ExtendScript

Adobe Photoshop has an EcmaScript 3 compatible scripting engine.
Its variant of EcmaScript is called ExtendScript.
ExtendScript files use the jsx file extension.
The ExtendScript Toolkit.app will let you experiment with this code.
It's installed at /Applications/Utilities/Adobe Utilities-CC.localized/ExtendScript Toolkit CC/ExtendScript Toolkit.app.

Here's some sample ExtendScript

function setColor_jsx(color){
  app.foregroundColor.rgb.red = color.red
  app.foregroundColor.rgb.green = color.green
  app.foregroundColor.rgb.blue = color.blue
  return app.foregroundColor.rgb.hexValue
}

var color = {
  red: Math.random() * 255,
  green: Math.random() * 255,
  blue: Math.random() * 255
}

setColor_jsx(color)

Photoshop / After Effects scripting with Node.js

Install

npm install photoshop

photoshop.createStream(jsx, [args])

This is almost always what you want to use.

photoshop.createStream creates a Node.js Stream. The first argument to createStream is an ExtendScript jsx function to evaluate in Photoshop. The jsx function will be called with an ExtendScript Socket instance and whatever additional arguments you supply as the second argument to createStream.

Writing to the socket from Photoshop will immediately stream that string back to node.

function streamColorChanges_jsx(writeStream, setColor_jsx, color){
  writeStream.write(setColor_jsx(color));
  alert("Photoshop won't return until this window is closed, but the stream already sent its data!");
}

var readStream = require('photoshop').createStream(streamColorChanges_jsx, [setColor_jsx, color]);

readStream.pipe(process.stdout);

readStream.on('end', function(){
  console.log('Done!')
});

aftereffects.createStream(jsx, [args])

require('photoshop/aftereffects').createStream(function jsx(stream, props){
  stream.writeln(JSON.stringify(props))
  alert("After Effects won't return until this window is closed, but the stream already sent its data!")
}, [{lulz:true}])
.pipe(process.stdout)

JSONify manythings!

require('./aftereffects').createStream(function jsx(stream, props){

  var composition = app.project.ao_comps()[0];
  var layer = composition.layers[1];

  stream.writeln(
    JSON.stringify(layer, null, 2)
  );

  stream.writeln(JSON.stringify(props))

}, [{lulz:Math.random(0)}])
.pipe(process.stdout)

photoshop.invoke(jsx, [args,] callback)

The invoke method evaluates the given ExtendScript script in Adobe Photoshop.
It handles serializing and deserializing the result of your script so you can return pretty much anything.
You can even return Photoshop host objects and it'll do its best to not completely wet its pants.
It includes es5shim and JSON2 so you can use normal JavaScript like Array map in your ExtendScript.

function recentFilesThatExist_jsx(){
  return app.recentFiles.map(File).filter(function(file){return file.exists})
}
require('photoshop').invoke(recentFilesThatExist_jsx, function(error, recentFiles){
  console.log(recentFiles)
})

The args argument is optional.
You can pass JSONable objects as arguments.

require('photoshop').invoke(setColor_jsx, [color], function(error, foregroundColor){
  console.log('#' + foregroundColor)
})