build.js

console.log('BUILD env=%s', process.env.NODE_ENV)

package.json

{
  "scripts": {
    "dev": "NODE_ENV=dev config_a__b=null npm start",
    "start": "npm run build && config_a__b__c__d=VALUE node start.js",
    "build": "node build.js"
  },
  "dependencies": {
    "rc": "^0.6.0"
  }
}

start.js

var config = require('rc')('config')
console.log('config', config)
console.log('START env=%s', process.env.NODE_ENV)