How to read file with async/await properly?

问题

I cannot figure out how async/await works. I slightly understands 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 burry 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 contain the dump of a buffer in binary and is stored on a SSD.

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

回答1:

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.

回答2:

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);
}

回答3:

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");

回答4:

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(file);
}

来源：https://stackoverflow.com/questions/46867517/how-to-read-file-with-async-await-properly

标签

node.js

asynchronous

readfile