Variables, Constants and Comments

Comments

Comments are notes we leave in the code for ourselves or other developers. They are not executed.

• Single line: Use //.
• Block (multiple lines): Use /* ... */.

// This is a single-line comment
/*
   This is a comment
   that spans multiple lines
*/

Variable Declaration

JavaScript has three keywords to create data containers:

1. var: Old way. Has function scope and can cause confusion (avoid using it).
2. let: Modern way. Allows reassigning values and has block scope.
3. const: For values that won’t change. Once assigned, it cannot be modified.

Scope

Scope is where a variable lives and is accessible.

• Global: Outside any function or block.
• Local / Block: Inside { }, like in an if or a function.

if (true) {
    let mensaje = "Hola"; // Only lives inside this block
    var saludo = "Buen día"; // Leaks out of the block (var problem)
}
console.log(saludo); // Works
console.log(mensaje); // Error! ReferenceError

Golden rule: Use const by default. If you need to change the value later, use let.