Unable to pass argument from outer scope in Node.js file system function?

I’ve noticed that arguments are not being recognized in Node.js filesystem when wrapped with (). Can anyone explain why this is happening please? My expectation was the even when wrapped in (), it will work just like forEach() function?

const fs = require('fs');
const path = require('path');
const root = path.resolve(__dirname);

fs.readdir(root, (error, filenames) => {
   filenames.forEach( filename => {
      const filePath = path.join(root, filename);
      fs.unlink( (filePath, err) => {
         // Throws error saying 'filePath' is not defined..?
      });
   });
});