Write a asynchronous code

Question

Im trying to check the file system on my server to check if a file exists or not. This simple problem became actually quite a challenging task. Here is my basic code which does not work:

  var fs = require('fs');
  var arrayLength = arr.length;
  for (var i = 0; i < arrayLength; i++) {
    var imgfile = arr[i].country
    fs.exists('/var/scraper/public/images/flags' + imgfile + ".png", (exists) => {
      console.log(exists ? 'it\'s there' : 'not here!');
    });                    
  }   

Its come to my attention that this is not asynchronous and will not work. I found this link: https://nodejs.org/dist/latest-v9.x/docs/api/fs.html#fs_fs_exists_path_callback

and my code should be built like this.

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  writeMyData(fd);
 });

I was hoping to get some help to help me rewrite my code to make it asynchronous?

any help at all with this would be much appreciated.

Answer 1

You can do this synchronously with no problem, it might just take awile. If the fs.exists function is not finding a file I'd just ensure your path is exactly correct. I added an extra '/' after /flags as I'm assuming the array of country names does not include this.

const fs = require('fs');
const path = require('path');
const imageDir = '/var/scraper/public/images/flags/';

var filesExist = arr.map((imgObj) => {
  const imgfile = imgObj.country;
  let filePath = path.join(imageDir, imgfile + ".png");
  console.log('Checking existance of file: ', filePath);
  // Return an object with 'exists' property and also the file path
  // To just return the path:
  // return fs.existsSync(filePath) ? filePath: path.join(imageDir, "noimg.png")};
  return { exists: fs.existsSync(filePath), path: fs.existsSync(filePath) ? filePath: path.join(imageDir, "noimg.png")};
});

console.log('Files exist: ', filesExist);

Answer 2

The simplest and lease time consuming solution to this problem would be to simply use fs.existsSync You can find docs for that method here. In fact, if you take a look at fs.exists in the docs you'll notice that fs.exists has been deprecated at least since the last v8 LTS release (this might be why you are having trouble with it).

If you are using a recent version of node with support for it, this is a great opportunity to leverage async/await. It might look something like this:

const fs = require('fs');
const { promisify } = require('util');
const existsAsync = promisify(fs.exists);

async function checkFiles (arr) {
  const arrayLength = arr.length;

  for (let i = 0; I < arrayLength; I++) {
    const countryName = arr[i].country;

    const filePath = `/var/scraper/public/images/flags${countryName}.png`; 
    const fileExists = await fs.existsAsync(filePath);

    console.log(`File flags${contryName}.png exists: ${fileExists}`);
  }
}