1.13 — Introduction to JavaScript: Quick Revision

Compact cheat sheet. Print-friendly.

How to use this material (instructions)

1. Skim top-to-bottom before interviews or labs.
2. If a row feels fuzzy — reopen the lesson: README.md → 1.13.a…1.13.h.
3. Drills — 1.13-Exercise-Questions.md.
4. Polished answers — 1.13-Interview-Questions.md.

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1.13.a What is JavaScript?

Concept	One-liner
JS role	Behavior — logic, events, DOM updates, network, apps
With HTML/CSS	Structure / presentation / behavior
ECMAScript	The language spec; “JS” = ES + host APIs
Not Java	Unrelated languages

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1.13.b Where JS runs

Browser  → DOM + Web APIs + window/document
Node.js  → V8 + files/HTTP/process (no document)

Term	Meaning
Runtime	Engine + host APIs + event loop (conceptually)
V8	Google’s JS engine (Chrome, Node)

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1.13.c <script> loading

Attribute	Behavior
(none, classic)	Fetch + run can block HTML parse
defer	Parallel fetch; run after parse; ordered
async	Parallel fetch; run ASAP; order not guaranteed between async scripts
type="module"	ES modules; deferred; import/export

Default choice for apps: <script src="app.js" defer></script> or type="module".

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1.13.d Console

Method	Use
console.log	General output (multiple args OK)
console.warn / error	Severity + stack styling
console.table	Tabular data
REPL	Type expressions at > in Console

Errors: read name + first stack frame first.

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1.13.e Variables & scope

Keyword	Reassign?	Scope	Hoisting / TDZ
const	No	Block	TDZ
let	Yes	Block	TDZ
var	Yes	Function	Hoisted; no TDZ

Rule: const → let → avoid var.

const object: cannot reassign binding; can mutate properties.

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1.13.f Naming

Style	Example	Use
camelCase	firstName	variables, functions
PascalCase	UserCard	classes / some components
SCREAMING_SNAKE	API_URL	fixed config constants

Booleans: is*, has*, should*, can*.

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1.13.g Types

Primitives (ES6+)

number · string · boolean · null · undefined · bigint · symbol

typeof cheatsheet

Expression	Result
typeof 1	"number"
typeof "x"	"string"
typeof true	"boolean"
typeof undefined	"undefined"
typeof null	"object" (quirk)
typeof {}	"object"
typeof NaN	"number"

Checks: x === null · Array.isArray(x) · prefer === / !==

null vs undefined: intentional empty vs not set / missing.

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1.13.h Mini programs

const n = Number(prompt("Number?"));  // string → number
if (prompt("x") === null) { /* cancelled */ }
alert(`Hello, ${name}!`);              // template literal

Production: replace prompt/alert with forms + DOM.

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One-liner definitions

Term	One-liner
DOM	Live tree of HTML nodes in memory
Engine	Executes JS (V8, SpiderMonkey, JSC)
Node.js	Server-side JS runtime (no DOM by default)
Primitive	Non-object atomic value
Reference type	Object (stored by reference)
Block scope	{ } limits visibility of let/const
TDZ	Cannot access let/const before declaration line
Hoisting	var declaration lifted; initializes as undefined
Template literal	Backtick string with ${expr} interpolation
Coercion	Implicit type conversion (==, some operators)

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Master workflow

1. HTML loads → parser hits <script> (mind defer/async/module).
2. JS runs in a host (browser: DOM; Node: core modules).
3. Use const/let, meaningful names, === for checks.
4. Know types (typeof quirks, array check, null vs undefined).
5. Debug with Console + errors; grow into Sources breakpoints.
6. I/O learning: prompt → string → Number() + validation.

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End of 1.13 quick revision.