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

ExampleRunnable

Try it Yourself »

undefined vs null

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.

Need a nudge?

Ask about this lesson or your next move. The guide explains the path without taking the work away from you.

Ready for the next lesson?

Mark this lesson complete when you have finished the exercise.

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Console

Output will appear here...