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Lesson 14 — Variables & Data Types Cheat Sheet

Full-Stack

In one line: const/let declare variables with block scope; JavaScript has six core data types; always compare with === not ==.

Key Ideas

1const. Can't be reassigned — the default choice.

2let. Can be reassigned, block-scoped.

3var. Function-scoped, legacy — avoid in new code.

4===. Strict equality — no type coercion.

5==. Loose equality — coerces types, causes bugs.

number, string, boolean

Primitives

array, object

Reference types

undefined / null

Absence of value

0 == "0"

true (coerced)

0 === "0"

false (strict)