1. What is a JavaScript Array?

 

An array is a special variable that can hold multiple values under a single name.

• Arrays can hold numbers, strings, objects, functions, or even other arrays.

• Arrays are zero-indexed, meaning the first element is at index 0.

Example:

let fruits = ["Apple", "Banana", "Orange"];
console.log(fruits[0]); // Apple
console.log(fruits[2]); // Orange

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2. Creating Arrays

a) Using square brackets [] (most common)

let numbers = [1, 2, 3, 4, 5];

b) Using Array constructor

let colors = new Array("Red", "Green", "Blue");

c) Creating an empty array

let emptyArr = [];

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3. Accessing Array Elements

• Use index to access elements.

let arr = ["A", "B", "C"];
console.log(arr[0]); // A
console.log(arr[2]); // C

• You can also modify elements using the index:

arr[1] = "Z";
console.log(arr); // ["A", "Z", "C"]

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4. Array Properties

a) .length

Returns the number of elements in the array.

let arr = [1, 2, 3];
console.log(arr.length); // 3

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5. Adding & Removing Elements

a) Add elements

• push() → add at the end

• unshift() → add at the beginning

let arr = [1, 2];
arr.push(3);       // [1,2,3]
arr.unshift(0);    // [0,1,2,3]

b) Remove elements

• pop() → remove from the end

• shift() → remove from the beginning

arr.pop();   // [0,1,2]
arr.shift(); // [1,2]

c) Remove/replace using splice()

let arr = [1,2,3,4];
arr.splice(1,2); // remove 2 elements starting from index 1 → [1,4]
arr.splice(1,1,9,8); // replace 1 element at index 1 with 9,8 → [1,9,8,4]

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6. Array Methods

Here are the most commonly used:

Method	Description	Example
concat()	Merge arrays	[1,2].concat([3,4]) → [1,2,3,4]
join()	Convert to string	[1,2,3].join("-") → "1-2-3"
slice()	Copy a part	[1,2,3,4].slice(1,3) → [2,3]
indexOf()	Find index	[1,2,3].indexOf(2) → 1
includes()	Check if exists	[1,2,3].includes(2) → true
reverse()	Reverse array	[1,2,3].reverse() → [3,2,1]
sort()	Sort array	[3,1,2].sort() → [1,2,3]
forEach()	Loop over items	[1,2,3].forEach(n => console.log(n))
map()	Transform array	[1,2,3].map(n => n*2) → [2,4,6]
filter()	Filter elements	[1,2,3,4].filter(n => n>2) → [3,4]
reduce()	Reduce to single value	[1,2,3].reduce((a,b)=>a+b) → 6

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7. Multi-Dimensional Arrays

Arrays can contain other arrays:

let matrix = [
  [1,2,3],
  [4,5,6],
  [7,8,9]
];
console.log(matrix[1][2]); // 6

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8. Checking Arrays

• Use Array.isArray() to check if a variable is an array:

let arr = [1,2,3];
console.log(Array.isArray(arr)); // true
console.log(Array.isArray(123)); // false

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9. Array Destructuring

let arr = [1,2,3];
let [a, b, c] = arr;
console.log(a, b, c); // 1 2 3

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10. Spread Operator with Arrays

• Expand array elements:

let arr1 = [1,2];
let arr2 = [3,4];
let arr3 = [...arr1, ...arr2]; // [1,2,3,4]

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11. Common Tricks

• Copy array: let copy = [...arr];

• Flatten array: arr.flat()

• Find max/min: Math.max(...arr) / Math.min(...arr)

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12. Array Iteration

let fruits = ["Apple","Banana","Cherry"];

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

// for..of loop
for(let fruit of fruits){
    console.log(fruit);
}

// forEach method
fruits.forEach(fruit => console.log(fruit));

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