What is First class function in JavaScript ?

In JavaScript, functions are first-class citizens. This means that functions in JavaScript are treated like any other variable. A first-class function can:

1. Be stored in a variable:

    let myFunction = function() { 
        console.log('Hello World'); 
    }
    myFunction();  // Outputs: Hello World
   

2. Be passed as an argument to another function:

    function sayHello() {
        return 'Hello, ';
    }
    function greet(helloFn, name) {
        console.log(helloFn() + name);
    }
    greet(sayHello, 'John'); // Outputs: Hello, John
   

3. Be returned from another function:

    function outerFunction() {
        return function innerFunction() {
            return 'Hello World';
        }
    }
    let myFunction = outerFunction();
    console.log(myFunction()); // Outputs: Hello World
   

4. Be stored in data structures such as an array, an object, or a map:

    let funcArray = [function(){ return 'Hello '}, function(){ return 'World!'}];
    console.log(funcArray[0]() + funcArray[1]()); // Outputs: Hello World!
   
    let funcObject = {sayHello: function(){ return 'Hello '}, sayWorld: function(){ return 'World!'}};
    console.log(funcObject.sayHello() + funcObject.sayWorld()); // Outputs: Hello World!
   

These characteristics are what make JavaScript a functional language, since it supports the concept of treating functions as first-class citizens.

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