Let Var Const

var, let, and const define variables with different scopes, hoisting behavior, and re-declaration capabilities.

var

• Functional scope.
• Hoisting with initialization to undefined.
• Allows re-definition and re-declaration.
• Considered obsolete in modern JS.

var name = "Alex";
var name = "Oleh"; // allowed

let

• Block scope.
• Subject to hoisting but falls into the TDZ (Temporal Dead Zone) until initialization → accessing the declaration causes an error.
• Can be changed but cannot be re-declared in the same block.

let age = 25;
age = 30;

const

• Block scope.
• Also has TDZ until initialization.
• Mandatory initialization upon declaration.
• Cannot be re-defined, but the contents of objects and arrays can be changed (without changing the reference).

const user = { name: "Anna" };
user.name = "Olha";

Key Ideas

• var — hoisting + undefined + function scope.
• let/const — block scope + TDZ until initialization.
• let changes, const — does not, but objects remain mutable.