You can use the path module to join the path of the directory in which helper1.js lives to the relative path of foobar.json. This will give you the absolute path to foobar.json.
var fs = require('fs');
var path = require('path');
var jsonPath = path.join(__dirname, '..', 'config', 'dev', 'foobar.json');
var jsonString = fs.readFileSync(jsonPath, 'utf8');
This should work on Linux, OSX, and Windows assuming a UTF8 encoding.