JavaScript Fundamentals · Variables & Data Types · Lesson 4 of 48
Primitive Data Types
Concept
Primitive Data Types
JavaScript has string, number, boolean, undefined, null, and more. The typeof operator tells you what type you are working with.
Unlike languages such as Java or C++, you never declare a type up front — a variable's type is simply whatever value is currently assigned to it.
By the end of this lesson
- Explain the core idea behind Primitive Data Types.
- Predict output before running a change.
- Test one realistic and one unusual input.
- Use the result to make the next decision.
How to study this page
- 1. Read one concept.
- 2. Change the example.
- 3. Run, compare, and explain.
- 4. Complete the challenge below.
Syntax
typeof valueExampleRunnable
let city = "Tashkent";
let year = 2026;
let isStudent = true;
console.log(typeof city, typeof year, typeof isStudent);undefined vs null
let a;
let b = null;
console.log(a, typeof a);
console.log(b, typeof b);Note: undefined means a variable was declared but never given a value. null is an explicit, intentional 'no value' that you assign yourself.
Self-check before continuing
Without looking at the example, describe what changes when you modify one input in Primitive Data Types. Then reopen the editor and prove your explanation with a small test.
You are ready to continue when you can predict, test, and explain the result.
Your turn
Create one variable of each primitive type and log its typeof.
Loading editor…
Console