Temporal Dead Zone

let and const declarations are hoisted, but not like var.

• The Temporal Dead Zone (TDZ) is the period in which a variable exists but cannot yet be accessed—the time in which it has not been initialized.
• The TDZ starts at the beginning of the scope, when memory is allocated, and ends when the declaration line is reached during execution.
• If we try to access these variables before they are initialized, we get a ReferenceError: a variable cannot be accessed before initialization.
• During the TDZ, variables are stored in the Environment Record, which is part of the Lexical Environment (Environment Record and Outer Reference).
• Redeclaring let or const variables gives a syntax error because the variable has already been declared, but we can redeclare var.

var x = 1
var x = 2 // no error, silently overwrites

let y = 1
let y = 2 // SyntaxError

Global Object

• var attaches to the global object.

var name = "John"
console.log(window.name) // "John"
console.log(globalThis.name) // "John"

• let and const are not attached to globalThis.

let name = "John"
console.log(window.name) // undefined
console.log(globalThis.name) // undefined

Global Object (window)
├── setTimeout
├── alert
├── var name = "John"    ← var lives HERE
└── ...

Script Scope
├── let name = "John"    ← let/const live HERE
└── const age = 25

← let/const live HERE └── const age = 25

└── const age = 25