1. Operators

JavaScript provides two primary equality comparison operators:

• == (loose equality): performs type coercion before comparing values.
• === (strict equality): compares both value and type without coercion.

Example:

let age1 = 20;
let age2 = "20";
console.log(age1 == age2);  // true (after string-to-number coercion)
console.log(age1 === age2); // false (string vs number)

Common type conversions:

• String to Number: Use parseInt() or unary +. Non-numeric strings yield NaN.
• Boolean to Number: true → 1, false → 0.
• Any to Boolean:
  • 0 and NaN → false
  • Empty string "" → false
  • null, undefined → false
  • All other values → true

Example usage for truthy/falsy checks:

let val = +"42";
console.log(parseInt(val) + 1); // 43

let flag = null;
if (!flag) {
  console.log("null is falsy"); // This runs
}

let input = "hello";
if (input) {
  console.log("Non-empty string is truthy"); // This runs
}

2. Control Flow Statements

JavaScript supports standard control structures similar to Java:

• if, else if, else
• switch
• for, while, do...while loops
• break and continue

Example:

for (let i = 0; i < 3; i++) {
  if (i === 1) continue;
  console.log(i); // Outputs: 0, 2
}

3. Functions

Functions can be declared or assigned as expressions (anonymous functions).

• Named function declaration
• Function expression (asssignable to variables)

Key characteristics:

• Parameter count is not enforced — extra arguments are ignored; missing ones become undefined.
• All arguments accessible via the arguments object (or modern spread syntax).

Example:

const sum = function(a, b) {
  return a + b;
};

console.log(sum(1, 2));      // 3
console.log(sum(1, 2, 9));   // 3 (third argument ignored)
console.log(sum(5));         // NaN (b is undefined)

4. Arrays

JavaScript arrays are dynamic, type-flexible, and zero-indexed.

Creation Methods

// Array constructor
let arr1 = new Array(10, 20, 30);

// Literal syntax (preferred)
let arr2 = [10, 20, 30];

Behavior and Features

• Dynamic length: Accessing beyond existing indices fills gaps with undefined.
• Heterogeneous types: Elements may hold different data types.
• length property reflects highest index + 1 (even if sparse).

Example:

let mixed = [1, "two", true, {x: 5}];
mixed[5] = "gap";
console.log(mixed[4]); // undefined
console.log(mixed.length); // 6

Common Methods

• push(...items) — append elements to end
• splice(start, deleteCount, ...items) — add/remove items at specific indices

Example usage:

let nums = [1, 2, 3];
nums.push(4);
console.log(nums); // [1, 2, 3, 4]

nums.splice(1, 2); // remove 2 items starting at index 1
console.log(nums); // [1, 4]