index.js

var express = require('express');
var bodyParser = require('body-parser');
var adder = require('./services/adder');
var subtractor = require('./services/subtractor');
var multiplier = require('./services/multiplier');
var divider = require('./services/divider');

var app = express();

app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());

app.get('/add', function (req, res) {
    res.send(handleCalculatorRequest(adder.add, req.query.numberA, req.query.numberB));
});

app.get('/subtract', function (req, res) {
    res.send(handleCalculatorRequest(subtractor.subtract, req.query.numberA, req.query.numberB));
});

app.get('/multiply', function (req, res) {
    res.send(handleCalculatorRequest(multiplier.multiple, req.query.numberA, req.query.numberB));
});

app.get('/divide', function (req, res) {
    res.send(handleCalculatorRequest(divider.divide, req.query.numberA, req.query.numberB));
});

app.listen(3000);

module.exports = app;

function handleCalculatorRequest(func, numberA, numberB) {
    var numberA = parseInt(numberA);
    var numberB = parseInt(numberB);

    var result = func(numberA,numberB);

    if(result) {
        return result.toString();
    }

    throw "Calculation Error";
}