My solution, as answered here, is to use:
var json = require('./data.json'); //with path
The file is loaded only once, further requests use cache.
edit To avoid caching, here’s the helper function from this blogpost given in the comments, using the fs module:
var readJson = (path, cb) => {
fs.readFile(require.resolve(path), (err, data) => {
if (err)
cb(err)
else
cb(null, JSON.parse(data))
})
}