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
*/

Declaring variables

JavaScript has three keywords to create data containers:

1. var: The old way. It has function scope and can cause confusing behavior (avoid using it).
2. let: The modern way. It allows reassignment and has block scope.
3. const: For values that should not change. Once assigned, it cannot be modified.

Scope

It is the place where a variable lives and can be accessed.

• Global: Outside any function or block.
• Local / Block: Inside { }, such as within an if or a function.

if (true) {
    let message = "Hello"; // Only lives inside this block
    var greeting = "Good morning"; // Escapes the block (problem with var)
}
console.log(greeting); // Works
console.log(message); // Error! ReferenceError

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