Arrays & Methods in JavaScript

Arrays are the workhorse of JavaScript

They let you store ordered collections of values and then process them with powerful built-in methods. Let’s go step by step.

📘 Arrays in JavaScript

1️⃣ Creating Arrays

// Using array literal
let fruits = ["apple", "banana", "cherry"];

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

// Empty array
let empty = [];

✅ Arrays can hold any type: numbers, strings, objects, even other arrays.

2️⃣ Common Iteration Methods

🔹 forEach()

Executes a function for each element in the array. Does not return a new array (returns undefined).

let fruits = ["apple", "banana", "cherry"];
fruits.forEach(fruit => console.log(fruit));
// apple
// banana
// cherry

🔹 map()

Transforms each element and returns a new array of the same length.

let numbers = [1, 2, 3, 4];
let squares = numbers.map(n => n * n);
console.log(squares); // [1, 4, 9, 16]

🔹 filter()

Returns a new array with elements that pass a condition.

let numbers = [5, 12, 8, 130, 44];
let bigNumbers = numbers.filter(n => n > 10);
console.log(bigNumbers); // [12, 130, 44]

🔹 reduce()

Reduces the array to a single value by applying a function.

let numbers = [1, 2, 3, 4, 5];
let sum = numbers.reduce((acc, curr) => acc + curr, 0);
console.log(sum); // 15

🔑 Key Points

• forEach() → executes a function for each element (no return value).
• map() → transforms elements and returns a new array.
• filter() → returns a new array with elements that meet a condition.
• reduce() → combines all elements into a single result (sum, average, etc.).

3️⃣ Comparison Table

Method	Returns	Use Case
forEach	undefined	Perform side effects (logging, updating external vars)
map	New array	Transform each element (e.g., square numbers)
filter	New array	Select elements that meet a condition
reduce	Single value	Accumulate into one result (sum, product, max, etc.)

4️⃣ Mini Project: Student Grades

let students = [
  { name: "Alice", marks: 85 },
  { name: "Bob", marks: 45 },
  { name: "Charlie", marks: 72 },
  { name: "David", marks: 90 }
];

// 1. Print all names
students.forEach(s => console.log(s.name));

// 2. Get array of marks squared
let squaredMarks = students.map(s => s.marks * s.marks);

// 3. Filter students who passed (>= 50)
let passed = students.filter(s => s.marks >= 50);

// 4. Calculate average marks
let total = students.reduce((acc, s) => acc + s.marks, 0);
let average = total / students.length;

console.log("Squared Marks:", squaredMarks);
console.log("Passed Students:", passed);
console.log("Average Marks:", average);

👉 Arrays + these methods = functional programming power in JavaScript. They let you write clean, declarative code instead of messy loops.