First-class Function

A programming language is said to have First-class functions when functions in that language are treated like any other variable. For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable.

Example | Assign a function to a variable

JavaScript

const foo = function() {
   console.log("foobar");
}
// Invoke it using the variable
foo();

We assigned an Anonymous Function in a Variable, then we used that variable to invoke the function by adding parentheses () at the end.

Example | Pass a function as an Argument

JavaScript

function sayHello() {
   return "Hello, ";
}
function greeting(helloMessage, name) {
  console.log(helloMessage() + name);
}
// Pass `sayHello` as an argument to `greeting` function
greeting(sayHello, "JavaScript!");

We are passing our sayHello() function as an argument to the greeting() function, this explains how we are treating the function as a value.

Example | Return a function

JavaScript

function sayHello() {
   return function() {
      console.log("Hello!");
   }
}

In this example; We need to return a function from another function - We can return a function because we treated function in JavaScript as a value.

Back to our example; Now, we need to invoke sayHello function and its returned Anonymous Function. To do so, we have two ways:

1- Using a variable

const sayHello = function() {
   return function() {
      console.log("Hello!");
   }
}
const myFunc = sayHello();
myFunc();

This way, it returns the Hello! message.

2- Using double parentheses

function sayHello() {
   return function() {
      console.log("Hello!");
   }
}
sayHello()();

We are using double parentheses ()() to invoke the returned function as well.