Section 1.1: Defining Higher-Order Functions

In JavaScript, functions hold the status of first-class citizens, meaning they can be manipulated just like any other data type. This characteristic allows functions to be passed as parameters to other functions or returned from them, giving rise to higher-order functions.

A higher-order function is one that accepts one or more functions as arguments or returns a function itself. This capability allows developers to create modular and reusable code. Consider this straightforward example:

function greet(name) {

return Hello, ${name}!;

}

function greetUser(greetFunction) {

return greetFunction('John');

}

console.log(greetUser(greet)); // Output: Hello, John!

Here, greetUser acts as a higher-order function, accepting greet as an argument and invoking it internally.

Section 1.2: Delving into Closures

Closures represent a crucial concept in JavaScript, enabling functions to maintain access to variables from their containing scope, even after the outer function has completed its execution. This feature is a result of how functions in JavaScript create closures around their defining environment.

Take a look at this example illustrating closures:

function createCounter() {

let count = 0;

return function() {

return ++count;

};

}

const counter = createCounter();

console.log(counter()); // Output: 1

console.log(counter()); // Output: 2

In this scenario, createCounter returns a function that increments and returns the count variable. The inner function keeps access to count, even after createCounter has finished executing.

Section 2.1: Array Manipulation with Higher-Order Functions

Here's how higher-order functions can be utilized for array manipulation:

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

const squaredNumbers = numbers.map(num => num * num);

console.log(squaredNumbers); // Output: [1, 4, 9, 16, 25]

const evenNumbers = numbers.filter(num => num % 2 === 0);

console.log(evenNumbers); // Output: [2, 4]

Section 2.2: Building a Counter Using Closures

To demonstrate closures further, consider this implementation of a counter:

function createCounter() {

let count = 0;

return function() {

return ++count;

};

}

const counter = createCounter();

console.log(counter()); // Output: 1

console.log(counter()); // Output: 2