JavaScript Basics

JavaScript = dynamic, interpreted, single-threaded language for web

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What is JavaScript?

JavaScript is a high-level, interpreted programming language that:

• Runs in browsers (client-side)
• Runs on servers (Node.js)
• Is dynamically typed
• Supports multiple paradigms (OOP, functional, procedural)

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Key Characteristics

Dynamic · Single-threaded · Event-driven

Feature	Meaning
Dynamic typing	Types determined at runtime
Single-threaded	One call stack, one execution
Event-driven	Asynchronous via event loop
Prototype-based	Objects inherit from prototypes
First-class functions	Functions are values

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Data Types

Primitives + Objects

Primitives (7 types)

• string
• number
• boolean
• undefined
• null
• symbol (ES6)
• bigint (ES2020)

Objects

• Arrays
• Functions
• Objects
• Dates
• RegExp

"JavaScript has 7 primitive types and objects, which are reference types."

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Variable Declarations

var = old, let/const = modern

Declaration	Scope	Hoisting	Reassignment
var	Function	Yes	Yes
let	Block	Temporal dead zone	Yes
const	Block	Temporal dead zone	No

"Use const by default, let when reassignment is needed, and avoid var."

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Hoisting

Declarations move to top

• var and function declarations are hoisted
• let and const are in temporal dead zone
• Only declarations hoisted, not initializations

"Hoisting moves variable and function declarations to the top of their scope before execution."

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Scope

Global · Function · Block

• Global scope: Accessible everywhere
• Function scope: var scoped to function
• Block scope: let and const scoped to {}

"Scope determines where variables are accessible, with block scope being preferred for let and const."

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Functions

Function declarations vs expressions

// Declaration (hoisted)
function greet() {}

// Expression (not hoisted)
const greet = function () {};

// Arrow function (ES6)
const greet = () => {};

"Function declarations are hoisted, while expressions and arrow functions are not."

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🧠 Ultra-Short Cheat Sheet

Dynamic typing
Single-threaded + event loop
7 primitives + objects
var/let/const
Hoisting
Scope (global/function/block)
Closures
this binding
Arrow functions