将异步函数的结果存储到变量中

const fs = require('fs');

let result;
fs.readFile('file.txt', 'utf8', function(err, data) {
  if (err) throw err;
  result = data;
  console.log(result);
});

const fs = require('fs');

function readFileAsync(filename) {
  return new Promise((resolve, reject) => {
    fs.readFile(filename, 'utf8', (err, data) => {
      if (err) {
        reject(err);
      } else {
        resolve(data);
      }
    });
  });
}

let result;
readFileAsync('file.txt')
  .then(data => {
    result = data;
    console.log(result);
  })
  .catch(err => {
    console.error(err);
  });