Data Types in JavaScript

What Are Data Types?

Data types tell JavaScript what kind of value you're working with. Different types allow different operations — you can add numbers, but you can't multiply text.

Primitive Data Types

JavaScript has 7 primitive data types (plus objects). Here are the ones you'll use constantly:

1. String — Text

Text wrapped in quotes. Use single quotes, double quotes, or backticks.

"Hello world"
'JavaScript'
`Template literals`

2. Number — Any Number

No separate type for decimals — everything is a Number.

42
3.14
-5
1_000_000  // readable with underscores

3. Boolean — True or False

Only two possible values: true or false.

let isLoggedIn = true;
let isGreater = 10 > 5;  // true

4. undefined — No Value Assigned

A variable that has been declared but not given a value.

let something;  // undefined

5. null — Intentional Empty Value

You explicitly set this to represent "no value."

let user = null;  // intentionally empty

6. Symbol — Unique Identifier

Creates a unique, immutable value. Used for object property keys.

The typeof Operator

Use typeof to check what type a value is:

typeof "hello"      // "string"
typeof 42           // "number"
typeof true         // "boolean"
typeof undefined    // "undefined"
typeof null         // "object" (this is a historical bug in JavaScript)

Common Mistakes

• Confusing undefined with null — they are not the same
• Using == instead of === (we'll cover this in the Operators lesson)
• Forgetting that typeof null === "object"
• Not wrapping strings in quotes

Quick Quiz

Summary

• Strings hold text, Numbers hold numeric values, Booleans hold true/false
• undefined means "no value assigned"
• null means "intentionally empty"
• Use typeof to check a value's type
• typeof null === "object" is a known JavaScript quirk