use of process.nextTick

const fs = require('fs');

function fileSize(fileName, cb) {
  if(typeof fileName !== 'string') {
    return process.nextTick(cb, new TypeError('argument should be string'));
  }
  
  fs.stat(fileName, (err, stats) => {
    if(err) {
      return cb(err);
    }
    
    cb(null, stats.size);
  });
}

fileSize(1, (err, stats) => {
  if(err) throw err;
  
  console.log(`Size in KB: ${size/1024}`);
});

console.log('Hello!');