#fundamentals

JavaScript has two core operators for checking what type a value is, and each answers a different question. typeof returns one of 8 strings and has known gotchas (null, arrays). instanceof walks the prototype chain and fails for objects from an embedded <iframe>. A note on when to use which, and why arrays have a dedicated Array.isArray.

Every object property in JS carries 4 flags under the hood (writable, enumerable, configurable, plus value or get/set). You rarely think about them - until something odd happens: an assignment doesn't change the value, delete doesn't remove the field, for...in skips the property. A note on what property descriptors are, how getters/setters work, and why Object.defineProperty has different defaults than an object literal.

JavaScript has a single numeric type - number - and plenty of things that trip up on it. A note on IEEE 754, NaN, precision limits, and BigInt. Why 0.1 + 0.2 !== 0.3 and why NaN === NaN is false.

Three methods on Function.prototype with the same goal - setting this. The differences are in (a) whether they invoke immediately, and (b) how you pass arguments. A quick rundown of which to use when.

In JavaScript an object can call a method it doesn't own. Class methods are defined once, on a shared prototype, and instances reach for them through a link. The mechanism is called the prototype chain. A note on how it works and why.

The classic explanation says == checks values and === checks both type and value. The spec says something different - both check the type. This note is about what the actual difference is.

The common advice says: always let, never var. But each of them communicates something different about the author's intent. A note on when var still makes sense and how to choose between them.

A common saying is that const means „immutable". In reality it only protects assignment to the name, not the value itself - that's why mutating an array declared with const works, but trying to assign it something new throws.

How does the engine know that one function can see another's variables, but not the other way around? In JavaScript it comes down to where the code sits, decided before anything runs. That's lexical scope.