Arrays

Arrays are ordered lists used to store multiple values in a single variable. They can contain any data type — strings, numbers, objects, functions, etc.

1. Creating Arrays

let arr = [10, 20, 30];
let empty = new Array(); // []

2. Accessing Items

arr[0];     // 10
arr.length; // 3
arr[arr.length - 1]; // Last item

3. Modifying Items

arr[1] = 99;       // [10, 99, 30]
arr.push(40);      // [10, 99, 30, 40]
arr.pop();         // Removes last → [10, 99, 30]
arr.shift();       // Removes first → [99, 30]
arr.unshift(5);    // Adds to front → [5, 99, 30]

4. Array Methods

push() / pop()

arr.push(100); // add to end
arr.pop();     // remove from end

shift() / unshift()

arr.shift();     // remove first
arr.unshift(0);  // add to start

slice(start, end)

Returns a shallow copy of a portion.

let sliced = arr.slice(1, 3); // [99, 30]

splice(start, deleteCount, ...items)

Modifies the array (insert/remove/replace).

arr.splice(1, 1);         // Remove 1 item at index 1
arr.splice(1, 0, 88);     // Insert 88 at index 1

indexOf() / includes()

arr.indexOf(30);   // 2
arr.includes(99);  // true

join()

arr.join(", "); // "5, 99, 30"

reverse()

arr.reverse(); // In-place reverse

sort()

arr.sort(); // Lexicographic sort
arr.sort((a, b) => a - b); // Numeric sort

5. Looping Arrays

for loop

for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

for...of

for (let val of arr) {
  console.log(val);
}

forEach()

arr.forEach((val, i) => {
  console.log(i, val);
});

6. Higher-Order Methods

map()

let doubled = arr.map(n => n * 2);

filter()

let evens = arr.filter(n => n % 2 === 0);

reduce()

let sum = arr.reduce((acc, curr) => acc + curr, 0);

7. Nested Arrays

let grid = [[1, 2], [3, 4]];
console.log(grid[1][0]); // 3