A function that can be called as a parameter, or argument to another function.

Example:

const functionFeeder = function(callback) {
	callback("Hello from the inside of Function Feeder.");
}

// Invocation
functionFeeder(function(greeting) {
	console.log(greeting);
}

Example 2:

Example 3:

// The callback function
function sayHello(name) {
	console.log(`Hello, ${name}`);
}

// The function that uses sayHello as an argument.
function callSayHelloWithLars(callback) {
	const innerName = "Lars";
	callback(innerName);
}

// Invoking 
callSayHelloWithLars(sayHello);

const elements = ["Fire", "water", "wind", "earth"]

// The function that uses the callback
const newForEach = (list, callback) => {
	for(let i = 0; i < list.length; i++) {
	callback(list[i], i);
	}
};

// Invoking newForEach
newForEach(elements, (item, index) => {
	console.log(item, index);
});

Example 3.1:

Example 4:

const elements = ["fire", "water", "wind", "earth"]

// The function that uses the callback
const newForEach = (list, callback) => {
	for(let i = 0; i < list.length; i++) {
	callback(list[i], i);
	}
};

// The callback
const iterator = (item, index) => {
	console.log(item, index);
}

// Invoking newForEach
newForEach(elements, iterator);

const elements = ["fire", "water", "wind", "earth"]

function showFirst(array, callback) {
	callback(array[0]);
}

// Invoking
showFirst(elements, // This is the callback: 
(firstItem) => {
	console.log(firstItem);
});