JavaScript is single-threaded — it runs one piece of code at a time. Yet it can handle clicks, timers, network requests, and animations without freezing. The event loop is how. It’s the mechanism that lets JavaScript juggle many tasks on a single thread. Key point: JavaScript has one call stack, one heap, and a task…

A generator is a function that can pause and resume. Where a regular function runs to completion, a generator produces a sequence of values, one at a time, on demand. The function* syntax and yield keyword make this possible. Key point: Generators return an iterator. You pull values out one at a time with .next(),…

A Symbol is a unique, immutable primitive value used as an object property key. Every Symbol is guaranteed to be different from every other Symbol — even if they have the same description. Symbols let you add properties to objects without any risk of collision, and they power many of JavaScript’s built-in protocols. Key point:…

BigInt is a numeric type for arbitrary-precision integers. Where regular JavaScript numbers lose precision past 2^53 – 1, BigInt keeps exact values no matter how large. It’s the tool for cryptography, large IDs, precise timestamps in nanoseconds, and anywhere exact integer math matters. Key point: BigInt and Number are different types. You can’t mix them…

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…

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…

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…

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 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…

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…