index.js

#!/usr/bin/env node

console.log(`my demo postinstall index is here`);

package.json

{
  "name": "my-demo-postinstall-plugin",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "bin": {
    "my-demo-postinstall-plugin": "index.js"
  },
  "files": ["index.js", "postinstall.js"],
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "postinstall": "node postinstall.js"
  },
  "keywords": [],
  "author": "Jarmo Isotalo <jarmo@isotalo.fi>",
  "license": "MIT"
}

postinstall.js

#!/usr/bin/env node

console.log(`my demo postinstall script is here`);