Reading csv files with async await functions in node.js. Undefined due to execution before promises resolve

Question

I'm reading files with Node in a 4 step process. Each step has to wait for the resolution of the function it calls. It seems like a very simple task to accomplish, but the async functions are not awaiting the result of their variable definitions ( i.e. even though the variable 'finish' is dependent on the variable processData which is awaiting an async function, it fires anyway.

I'm guessing I have a butchered understanding of the fundamental behavior if my conclusions aren't correct. And I know there are a ton of threads with this issue, but I would appreciate some help. Oh, and if I log the results from the fs.readFile method in its callback, the data shows up. But the data is returned undefined from that block. Code below:

async function formatData(){ 
    const processData = await getFiles()
    const finish = await printData(processData)
}


async function getFiles(){
    const checkFile = await getFileData('./csvs/checkfile_for_node_test.csv')
    const scheduleFile = await getFileData('./csvs/schedules_for_node_test.csv')

    console.log('returning files')
    return {checks: checkFile, schedules: schedFile}
}

async function getFileData(file){
     const fileData = await fs.readFile(file, (err, data) => data ) 
     console.log(`file data: ${fileData}`)
     return fileData
}

function printData(data){
    console.log(data)
    return data
}

formatData()

You are running await on printData which is not an async function. Please make it async and check. You can also simply remove async from printData call. — Sachin Singh
– Sachin Singh, Commented Jul 24, 2020 at 18:05
Are you using a Promisified version of readFile? It doesn't look that way, in which case await fs.readFile(...) won't do what you hope. — Scott Sauyet
– Scott Sauyet, Commented Jul 24, 2020 at 18:07
So you're getting file data: undefined in your logs, right? — Bergi
– Bergi, Commented Jul 24, 2020 at 18:47

Nadeem Khan · Accepted Answer · 2020-07-24 18:26:33Z

2

I believe you are missing promise from getFileData function. Below is the modified code

const fs = require("fs");

async function formatData() {
  const processData = await getFiles();
  const finish = await printData(processData);

  console.log(finish);
}

async function getFiles() {
  const checkFile = await getFileData("./hello.txt");
  const scheduleFile = await getFileData("./world.txt");

  console.log("returning files");
  return { checks: checkFile, schedules: scheduleFile };
}

async function getFileData(file) {
  return new Promise((res, rej) => {
    fs.readFile(file, (err, data) => {
      if (err) rej(err);
      console.log(`file data: ${data}`);
      res({ fileData: data });
    });
  });

  return fileData;
}

async function printData(data) {
  console.log(data);
  return data;
}

formatData();

answered Jul 24, 2020 at 18:26

Nadeem Khan

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3 Comments

Bergi Over a year ago

Don't forget to put the resolve() call in an else statement

Nadeem Khan Over a year ago

Thanks, @Bergi, I have only renamed the resolve to res, in above code. Is it still incorrect?

Bergi Over a year ago

No, it's not correct. You're still calling the console.log and the res even if there was an error.

Sakibur Rahman · Accepted Answer · 2020-07-24 18:24:26Z

1

I don't know if the code you provided is the actual code, but there seems to be a lot of issues. I will break down them one by one for you:

When you are calling formatData you should use await keyword before it because it is an async method.
At the last line of getFiles method, you mistakenly placed schedFile instead of scheduleFile.
You are trying to use fs.readFile with await. It does not work that way. You have to either promisify it or use readFileSync.

All put together in code:

async function formatData(){ 
    const processData = await getFiles()
    const finish = await printData(processData)
}


async function getFiles(){
    const checkFile = await getFileData('./csvs/checkfile_for_node_test.csv')
    const scheduleFile = await getFileData('./csvs/schedules_for_node_test.csv')

    console.log('returning files')
    return {checks: checkFile, schedules: scheduleFile}
}

async function getFileData(file){
     const readFileAsync = promisify(fs.readFile); 
     const fileData = await readFileAsync(file, { encoding: 'utf-8' });
     console.log(`file data: ${fileData}`)
     return fileData
}

function printData(data){
    console.log(data)
    return data
}

window.addEventListener('load', async function () {
   await formatData();
});

I hope it works.

answered Jul 24, 2020 at 18:24

Sakibur Rahman

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2 Comments

Bergi Over a year ago

4. printData is synchronous, there's no reason to await the call

Bergi Over a year ago

"When you are calling formatData you should use await keyword" - not necessarily, it appears to be the top-level call. What's more important is to handle errors from it, using e.g. formatData().catch(console.error). Btw, this is node.js code, which doesn't have a window and there are no load events to listen for.

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