calling a function on a function is it possible?

Question

in the following code

const express = require('express');
const app = express()
app.use(express.static('public'));

express is a function so how it call "express.static('public')" method on it? is it possible in JavaScript to call a function which is inside a function

Answer 1

A functions is not only a first class function, but also a first class object and can contain properties.

function foo() {}

foo.bar = function () { console.log('i am bar'); };

foo.bar();

Answer 2

You can attach a function as member data to another function (which is what is done in your example).

const express = () => {};
express.static = argument => console.log('argument');
express.static('public'); # console >>> 'public'

However, you cannot readily access a variable that is defined in a function body.

const express = () => {
    const static = argument => console.log('argument');
};
express.static('public'); # console >>> Error: undefined in not a function

There is a signifiant difference between member data attached to a function (the first example) and the closure that wraps the body of the function (the second example).

So, to answer your question "is it possible in JavaScript to call a function which is inside a function?" No, this is not readily possible with the important note that this is not what is being done in your example.