calc.js

module.exports = {
  add: function(a, b) {
    return a + b;
  },
  sub: function(a, b) {
    return a - b;
  },
  multi: function(a, b) {
    return a * b;
  },
  div: function(a, b) {
    return a / b
  }
}

index.js

var prompt = require('prompt');
var calc = require('./calc')
prompt.colors = false;
prompt.start();

prompt.get(['operator', 'number1', 'number2'], function (err, result) {
  if (err) { return onErr(err); }
  console.log('Output:');
  console.log('  operator: ' + result.operator);
  console.log('  answer: ' + doCalc(result.operator, Number(result.number1), Number(result.number2)));
});

var doCalc = function(operator, a, b) {
  switch (operator) {
    case "+":
      return calc.add(a, b);
    case "-":
      return calc.sub(a, b);
    case "*":
      return calc.multi(a, b);
    case "/":
      return calc.div(a, b);
  }
}

package.json

{
  "name": "calc",
  "version": "1.0.0",
  "description": "a calculator to beat davids",
  "main": "index.js",
  "dependencies": {
    "prompt": "^0.2.14"
  },
  "devDependencies": {},
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "alex",
  "license": "MIT"
}