我在package.json中发现这些属性以下划线为前缀。它们是用来干什么的？为什么在属性上加下划线？
"_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"
},