First-class Function in JavaScript

In JavaScript functions are treated as First-class Function when a function can be assigned as a value to variable or it can be passed as arguments to other function or can be returned by another function.

Example: Assign a function to variable

const name = function() {
      console.log("Rishi");
}

name();  // invoking it using variable

// Rishi

Note: You can name function, still you can invoke by using variable.

Example: Pass a function as an Argument

function hello() {
     console.log("Hello, ")
}

function greetingUser(helloMessage, user) {
     console.log(helloMessage() + user);
}

//pass hello as argument
greetingUser(hello, "Rishi");

// Hello, Rishi

Example: Return a Function

1. Using Variable

const name = function() {
      return function(){
          console.log("Rishi");
}

// invoking function using variable

const myName = name();
myName();

1. Using double parentheses

function name() {
     return function(){
          console.log("Rishi");
}

// invoking function using double parentheses

name()();