What are these properties prefixing underscore used for in package.json?

Question

I found these properties prefixing underscore in package.json. What are they used for? why prefixing underscore on properties?

  "_from": "react-scripts@0.9.3",
  "_id": "react-scripts@0.9.3",
  "_inCache": true,
  "_location": "/react-scripts",
  "_nodeVersion": "6.0.0",
  "_npmOperationalInternal": {
    "host": "packages-18-east.internal.npmjs.com",
    "tmp": "tmp/react-scripts-0.9.3.tgz_1488303928068_0.6415497597772628"
  },
  "_npmUser": {
    "name": "gaearon",
    "email": "dan.abramov@gmail.com"
  },

Answer 1

Those are meta data reserved for package registries. All properties beginning with _ or $ are reserved for package registries to use at their discretion. wiki common "Reserved Properties" section explains it.

Answer 2

Underscore is the reserved property of package.json, it is used for reserved keys. So, it is clearly mentioned in Documentation of package.json , we can't start a name with an underscore.