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Blocking vs Non-blocking

Synchronous file read:

const fs = require('fs');

const data = fs.readFileSync('/file.md'); // blocks here until file is read

console.log(data);

// moreWork(); will run after console.log

Asynchronous file read:

const fs = require('fs');

fs.readFile('/file.md', (error, data) => {

  if (error) throw error; // throw error or use logger to log
  console.log(data);
});

// moreWork(); will run before console.log

Warning on mixing Blocking and Non-blocking code​

Problem:

const fs = require('fs');

fs.readFile('/file.md', (err, data) => {

  if (err) throw err;
  console.log(data);
});

fs.unlinkSync('/file.md'); // File will get delete first

Solution: Non-blocking with correct order of execution

const fs = require('fs');

fs.readFile('/file.md', (readFileErr, data) => {

  if (readFileErr) throw readFileErr;
  console.log(data);

  fs.unlink('/file.md', (unlinkErr) => {
    if (unlinkErr) throw unlinkErr;
  });
});