use import and export to share code between files, and they’ve replaced the older CommonJS (require/module.exports) approach in modern JavaScript. Key point: ES Modules are static — imports are resolved at load time, not runtime (except for dynamic import())….

Arrays come with dozens of built-in methods — the day-to-day tools that make JavaScript expressive. Some mutate the array in place; others return a new array or a value. Knowing which is which, and when to use what, is the difference between clean code and buggy code. Key point: Every array method falls into one…

The Object global is a toolbox — it’s the constructor for objects, but it also holds static methods that work on any object. These methods are how you enumerate, copy, freeze, compare, and inspect objects. They’re the foundation of modern JavaScript patterns like immutability and destructuring. Key point: Most Object.* methods work on own enumerable…

A Set is a collection of unique values. Unlike arrays, it never allows duplicates and provides fast lookup with .has(). A WeakSet is a special variant that only holds objects and lets them be garbage-collected when no other references remain. Key point: Use Set when you need uniqueness or fast membership checks on primitives. Use…

The Date object represents a single moment in time. It’s been in JavaScript since the beginning, and it carries some quirks — zero-indexed months, mutable objects, inconsistent parsing. But it’s also unavoidable, because every application eventually needs to show, compare, or manipulate dates. Key point: A Date is stored as milliseconds since the Unix epoch…

Regular expressions (regex) are patterns for matching text. They look cryptic at first — /^\d{3}-\d{4}$/ — but they’re the most powerful way to search, validate, and transform strings. Every language has them, and JavaScript’s are among the most capable. Key point: A regular expression is an object that describes a pattern. You can create one…

JavaScript has one number type — a 64-bit floating-point value. There’s no separate integer type (except BigInt). Understanding how it behaves — including its limits and quirks — is essential for writing correct numeric code. Key point: All JavaScript numbers are IEEE 754 double-precision floating-point. This means integers up to 2^53 are exact, but floating-point…