Node.js使用async/await读取文件 [英] Node.js read file using async/await

问题描述

我正在尝试使用异步/等待代码来读取文件. 这是我的代码:

I am experimenting with async/await code to read file. Here's my code:

var fs = require('fs');

function readFile(fileName) {
  return new Promise(resolve => {
    //console.log(test);
    fs.readFile(fileName, 'utf8', function (err, data) {
      if (err) throw err;

      console.log(fileName)
      console.log(data)
    })
    resolve();
  });
}

async function run() {
  await readFile('file1.txt');
  await readFile('file2.txt');
  readFile('file3.txt');
}

run();

但是结果仍然是随机的.这意味着file3有时会在file2之前读取.我在哪里做错了?

But the result is still random. It means file3 sometime read before file2. Where am I doing wrong?

推荐答案

有很多方法可以实现.

此链接

我会写一个简单的:

1)使用util.promisify将回调方法转换为promise:

1) using util.promisify to convert callback method to promise:

const fs = require('fs');
const util = require('util');
const readFile = (fileName) => util.promisify(fs.readFile)(fileName, 'utf8');

(async () => {
  try {
    const files = ['file1.txt', 'file2.txt', 'file3.txt'];
    for (const file of files) {
      console.log(
        await readFile(file)
      );
    }
  }
  catch (error) {
    console.error(error);
  }
})();

2)*Sync方法.由于您的代码未处理并发，因此可以使用*Sync方法:

2) *Sync methods. Since Your code is not dealing with concurrency You can use *Sync methods:

const fs = require('fs');

try {
  const files = ['file1.txt', 'file2.txt', 'file3.txt'];
  for (const file of files) {
    console.log(
      fs.readFileSync(file, 'utf8')
    );
  }
}
catch (error) {
  console.error(error);
}

顺便说一句.这是您的固定代码:

BTW. Here is Your fixed code:

var fs = require('fs');

function readFile(fileName) {
  return new Promise((resolve, reject) => {
    fs.readFile(fileName, 'utf8', function (error, data) {
      if (error) return reject(error);

      console.log(fileName)
      console.log(data)

      resolve();
    })
  });
}

async function run() {
  await readFile('file1.txt');
  await readFile('file2.txt');
  await readFile('file3.txt');
}

run();

因为您正在调用readFile并以相同的异步序列解析它在同一时间被调用，这是race condition的原因.

since You're calling readFile and resolve at same async sequence it's being called at same time which is reason of race condition.

您必须等待回调处理，然后再解决它(在回调作用域内).

You've to wait for callback handling and then resolve it (inside callback scope).

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