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

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

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

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

• javascript
• node.js
• reactjs
• npm
• package.json

2 Answers 2

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

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.