const fs = require('fs');

fs.readFile('./test.txt', 'utf-8', (err, data) => {

  err

  ? console.error(err)

  : console.log(data.toString()+'1');

});

console.log("over1");

fs.readFile('./test.txt', 'utf-8', (err, data) => {

  setTimeout(() => {

    err

    ? console.error(err)

    : console.log(data.toString()+'2');

  }, 100);

});

console.log("over2");

fs.readFile('./test.txt', 'utf-8', (err, data) => {

  err

  ? console.error(err)

  : console.log(data.toString()+'3');

});

console.log("over3");

var data = fs.readFileSync('./test.txt');

console.log(data.toString()+'4');

console.log("over4");

/*------------------------

输出结果如下

over1

over2

over3

hello node dkplus!4

over4

hello node dkplus!1

hello node dkplus!3

hello node dkplus!2

------------------------*/