I cannot figure out how async/await works. I slightly understand it but I can't make it work.

function loadMonoCounter() {
    fs.readFileSync("monolitic.txt", "binary", async function(err, data) {
       return await new Buffer( data);
  });
}

module.exports.read = function() {
  console.log(loadMonoCounter());
};

I know, I could use readFileSync, but if I do, I know I'll never understand async/await and I'll just bury the issue.

Goal: Call loadMonoCounter() and return the content of a file.

That file is incremented every time incrementMonoCounter() is called (every page load). The file contains the dump of a buffer in binary and is stored on a SSD.

No matter what I do, I get an error or undefined in the console.

Jeremy Dicaire

Ответов: 12

Ответы (12)

To use await/async you need methods that return promises. The core API functions don't do that without wrappers like promisify:

const fs = require('fs');
const util = require('util');

// Convert fs.readFile into Promise version of same    
const readFile = util.promisify(fs.readFile);

function getStuff() {
  return readFile('test');
}

// Can't use `await` outside of an async function so you need to chain
// with then()
getStuff().then(data => {
  console.log(data);
})

As a note, readFileSync does not take a callback, it returns the data or throws an exception. You're not getting the value you want because that function you supply is ignored and you're not capturing the actual return value.

This is TypeScript version of @Joel's answer. It is usable after Node 11.0:

import { promises as fs } from 'fs';

async function loadMonoCounter() {
    const data = await fs.readFile('monolitic.txt', 'binary');
    return Buffer.from(data);
}

From node v14.0.0

const {readFile} = require('fs/promises');

const myFunction = async()=>{
    const result = await readFile('monolitic.txt','binary')
    console.log(result)
}

myFunction()

To keep it succint and retain all functionality of fs:

const fs = require('fs');
const fsPromises = fs.promises;

async function loadMonoCounter() {
    const data = await fsPromises.readFile('monolitic.txt', 'binary');
    return new Buffer(data);
}

Importing fs and fs.promises separately will give access to the entire fs API while also keeping it more readable... So that something like the next example is easily accomplished.

// the 'next example'
fsPromises.access('monolitic.txt', fs.constants.R_OK | fs.constants.W_OK)
    .then(() => console.log('can access'))
    .catch(() => console.error('cannot access'));

see this example https://www.geeksforgeeks.org/node-js-fs-readfile-method/

// Include fs module
var fs = require('fs');
  
// Use fs.readFile() method to read the file
fs.readFile('demo.txt', (err, data) => {
  console.log(data);
})

const fs = require("fs");
const util = require("util");
const readFile = util.promisify(fs.readFile);
const getContent = async () => {
let my_content;
try {
  const { toJSON } = await readFile("credentials.json");
  my_content = toJSON();
  console.log(my_content);
} catch (e) {
  console.log("Error loading client secret file:", e);
 }
};

You can use fs.promises available natively since Node v11.0.0

import fs from 'fs';

const readFile = async filePath => {
  try {
    const data = await fs.promises.readFile(filePath, 'utf8')
    return data
  }
  catch(err) {
    console.log(err)
  }
}

Since Node v11.0.0 fs promises are available natively without promisify:

const fs = require('fs').promises;
async function loadMonoCounter() {
    const data = await fs.readFile("monolitic.txt", "binary");
    return new Buffer(data);
}

I read file by using the Promise. For me its properly:

const fs = require('fs')

//function which return Promise
const read = (path, type) => new Promise((resolve, reject) => {
  fs.readFile(path, type, (err, file) => {
    if (err) reject(err)
    resolve(file)
  })
})

//example how call this function
read('file.txt', 'utf8')
    .then((file) => console.log('your file is '+file))
    .catch((err) => console.log('error reading file '+err))

//another example how call function inside async
async function func() {
  let file = await read('file.txt', 'utf8')
  console.log('your file is '+file)
}

You can find my approach below: First, I required fs as fsBase, then I put the "promises" inside fs variable.

const fsBase = require('fs');
const fs = fsBase.promises

const fn = async () => {
    const data = await fs.readFile('example.txt', 'utf8');
    console.log(data);
};

fn();

There is a fs.readFileSync( path, options ) method, which is synchronous.

You can easily wrap the readFile command with a promise like so:

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

then use:

await readFile("path/to/file");

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