JavaScript：读取文件后尝试返回字符串时未定义类型

Question

我是 JavaScript 新手，在 Node.JS v12.13.1 中编写 JS 代码。

我正在尝试异步读取文件并通过承诺返回字符串值。

到目前为止，大多数函数返回 undefined 值，而其他函数返回 [object Promise] 值。

writeFile 函数有效，我可以验证 out.txt 是否包含“测试数据”

我对堆栈溢出帖子进行了研究，我只是感到困惑。
How to read file with async/await properly?
Using filesystem in node.js with async / await
Using Promises with fs.readFile in a loop

API 文档：
https://nodejs.org/api/fs.html#fs_fs_promises_api

有人可以告诉我我缺少什么吗？我将不胜感激，因为我已经进行了大约 3 天的反复试验研究。

谢谢！

function writeFile(data) {
    const fs = require('fs');
    fs.writeFile('out.txt', data, (err) => {
        if (err) throw err;
    });
}

function readFile1() {
    const fs = require('fs').promises;
    (async() =>  {
    try {
      const result = await fs.readFileSync('out.txt', 'utf8');
      console.log(result);
   } catch(e) {
      console.error(e);
   }
    })();
}

function readFile2() {
    const fs = require('fs');
    const { promisify } = require('util');
    const readFile = promisify(fs.readFile);

    (async() =>  {
    try {
        const result = await readFile('out.txt', 'utf8');
        console.log(result);
    } catch(e) {
        console.error(e);
    }
    })();
}

function readFile3() {
    const fs = require('fs');
    const util = require('util');
    const readFile = util.promisify(fs.readFile);

    function getStuff() {
        return readFile('out.txt', 'utf8');
    }
    getStuff().then(data => {
        console.log("Inside getStuff Method: "+data);
    })
}

function readFile4() {
    const fs = require('fs');    
    const fsPromises = fs.promises;
  try {  
    return fsPromises.readFile('out.txt');
  } catch (err) {
    console.error('Error occured while reading file!', err);
  }
}

async function readFile5() {
    const fs = require('fs');
    const fsPromises = fs.promises;
    let fileHandle = null;
    try {
        fileHandle = await fsPromises.open('out.txt');
        await fileHandle.readFile();
    } finally {
        if (fileHandle) {
            await fileHandle.close();
        }
    }
    return fs.readFileSync('out.txt', 'utf8');
}

writeFile("Test Data");
myString = readFile5();
console.log("The String Value is: " + myString);

Answer 1

以读取模式打开文件

fileHandle = await fsPromises.open('out.txt','r');

那么。

你可以像下面那样做

 async function getPromiseResolved() {
    writeFile("Test Data");
    myString =  await readFile5();
    console.log("The String Value is: " + myString);
    }

getPromiseResolved()

https://stackoverflow.com/a/29516570/6302996

完整代码

// This is your Editor pane. Write your JavaScript here and 
// use the command line to execute commands

function writeFile(data) {
  const fs = require('fs');
  fs.writeFile('out.txt', data, (err) => {
    if (err) throw err;
  });
}

async function readFile5() {
  const fs = require('fs');
  const fsPromises = fs.promises;
  let fileHandle = null;
  try {
    fileHandle = await fsPromises.open('out.txt', 'r');
    await fileHandle.readFile();
  } finally {
    if (fileHandle) {
      await fileHandle.close();
    }
  }
  var myString = await fs.readFileSync('out.txt', 'utf8');
  return myString;
}
async function getPromiseResolved() {
  writeFile("Test Data");
  myString = await readFile5();

  console.log("The String Value is: " + myString);
}

getPromiseResolved()

Answer 2

你可以这样做：

const fs = require('fs')

const readFile = filepath =>
   new Promise((resolve, reject) => 
      fs.readFile(filepath, 'utf-8', 
         (err, result) => err ? reject(err): resolve(result)))

这基本上就是promisify 对方法所做的。这就是您可以将任何回调样式函数转换为返回 Promise 的方法。