04 — Variables and scope

One line: A JavaScript variable is a binding in a scope, and nearly every scope surprise comes down to when a binding is created, how far it reaches, and whether a loop turn or a function call gets its own.

The chapter starts with the three declarations and the two kinds of scope they create, then what const does and does not fix. Hoisting and the temporal dead zone explain when a name exists before its line runs. Closures follow, because a function keeps the bindings of the scope it was made in, and then the loop where that matters most. It ends with the global object and with destructuring, which declares several bindings at once.

The pages

Page	Level	In one line	Status
var, let and const	101	var belongs to the whole function, so a var declared inside an if block is still there after the block ends, while let and const stop at the closing brace; const also refuses reassignment, with a TypeError at run time.	stub
const is not frozen	101	const fixes the binding, not the value: const list = [1] still lets you push, only list = [] throws, and even Object.freeze stops at the first level, so a frozen object's array property still takes new items.	stub
Hoisting and the temporal dead zone	201	Every declaration is known from the top of its scope: a var reads as undefined before its line, a function declaration can already be called, and a let or const throws a ReferenceError until its line has run — a matter of time, not position.	stub
Closures	201	A closure keeps the variable, not a copy of its value: a function made before greeting changes sees the new value, and two functions returned by one call share one count, while a second call gets a fresh one.	stub
Closures in loops	201	Functions made in a for (var i ...) loop all see the one i the loop finished with, so they return 3, 3, 3; for (let i ...) gives each turn its own copy of i, so they return 0, 1, 2.	stub
The global object	201	In a classic script, a top-level var or function declaration becomes a property of globalThis but let, const and class do not; in a module none of them do, so var x at the top of an .mjs file never reaches globalThis.x.	stub
Destructuring	101	A destructuring default fires only for undefined: const { a = 1 } = { a: null } leaves a as null and const [c = 1] = [0] keeps 0; and destructuring null itself throws a TypeError, even when the pattern asks for nothing.	stub

Boundaries

this and function parameters are chapter 5, and the scope of a module and its imports is chapter 14.