NodeJS: downloading NodeJS and exporting modules

app.js

let calc = require("./calcMethod");
let personalData = require("./personData")

// <------------- demo 1 -----------------> // 
console.log(calc.math("A",1,calc.add));
console.log(calc.math(1,3,calc.subtract));
console.log(calc.math(1,3,calc.multiply));

// <------------- demo 2 -----------------> // 
console.log(personalData("Hegabovic",1995));

calculator.js

// using function callback
let add = function add(num1, num2) {
    return num1 + num2;
}

let subtract = function subtract(num1, num2) {
    return num1 - num2;
}

let multiply = function multiply(num1, num2) {
    return num1 * num2;
}

let math = function math(num1, num2, func) {
    if( isNaN(num1) || isNaN(num2) ){
        return "please enter a valid number";
    }else {
        return func(num1, num2);
    }
}

module.exports = {
    add,
    subtract,
    multiply,
    math
}

personalData.js

let personDate = function personData(name, date) {
    let currentYear = new Date();
    currentYear.getFullYear()
    if (isNaN(name)) {
        let age = currentYear.getFullYear() - date;
        return `Hello ${name} and your age now ${age}`;
    }
}

// console.log(personData("Hegabovic",1995))

module.exports = personDate