Data Types

Primitive Types

• Number: Numbers of any kind (integers or decimals).
• String: Text values.
• Boolean: true or false.

The “Weird Values”

JavaScript has unique behavior with certain values that can be confusing at first:

1. NaN (Not-a-Number)

It happens when you perform an invalid math operation (for example, "hello" * 2).

• Fun fact: typeof NaN is "number".
• NaN === NaN is false! You must use isNaN() to check it.

2. null

It represents the intentional absence of a value. It is an object (typeof null is "object").

3. undefined

It means a variable has been declared but has not been assigned any value yet.

4. Falsy Values

In boolean contexts (such as an if), these values evaluate to false:

• 0
• "" (empty string)
• null
• undefined
• NaN
• false

The typeof operator

It is used to know the type of data a variable has at that moment.

let x = 10;
console.log(typeof x); // "number"