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)

Type	Example	Notes
Number	42, 3.14, NaN, Infinity	64-bit floating point (IEEE 754)
BigInt	9007199254740993n	Arbitrary-precision integers
String	"hello", 'world', template	Immutable sequence of characters
Boolean	true, false	Logical values
undefined	undefined	Variable declared but not assigned
null	null	Intentional absence of value
Symbol	Symbol("id")	Unique, immutable identifier

Object Type (1)

Everything that is not a primitive is an object: arrays, functions, dates, regular expressions, and plain objects ({}).

const person = { name: "Alice", age: 30 };   // object
const colours = ["red", "green", "blue"];     // array (a special object)

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The typeof Operator

Use typeof to check a value's type at runtime:

typeof 42;            // "number"
typeof "hello";       // "string"
typeof true;          // "boolean"
typeof undefined;     // "undefined"
typeof null;          // "object"   ← historical bug, never fixed
typeof {};            // "object"
typeof [];            // "object"   ← use Array.isArray() instead
typeof function(){}   // "function"

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Strings and Template Literals

Strings can be created with single quotes, double quotes, or backticks.

const single = 'Hello';
const double = "World";

Template literals (backticks) allow embedded expressions and multi-line strings:

const name = "Alice";
const greeting = `Hello, \${name}! You have \${2 + 3} new messages.`;
// "Hello, Alice! You have 5 new messages."

const multiLine = `Line one
Line two
Line three`;

Useful String Methods