What is an Array?

• • An array is like a numbered list where each element has an index that identifies its position.
  • Every item has an index, and the index represents the position in the list, so you know exactly how data is organized.
  • A car has a number, and that number can be compared to an index because both identify a specific element.
  • Arrays work by using the index to access an element directly instead of checking every item one by one.
  • You can Think of reading an array like taking a train to a chosen stop instead of a walk through every element, because the index points to the required position.
  • Important: Arrays use zero-based indexing, so the first element is stored at index 0.

How to Create Arrays

Arrays are created by placing one or more values inside square brackets ([]), with each value separated by a comma.

Example:

// Example 1: Array of numbers

let numbers = [10, 20, 30, 40]; 

// Example 2: Array of strings

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

// Example 3: Array of mixed values

let data = [1, "Book", true, 3.5];

Common Array Methods

Updating Array Elements

Change an array element by assigning a new value to its index.

let cities = ["London", "Paris", "Tokyo"];

// Update an existing element

cities[0] = "Berlin";

console.log(cities); // ["Berlin", "Paris", "Tokyo"]

// Add a new element using an index

cities[3] = "Sydney";

console.log(cities); // ["Berlin", "Paris", "Tokyo", "Sydney"] 

Iterating Over an Array

let scores = [85, 92, 78];

// for...of loop

for (let score of scores) {

  console.log(score);

}

// forEach method

scores.forEach(function(score, index) {

  console.log("Score " + index + ": " + score);

});

// Traditional for loop

for (let i = 0; i < scores.length; i++) {

  console.log(scores[i]);

} 

Useful Array Methods

map()

Creates a new array by applying a function to every element.

JavaScript

let prices = [10, 20, 30];

let discounted = prices.map(price => price - 5);

console.log(discounted); // [5, 15, 25]

filter()

Creates a new array containing only the elements that match a condition.

JavaScript

let ages = [12, 18, 25, 15];

let adults = ages.filter(age => age >= 18);

console.log(adults); // [18, 25]

find()

Returns the first element that satisfies a condition.

JavaScript

let temperatures = [18, 22, 27, 30];

let hotDay = temperatures.find(temp => temp > 25);

console.log(hotDay); // 27

reduce()

Combines all array elements into a single value.

JavaScript

let marks = [70, 85, 90];

let totalMarks = marks.reduce((total, mark) => total + mark, 0);

console.log(totalMarks); // 245

slice()

Returns a selected portion of an array without changing the original array.

JavaScript

let months = ["Jan", "Feb", "Mar", "Apr"];

let firstTwoMonths = months.slice(0, 2);

console.log(firstTwoMonths); // ["Jan", "Feb"]

splice()

Adds, removes, or replaces elements in the original array.

JavaScript

let animals = ["Cat", "Dog", "Bird"];

animals.splice(1, 1, "Rabbit");

console.log(animals); // ["Cat", "Rabbit", "Bird"]

What are the key takeaways?

• Arrays store multiple values in a single variable, making related items available through an index.
• Arrays are zero-indexed, so the first item uses index 0 instead of one.
• push() and pop() add/remove elements at the end of an array.
• The length property gets the number of items currently stored in an array.
• map(), filter(), and reduce() are powerful array methods that process data in different ways.
• Some methods modify the original array, while others return a new one with updated values.