If a programming language treats functions as any other variable, that particular programming language is considered to have first-class functions.

In a programming language with first-class functions,

• A function can be assigned as a value to a variable.
• A function can be passed as an argument to another function.
• Another function can return a function.

Assign a function to a variable.

const helloWorld = function() {
	console.log(“Hello worldz!”);
}

helloWorld();

Pass a function as an argument to another function.

function sayHello() {
	Return “hello”;
}

function greeting(message, name) {
	console.log(message() + name );
}

greeting(sayHello, “world!”);

The above sayHello function is passed as an argument to the greeting function. When a function is used as an argument to another function, that function is called a callback function. Therefore sayHello() is a callback function.

Return a function from another function.

In a programming language with first-class functions, functions are treated as a value. So a function can be returned from another function.

function sayHelloWorld() {
	return function() {
		console.log(“Hello world!”);
    }
}

If a function returns another function, that function is called a Higher-Order function.

To invoke a Higher-Order function, there are two ways.

1. Using Variables.

const sayHelloWorld() {
	retun function() {
	console.log(“Hello World!”);
    }
}

const func = sayHello();
func();

1. Using double parentheses.

function sayHelloWorld() {
	return function() {
		console.log(“Hello world!”);
    }
}

sayHelloWorld()();

For the above examples, I have Used JavaScript. JavaScript is not the only language with first-class functions.

Most functional programming languages (Haskell, Scala, Julia …) and scripting languages (JavaScript, Python, Perl, PHP … ) are considered to have first-class functions.