1// Each line below is a statement
2let name = 'Alice';          // declaration statement
3name = 'Bob';                // assignment statement
4console.log(name);           // expression statement
5
6if (name === 'Bob') {        // if statement (block)
7  console.log('Hello Bob!');
8}
9
10for (let i = 0; i < 3; i++) { // for statement (block)
11  console.log(i);
12}
13
14// Multiple statements can be on one line (not recommended)
15let x = 1; let y = 2; console.log(x + y);
16
17// A block { } groups multiple statements together
18{
19  let temp = 10;
20  console.log(temp);
21}

1// ✅ With semicolons — explicit, clear, no surprises
2const a = 5;
3const b = 10;
4console.log(a + b);
5
6// ✅ Without semicolons — works fine (ASI inserts them)
7const c = 5
8const d = 10
9console.log(c + d)
10
11// ❌ ASI FAILURE — classic bug
12// You might expect: const x = 1; [1, 2, 3].forEach(...)
13// JS reads it as:   const x = 1[1, 2, 3].forEach(...) → TypeError!
14const arr = [1, 2, 3]
15;[1, 2, 3].forEach(n => console.log(n)) // workaround: leading semicolon
16
17// ❌ Another ASI trap with return
18function getValue() {
19  return   // ASI inserts ; here!
20  {         // this object is never returned
21    value: 42
22  }
23}
24console.log(getValue()); // undefined — not { value: 42 }!

1// This is a single-line comment — everything after // is ignored
2console.log('Single-line comment above me');
3
4/*
5  This is a multi-line comment.
6  It can span as many lines as you need.
7  Useful for longer explanations or temporary code blocks.
8*/
9console.log('Multi-line comment above me');
10
11// Inline comment — after valid code
12const TAX_RATE = 0.18; // 18% GST — update if rate changes
13
14// ✅ Good comment — explains WHY, not WHAT
15const delay = 300; // Debounce delay in ms — prevents API spam on keypress
16
17// ❌ Bad comment — just repeats the code
18const price = 99; // set price to 99
19
20// Commenting out code temporarily (use sparingly)
21// console.log('debug: checking value here');
22console.log('All comments above are ignored by the engine');

1// ✅ These are THREE different variables
2let color  = 'red';
3let Color  = 'green';
4let COLOR  = 'blue';
5console.log(color, Color, COLOR); // red green blue
6
7// ✅ Keywords must be lowercase
8let x = 1;     // ✅ correct
9// Let x = 1;  // ❌ SyntaxError — 'Let' is not a keyword
10// LET x = 1;  // ❌ SyntaxError
11
12// ✅ Built-in objects are case-sensitive too
13console.log(typeof Math.PI);     // 'number' ✅
14// console.log(math.PI);         // ❌ ReferenceError: math is not defined
15// console.log(Math.pi);         // ❌ undefined — 'pi' not 'PI'
16
17// ✅ Function names are case-sensitive
18function greet() { return 'hello'; }
19// greet() ✅    Greet() ❌   GREET() ❌
20console.log(greet());
21
22// ✅ HTML attributes are NOT case-sensitive, but JS is
23// onclick vs onClick vs ONCLICK — all work in HTML
24// but in JavaScript: element.onclick (all lowercase)

1// ✅ Valid identifiers
2let name        = 'Alice';
3let userName    = 'bob123';
4let _private    = 'hidden';   // underscore prefix = convention for private
5let $element    = null;       // $ prefix = common in jQuery / DOM libs
6let camelCase   = true;
7let CONSTANT    = 3.14;       // all-caps = convention for constants
8let _123        = 'valid';    // digits allowed after first char
9
10// ❌ Invalid identifiers (would cause SyntaxError)
11// let 1name   = 'bad';   // starts with a number
12// let my-var  = 'bad';   // hyphens not allowed
13// let my var  = 'bad';   // spaces not allowed
14// let for     = 'bad';   // reserved keyword
15// let class   = 'bad';   // reserved keyword
16
17console.log(name, userName, _private, $element);
18console.log(CONSTANT);

1// These are all identical to the JavaScript engine:
2console.log(1 + 2);
3console.log(1+2);
4console.log( 1 + 2 );
5console.log(
6  1 +
7  2
8);
9
10// ✅ Use whitespace to make code readable
11const result = (10 + 5) * 2 / 3;
12
13// ❌ Technically valid but unreadable
14const result2=(10+5)*2/3;
15
16// Whitespace INSIDE strings IS significant
17console.log('hello world');  // 'hello world' — space preserved
18console.log('hello  world'); // 'hello  world' — two spaces preserved
19
20console.log(result, result2); // both equal 10

1// Statements — perform actions
2let x = 10;                         // declaration statement
3if (x > 5) { console.log('big'); }  // if statement
4for (let i = 0; i < 2; i++) {}      // for statement
5
6// Expressions — produce values (usable inline)
7const a = 5 + 3;           // 5 + 3 is an expression → value 8
8const b = x > 5 ? 'big' : 'small';  // ternary expression → string
9const c = Math.max(1, 2, 3);        // function call expression → 3
10const d = [1, 2, 3].length;         // property access expression → 3
11
12console.log(a, b, c, d);
13
14// Expression statement — an expression used as a statement
15console.log('hello'); // function call (expression) used as a statement
16x++;                  // increment (expression) used as a statement
17
18// Can't use a statement where an expression is expected:
19// const val = if (true) { 5 };   // ❌ SyntaxError — if is a statement
20const val = true ? 5 : 0;         // ✅ ternary is an expression
21console.log(val);

1// Block with if
2if (true) {
3  console.log('Line 1 of block');
4  console.log('Line 2 of block');
5} // end of block
6
7// Block with for
8for (let i = 0; i < 3; i++) {
9  const msg = `iteration ${i}`; // block-scoped variable
10  console.log(msg);
11}
12// console.log(msg); // ❌ ReferenceError — 'msg' is scoped to the for block
13
14// Block with function
15function add(a, b) {
16  const result = a + b;         // scoped to this block
17  return result;
18}
19console.log(add(3, 4));
20
21// Standalone block (rarely used, but valid)
22{
23  const temp = 'only in this block';
24  console.log(temp); // ✅
25}
26// console.log(temp); // ❌ ReferenceError — out of scope
27
28// Single-line if WITHOUT block — valid but risky
29if (true) console.log('no braces — only one statement allowed');
30// Recommendation: always use braces, even for single statements

1// Number literals
2console.log(42);          // integer
3console.log(3.14);        // decimal (float)
4console.log(-7);          // negative
5console.log(1_000_000);   // numeric separator (ES2021) — same as 1000000
6console.log(0xFF);        // hexadecimal — 255
7console.log(0b1010);      // binary — 10
8console.log(0o17);        // octal — 15
9console.log(2e6);         // scientific notation — 2000000
10
11// String literals
12console.log('single quotes');
13console.log("double quotes");
14console.log(`template literal — can span
15multiple lines`);
16console.log(`Hello ${'World'}!`); // template with expression
17
18// Boolean literals
19console.log(true);
20console.log(false);
21
22// Null & Undefined literals
23console.log(null);        // intentional absence of value
24console.log(undefined);   // variable declared but not assigned
25
26// Array literal
27console.log([1, 'two', true, null]);
28
29// Object literal
30console.log({ name: 'Alice', age: 25 });
31
32// RegExp literal
33console.log(/^[a-z]+$/i); // regular expression
34
35// BigInt literal (ES2020)
36console.log(9007199254740993n); // n suffix = BigInt

1// ── Arithmetic operators ─────────────────────────────────
2console.log(10 + 3);   // 13  — addition
3console.log(10 - 3);   // 7   — subtraction
4console.log(10 * 3);   // 30  — multiplication
5console.log(10 / 3);   // 3.3… — division
6console.log(10 % 3);   // 1   — modulus (remainder)
7console.log(10 ** 3);  // 1000 — exponentiation (ES2016)
8
9let n = 5;
10n++;  console.log(n); // 6 — post-increment
11++n;  console.log(n); // 7 — pre-increment
12n--;  console.log(n); // 6 — post-decrement
13
14// ── Assignment operators ──────────────────────────────────
15let x = 10;        // assign
16x += 5;  console.log(x);  // 15  — x = x + 5
17x -= 3;  console.log(x);  // 12  — x = x - 3
18x *= 2;  console.log(x);  // 24  — x = x * 2
19x /= 4;  console.log(x);  // 6   — x = x / 4
20x %= 4;  console.log(x);  // 2   — x = x % 4
21x **= 3; console.log(x);  // 8   — x = x ** 3
22
23// ── Comparison operators ──────────────────────────────────
24console.log(5 == '5');   // true  — loose (type coercion)
25console.log(5 === '5');  // false — strict (no coercion) ← always use this
26console.log(5 != '5');   // false — loose not-equal
27console.log(5 !== '5');  // true  — strict not-equal ← always use this
28console.log(5 > 3);      // true
29console.log(5 < 3);      // false
30console.log(5 >= 5);     // true
31console.log(5 <= 4);     // false
32
33// ── Logical operators ─────────────────────────────────────
34console.log(true && false);  // false — AND
35console.log(true || false);  // true  — OR
36console.log(!true);          // false — NOT
37
38// ── String operator ───────────────────────────────────────
39console.log('Hello' + ' ' + 'World'); // 'Hello World' — concatenation
40
41// ── Ternary operator ──────────────────────────────────────
42const age = 20;
43const status = age >= 18 ? 'adult' : 'minor';
44console.log(status); // 'adult'
45
46// ── typeof operator ───────────────────────────────────────
47console.log(typeof 42);          // 'number'
48console.log(typeof 'hello');     // 'string'
49console.log(typeof true);        // 'boolean'
50console.log(typeof undefined);   // 'undefined'
51console.log(typeof null);        // 'object' ← famous bug
52console.log(typeof {});          // 'object'
53console.log(typeof []);          // 'object'
54console.log(typeof function(){}); // 'function'

1// Single and double quotes — identical behaviour
2const a = 'Hello';
3const b = "World";
4console.log(a + ' ' + b);  // Hello World
5
6// Use the other quote to avoid escaping
7const c = "It's a great day";  // ✅ no escape needed
8const d = 'She said "hello"';  // ✅ no escape needed
9const e = 'It\'s escaped';     // ✅ escaped apostrophe
10
11// Template literals — backticks
12const name = 'Alice';
13const age  = 25;
14
15// Embed expressions with ${}
16console.log(`Name: ${name}, Age: ${age}`);
17console.log(`In 10 years: ${age + 10}`);
18console.log(`Is adult: ${age >= 18 ? 'yes' : 'no'}`);
19
20// Multi-line strings — only works with backticks
21const multiLine = `Line 1
22Line 2
23Line 3`;
24console.log(multiLine);
25
26// Escape sequences — work in all three quote styles
27console.log('Tab:\there');
28console.log('Newline:\nhere');
29console.log('Backslash: \\');
30console.log('Unicode: \u2764'); // ❤

1// ── const — block-scoped, cannot be reassigned ───────────
2const PI = 3.14159;
3console.log(PI);
4// PI = 3; // ❌ TypeError: Assignment to constant variable
5
6// const with objects — the binding is constant, not the object
7const user = { name: 'Alice' };
8user.name = 'Bob'; // ✅ mutating the object is fine
9// user = {};       // ❌ reassigning the binding is not
10console.log(user.name); // 'Bob'
11
12// ── let — block-scoped, can be reassigned ─────────────────
13let score = 0;
14score = 10;    // ✅ reassignment allowed
15score += 5;
16console.log(score); // 15
17
18if (true) {
19  let blockVar = 'inside block';
20  console.log(blockVar); // ✅
21}
22// console.log(blockVar); // ❌ ReferenceError — block scoped
23
24// ── var — function-scoped, avoid in modern code ───────────
25var oldStyle = 'legacy';
26console.log(oldStyle);
27
28if (true) {
29  var leaksOut = 'not block scoped!';
30}
31console.log(leaksOut); // ✅ 'not block scoped!' — var ignores blocks!
32
33// var hoisting — can use before declaration (bad)
34console.log(hoisted);  // undefined (not an error!)
35var hoisted = 'value';
36console.log(hoisted);  // 'value'

1// ── 1. Function Declaration ───────────────────────────────
2// Hoisted — can be called before it's defined in the file
3function add(a, b) {
4  return a + b;
5}
6console.log(add(2, 3)); // 5
7
8// ── 2. Function Expression ────────────────────────────────
9// Not hoisted — must be defined before calling
10const multiply = function(a, b) {
11  return a * b;
12};
13console.log(multiply(4, 3)); // 12
14
15// ── 3. Arrow Function (ES6) ───────────────────────────────
16// Concise syntax — no own 'this' binding
17const subtract = (a, b) => a - b;      // implicit return (one expression)
18const square   = x => x * x;           // single param — no parens needed
19const greet    = () => 'Hello!';        // no params — empty parens needed
20const divide   = (a, b) => {           // block body — explicit return needed
21  if (b === 0) return 'Cannot divide by zero';
22  return a / b;
23};
24
25console.log(subtract(10, 3)); // 7
26console.log(square(5));       // 25
27console.log(greet());         // 'Hello!'
28console.log(divide(10, 2));   // 5
29
30// ── 4. Method Shorthand (inside objects) ──────────────────
31const calculator = {
32  value: 0,
33  add(n)    { this.value += n; return this; },   // method shorthand
34  subtract(n) { this.value -= n; return this; },
35  result()  { return this.value; },
36};
37console.log(calculator.add(10).subtract(3).result()); // 7
38
39// ── Default Parameters ────────────────────────────────────
40function createUser(name, role = 'viewer', active = true) {
41  return { name, role, active };
42}
43console.log(createUser('Alice'));            // { name:'Alice', role:'viewer', active:true }
44console.log(createUser('Bob', 'admin'));     // { name:'Bob', role:'admin', active:true }

1// ── if / else if / else ───────────────────────────────────
2const score = 72;
3
4if (score >= 90) {
5  console.log('A');
6} else if (score >= 80) {
7  console.log('B');
8} else if (score >= 70) {
9  console.log('C');
10} else {
11  console.log('F');
12} // → C
13
14// ── Ternary (shorthand if/else for expressions) ───────────
15const pass = score >= 50 ? 'Pass' : 'Fail';
16console.log(pass); // 'Pass'
17
18// ── switch ────────────────────────────────────────────────
19const day = 'Mon';
20switch (day) {
21  case 'Mon':
22  case 'Tue':
23  case 'Wed':
24  case 'Thu':
25  case 'Fri':
26    console.log('Weekday');
27    break;
28  case 'Sat':
29  case 'Sun':
30    console.log('Weekend');
31    break;
32  default:
33    console.log('Unknown');
34}
35
36// ── for loop ──────────────────────────────────────────────
37for (let i = 0; i < 4; i++) {
38  process.stdout ? process.stdout.write(i + ' ') : console.log(i);
39}
40console.log();
41
42// ── for...of — iterates values (arrays, strings) ──────────
43const fruits = ['apple', 'banana', 'cherry'];
44for (const fruit of fruits) {
45  console.log(fruit);
46}
47
48// ── for...in — iterates keys (object properties) ──────────
49const obj = { a: 1, b: 2, c: 3 };
50for (const key in obj) {
51  console.log(`${key}: ${obj[key]}`);
52}
53
54// ── while ─────────────────────────────────────────────────
55let count = 3;
56while (count > 0) {
57  console.log(`Countdown: ${count}`);
58  count--;
59}
60
61// ── break and continue ────────────────────────────────────
62for (let i = 0; i < 10; i++) {
63  if (i === 3) continue; // skip 3
64  if (i === 6) break;    // stop at 6
65  console.log(i);        // 0, 1, 2, 4, 5
66}

1// ── try / catch ───────────────────────────────────────────
2try {
3  const result = JSON.parse('{ invalid json }'); // throws SyntaxError
4  console.log(result); // never reached
5} catch (error) {
6  console.log('Caught:', error.message);
7  console.log('Type:', error.constructor.name); // SyntaxError
8}
9
10// ── finally — runs regardless of error ────────────────────
11function fetchData() {
12  try {
13    console.log('Fetching...');
14    throw new Error('Network failure'); // simulate error
15    console.log('Success');             // never runs
16  } catch (err) {
17    console.log('Error:', err.message);
18  } finally {
19    console.log('Cleanup — always runs (close DB, hide loader, etc.)');
20  }
21}
22fetchData();
23
24// ── throw — create your own errors ────────────────────────
25function divide(a, b) {
26  if (b === 0) throw new Error('Cannot divide by zero');
27  if (typeof a !== 'number' || typeof b !== 'number') {
28    throw new TypeError('Both arguments must be numbers');
29  }
30  return a / b;
31}
32
33try {
34  console.log(divide(10, 2));  // 5
35  console.log(divide(10, 0)); // throws
36} catch (e) {
37  console.log('Caught:', e.message);
38}
39
40// Built-in error types
41// TypeError    — wrong type
42// RangeError   — value out of range
43// SyntaxError  — invalid syntax
44// ReferenceError — variable not defined

1// ── math.js — EXPORTING ───────────────────────────────────
2
3// Named exports — export multiple things
4export const PI = 3.14159;
5
6export function add(a, b) {
7  return a + b;
8}
9
10export function subtract(a, b) {
11  return a - b;
12}
13
14// Default export — one per file
15export default function multiply(a, b) {
16  return a * b;
17}
18
19
20// ── app.js — IMPORTING ────────────────────────────────────
21
22// Import named exports — use exact names in { }
23import { PI, add, subtract } from './math.js';
24
25// Import default export — any name you choose
26import multiply from './math.js';
27
28// Import both at once
29import multiply, { PI, add } from './math.js';
30
31// Import everything as a namespace
32import * as Math from './math.js';
33Math.add(1, 2);    // 3
34Math.PI;           // 3.14159
35
36// Rename on import
37import { add as sum } from './math.js';
38console.log(sum(3, 4)); // 7
39
40// Dynamic import — load on demand (async)
41async function loadHeavyModule() {
42  const { heavyFunction } = await import('./heavy.js');
43  heavyFunction();
44}

1// Enable strict mode for the whole file
2'use strict';
3
4// Or just for a function
5function strictFn() {
6  'use strict';
7  // ...
8}
9
10// What strict mode changes:
11
12// 1. Using undeclared variables throws ReferenceError
13try {
14  // undeclaredVar = 5;  // ❌ ReferenceError in strict mode
15  // (silently creates a global in sloppy mode)
16  console.log('Undeclared var would throw in strict mode');
17} catch (e) { console.log(e.message); }
18
19// 2. Duplicate parameter names throw SyntaxError
20// function dup(a, a) {} // ❌ SyntaxError in strict mode
21
22// 3. 'this' in regular functions is undefined (not window)
23function checkThis() {
24  'use strict';
25  console.log('this in strict function:', this); // undefined
26}
27checkThis();
28
29// 4. Writing to read-only properties throws TypeError
30// const obj = Object.freeze({ x: 1 });
31// obj.x = 2; // ❌ TypeError in strict mode (silently ignored otherwise)
32
33console.log('Strict mode demonstrated!');
34
35// NOTE: ES Modules and class bodies are ALWAYS in strict mode
36// You don't need 'use strict' in .mjs files or inside class definitions