JavaScript Variables, Data Types, and Operators

Variable Declarations

// var — function-scoped, hoisted, avoid in modern JS
var x = 10;

// let — block-scoped, can be reassigned
let count = 0;
count = 5;

// const — block-scoped, cannot be reassigned (but objects can be mutated)
const PI = 3.14;
const user = { name: "Alice" };
user.name = "Bob"; // OK — mutating the object

Data Types

typeof 42;          // "number"
typeof "hello";     // "string"
typeof true;        // "boolean"
typeof null;        // "object" (historical bug)
typeof undefined;   // "undefined"
typeof {};          // "object"
typeof [];          // "object"
typeof function(){}; // "function"

Template Literals

const name = "Alice";
const age = 25;
const msg = `Hello, ${name}! You are ${age} years old.`;
console.log(msg); // Hello, Alice! You are 25 years old.

// Multi-line strings
const html = `
  <div>
    <p>Hello ${name}</p>
  </div>
`;

Operators

// Arithmetic
let a = 10 + 3;   // 13
let b = 10 - 3;   // 7
let c = 10 * 3;   // 30
let d = 10 / 3;   // 3.333...
let e = 10 % 3;   // 1 (modulus)
let f = 2 ** 3;   // 8 (exponent)

// Assignment
x += 5;  x -= 2;  x *= 3;  x /= 2;  x **= 2;

// Comparison
5 == "5"   // true  (loose, type coercion)
5 === "5"  // false (strict, no coercion)
5 != "5"   // false
5 !== "5"  // true
5 > 3      // true

// Logical
true && false  // false (AND)
true || false  // true  (OR)
!true          // false (NOT)

// Nullish coalescing
let val = null ?? "default";  // "default"

// Optional chaining
let city = user?.address?.city;  // undefined (no error)

Type Coercion Examples

"5" + 3    // "53" (string concatenation)
"5" - 3    // 2   (numeric operation)
"5" * 2    // 10
Boolean(0) // false
Boolean("") // false
Boolean([]) // true

Key Takeaway: Use const by default, let when reassignment is needed, and avoid var. Always use === for comparison to avoid type coercion surprises.