1// ✅ Valid identifiers
2let name       = 'Alice';
3let userName   = 'bob123';
4let _private   = 'hidden';   // starts with underscore — OK
5let $price     = 9.99;       // starts with $ — OK
6let score1     = 100;        // digit after first char — OK
7let LIMIT      = 500;        // all uppercase — OK
8let camelCase  = true;
9
10// ❌ Invalid identifiers — these cause SyntaxError
11// let 1name  = 'bad';   // starts with a digit
12// let my-var = 'bad';   // hyphen not allowed
13// let my var = 'bad';   // space not allowed
14// let for    = 'bad';   // reserved keyword
15// let class  = 'bad';   // reserved keyword
16// let @tag   = 'bad';   // @ not allowed
17
18console.log(name, userName, _private, $price, score1);

1// ❌ You cannot use keywords as identifier names:
2// let let = 5;       // SyntaxError
3// let const = 5;     // SyntaxError
4// let for = 5;       // SyntaxError
5// let if = 5;        // SyntaxError
6// let class = 5;     // SyntaxError
7// let return = 5;    // SyntaxError
8
9// ✅ But you can use them as PART of a name:
10let letValue = 5;      // 'let' is part of the name — OK
11let forEach = true;    // 'for' is part of the name — OK
12let myClass = 'math';  // 'class' is part of the name — OK
13let returnValue = 42;  // 'return' is part of the name — OK
14
15console.log(letValue, forEach, myClass, returnValue);
16
17// ✅ These words are NOT reserved (but avoid them — confusing):
18let undefined2 = 'not a keyword technically';
19let NaN2 = 99;
20console.log(undefined2, NaN2);

1// camelCase — variables and functions
2let firstName     = 'Alice';
3let totalPrice    = 49.99;
4let isLoggedIn    = true;
5let itemCount     = 0;
6
7function getUserData() {
8  return { name: firstName };
9}
10function calculateTotal(price, tax) {
11  return price + price * tax;
12}
13
14// PascalCase — classes
15class UserProfile {
16  constructor(name) {
17    this.name = name;
18  }
19  greet() {
20    return `Hello, ${this.name}!`;
21  }
22}
23
24// UPPER_SNAKE_CASE — constants that never change
25const MAX_ITEMS     = 100;
26const TAX_RATE      = 0.18;
27const API_URL       = 'https://api.example.com';
28const DEFAULT_LANG  = 'en';
29
30// _leadingUnderscore — private / internal (convention only)
31class Counter {
32  constructor() {
33    this._count = 0;  // signals: don't touch this directly
34  }
35  increment() {
36    this._count++;
37  }
38  get value() {
39    return this._count;
40  }
41}
42
43// Run it
44const user = new UserProfile('Bob');
45console.log(user.greet());            // Hello, Bob!
46console.log(calculateTotal(100, TAX_RATE)); // 118
47
48const counter = new Counter();
49counter.increment();
50counter.increment();
51console.log('Count:', counter.value); // 2

1// ❌ Bad names — impossible to understand
2let x = 25;
3let d = new Date();
4let a = [];
5let temp = fetchUser();
6function calc(v, r) {
7  return v * r;
8}
9
10// ✅ Good names — reads like English
11let userAge     = 25;
12let currentDate = new Date();
13let cartItems   = [];
14let currentUser = fetchUser();
15function calculateTax(price, taxRate) {
16  return price * taxRate;
17}
18
19// ✅ Boolean names — use is/has/can/should prefix
20let isActive      = true;
21let hasPermission = false;
22let canEdit       = true;
23let shouldRedirect = false;
24
25// ✅ Function names — start with a verb
26function getUser()     {}   // get something
27function setTheme()    {}   // set something
28function createPost()  {}   // create something
29function deleteItem()  {}   // delete something
30function validateForm() {}  // validate something
31function isAdmin()     {}   // check something (returns boolean)
32
33console.log('userAge:', userAge);
34console.log('isActive:', isActive);
35console.log('Tax on 100:', calculateTax(100, 0.18));
36
37function fetchUser() { return { name: 'Alice' }; }
38console.log('currentUser:', currentUser);