Variable scope (var, let, and const)

When we declare a variable in TypeScript, we can use the var, let, or const keywords:

var myNumber: number = 1; 
let isValid: boolean = true; 
const apiKey: string = "0E5CE8BD-6341-4CC2-904D-C4A94ACD276E"; 

Variables declared with var are scoped to the nearest function block (or global, if outside a function block).

Variables declared with let are scoped to the nearest enclosing block (or global, if outside any block), which can be smaller than a function block.

The const keyword creates a constant that can be global or local to the block in which it is declared. This means that constants are block-scoped.