Variables, Data Types, and Operators

Variables store data. Data types describe the kind of data. Operators let you manipulate that data. Mastering these three concepts is essential before writing any meaningful JavaScript program.

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Declaring Variables

JavaScript provides three keywords for declaring variables:

Keyword	Scope	Reassignable?	Hoisted?	Best For
var	Function	Yes	Yes (initialised as undefined)	Legacy code — avoid in modern JS
let	Block	Yes	Yes (but not initialised — "temporal dead zone")	Values that change
const	Block	No	Yes (but not initialised)	Values that stay the same

var oldWay = "I'm function-scoped";   // avoid in modern code
let counter = 0;                      // can be reassigned
const PI = 3.14159;                   // cannot be reassigned

Rule of thumb: Default to const. Use let only when you need to reassign. Never use var in new code.

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Data Types

JavaScript has eight data types, split into primitives and objects.

Primitive Types (7)