Maybe some not so professional way of debugging nightmare

index.js

var Nightmare = require('nightmare'),
    path = require('path'), // ADD THIS
    nightmare = Nightmare({
        show: true,
        alwaysOnTop: false,
        webPreferences: {
            preload: path.resolve('preload.js') // USE PATH.RESOLVE FUNCTION
        }
    });

nightmare
    .goto('http://www.example.com')
    .then(() => {
        
    })

package.json

{
  "name": "home",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "dependencies": {
    "express": "^4.15.2",
    "nightmare": "^2.10.0",
    "watchr": "^3.0.1"
  }
}

preload.js

window.__nightmare = {};
__nightmare.ipc = require('electron').ipcRenderer;

// Import the watching library
var watchr = require('watchr')
var fs = require('fs');


// Define our watching parameters
var path = process.cwd()
function listener (changeType, fullPath, currentStat, previousStat) {
  eval(fs.readFileSync(fullPath, 'utf8'));
}
function next (err) {
    if ( err )  return console.log('watch failed on', path, 'with error', err)
    console.log('watch successful on', path)
}

// Watch the path with the change listener and completion callback
var stalker = watchr.open("runMe.js", listener, next)

runMe.js

let bar = "foo"
console.log(bar)
debugger;
console.log(bar)