1const single = 'Hello';
2const double = "Hello";
3 
4// Template literals (backticks) - most powerful
5const name = "World";
6const greeting = `Hello, ${name}!`;  // "Hello, World!"
7 
8// Multi-line
9const multiline = `
10Line 1
11Line 2
12`;
13 
14console.log("JavaScript".length); // 10

1const integer = 42;
2const decimal = 3.14;
3const scientific = 2.5e6;  // 2,500,000
4 
5// Special values
6console.log(Infinity);
7console.log(NaN);
8console.log(0.1 + 0.2);         // 0.30000000000000004
9console.log(0.1 + 0.2 === 0.3); // false!

1const isLoggedIn = true;
2const hasPermission = false;
3console.log(5 > 3);      // true
4console.log(10 === 20);  // false

1// undefined = declared but not assigned
2let x;
3console.log(x);         // undefined
4 
5// null = intentionally empty
6let user = null;
7console.log(user);       // null
8 
9console.log(null == undefined);  // true  (loose)
10console.log(null === undefined); // false (strict)

1console.log(typeof "hello");      // "string"
2console.log(typeof 42);           // "number"
3console.log(typeof true);         // "boolean"
4console.log(typeof undefined);    // "undefined"
5console.log(typeof null);         // "object" ⚠️
6console.log(typeof {});           // "object"
7console.log(typeof []);           // "object"
8console.log(typeof function(){}); // "function"
9 
10// Better array check
11console.log(Array.isArray([]));   // true

1// + with a string converts everything to string
2console.log("5" + 3);      // "53"
3console.log("Hello" + 42); // "Hello42"
4console.log("" + true);    // "true"
5 
6// Explicit
7console.log(String(42));    // "42"

1// Math operators (except +) convert to numbers
2console.log("6" - 2);     // 4
3console.log("6" * "3");   // 18
4console.log(true + 1);    // 2
5console.log(null + 5);    // 5
6 
7// Explicit
8console.log(Number("42"));      // 42
9console.log(Number("hello"));   // NaN
10console.log(Number(true));      // 1
11console.log(Number(""));        // 0
12console.log(Number(undefined)); // NaN

1// The 8 FALSY values:
2Boolean(false);      // false
3Boolean(0);          // false
4Boolean("");         // false
5Boolean(null);       // false
6Boolean(undefined);  // false
7Boolean(NaN);        // false
8Boolean(-0);         // false
9Boolean(0n);         // false
10 
11// Everything else is TRUTHY:
12Boolean("hello");    // true
13Boolean(42);         // true
14Boolean([]);         // true  ← empty array is truthy!
15Boolean({});         // true  ← empty object is truthy!
16Boolean("0");        // true  ← string "0" is truthy!

1// == performs type coercion
2console.log(5 == "5");      // true
3console.log(0 == false);    // true
4console.log("" == 0);       // true
5 
6// === checks type AND value (no coercion)
7console.log(5 === "5");     // false
8console.log(0 === false);   // false
9console.log("" === 0);      // false

1function validateAge(input) {
2const age = Number(input);
3 
4if (Number.isNaN(age)) return "Enter a valid number";
5if (age < 0 || age > 150) return "Age must be 0-150";
6if (!Number.isInteger(age)) return "Must be a whole number";
7 
8return "Valid age: " + age;
9}
10 
11console.log(validateAge("25"));   // "Valid age: 25"
12console.log(validateAge("abc"));  // "Enter a valid number"
13console.log(validateAge("25.5")); // "Must be a whole number"