// package.json
{
  (...)
  "scripts": {
    "dev": "nodemon index.js",
    "staging-dev": "NODE_ENV=staging nodemon index.js"
  },
  (...)
}

// foo.js
const config = require("./knexfile");
const env = process.env.NODE_ENV || "development";
const knex = require("knex")(config[env]);

//knexfile.js
module.exports = {
  development: {
    client: 'postgresql',
    connection: {
      database: 'my_db_dev',
      user:     'postgres',
      password: 'postgres'
    },
    pool: {
      min: 2,
      max: 10
    },
    migrations: {
      tableName: 'knex_migrations'
    }
  },
  staging: {
    client: 'postgresql',
    connection: {
      database: 'my_db',
      user:     'postgres',
      password: 'postgres'
    },
    pool: {
      min: 2,
      max: 10
    },
    migrations: {
      tableName: 'knex_migrations'
    }
  }
};