Variables & Data Types (let, const, typeof)

In Plain English

JavaScript features 7 primitive types: number, string, boolean, null, undefined, symbol, and bigint. Variables are declared with const (immutable binding) or let (reassignable), while typeof inspects the runtime type.

The Mental Model

"A variable is a labeled container. const is a welded box whose contents cannot be swapped for a new item, whereas let is an open jar where you can place different values as needed."

Deep Dive: How It Works

const vs let: Use const by default. Use let only when you know the variable will be reassigned (like loop counters).

Numbers: All numbers in JavaScript are IEEE 754 double-precision floats (no separate int vs float type for standard arithmetic).

null vs undefined: undefined means a variable has been declared but not assigned. null is an intentional empty value.

Syntax Blueprint

const name = "Ada"; let score = 100; typeof score ──▶ "number"

Use const for immutable bindings, let for reassignments, and typeof to inspect data types.

Core Rules to Remember

let: Block-scoped, reassignable variable declaration.

const: Block-scoped, non-reassignable constant reference.

typeof Operator: Returns a string indicating the data type (e.g., "string", "number", "boolean").

Common Beginner Traps & How to Fix Them

Mistake:Attempting to reassign a const variable.

Why it happens: const prevents reassignment to a new value: TypeError: Assignment to constant variable.

How to fix: If a variable must change its value over time, declare it with let instead.

Pro Tip: typeof null returns "object"—this is a historic 1995 JavaScript bug that cannot be changed for backwards compatibility.

Live Interactive Example

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Inspecting Types

JavaScript ES2024

Output Console

Click "Run Code" to view the rendered output.

How it works: Demonstrates JavaScript primitive types and the output of typeof.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Declare variables and inspect type

Declare const language = "JavaScript";
Declare let activeUsers = 42000;
Print "Type of language: string" using typeof language.
Print "Type of activeUsers: number" using typeof activeUsers.

Sandbox Output

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Deep Dive: How It Works

const vs let: Use const by default. Use let only when you know the variable will be reassigned (like loop counters).

Numbers: All numbers in JavaScript are IEEE 754 double-precision floats (no separate int vs float type for standard arithmetic).

null vs undefined: undefined means a variable has been declared but not assigned. null is an intentional empty value.