Node Function Returning Empty Array

Question

In the following node function, it is returning an empty array. Not sure why its doing that. Could this be a async await issue? Would appreciate any help. Thank you

const folderPath = '/public/home.html'

function getCircuitAndFuse(folderPath){
  //List containing circuit name with its fuse
  let temporaryList = [];
  let finalCircuitAndFuseList = []

  fs.readFile(__dirname + folderPath, (error, data)=>{
    if(error){
      console.log(`Unable to read file: ${error}`)
    }else{

      var $ = cheerio.load(data)

      $('img').each(function(index, element){
        let getClassAtr = element.attribs.class
        temporaryList.push(getClassAtr.slice(0, getClassAtr.lastIndexOf(" ")))
      })

      finalCircuitAndFuseList = [...new Set(temporaryList)]
    }
  })
return finalCircuitAndFuseList;
}
let getInfo = getCircuitAndFuse(folderPath)

// Returning empty array
console.log(getInfo)

***Server code****
const server = http.createServer(function(req, res){
  res.writeHead(200, {'Content-Type': 'text/plain'})
  res.end()
}).listen(port, ()=>{
  console.log(`Server listening on port ${port}. Press Ctrl-C to terminate...`)
})

Your function returns before fs.readFile has finished. Since that method is asynchronous, Node executes it and immediately goes to the bottom of the function and returns you your empty array every time. — Dan Zuzevich
– Dan Zuzevich, Commented Sep 21, 2021 at 16:43
I wouldd say yes. your console.log is running before the function completes. you have an async call (readFile), that needs to finish before you try and access the return value. — G-Force
– G-Force, Commented Sep 21, 2021 at 16:45

Faruk · Accepted Answer · 2021-09-21 16:45:15Z

2

getCircuitAndFuse must return Promise like this:

function getCircuitAndFuse(folderPath) {
  return new Promise((resolve, reject) => {
    //List containing circuit name with its fuse
    let temporaryList = [];

    fs.readFile(__dirname + folderPath, (error, data) => {
      if (error) {
        console.log(`Unable to read file: ${error}`);
      } else {
        var $ = cheerio.load(data);

        $('img').each(function (index, element) {
          let getClassAtr = element.attribs.class;
          temporaryList.push(
            getClassAtr.slice(0, getClassAtr.lastIndexOf(' '))
          );
        });

        resolve([...new Set(temporaryList)]);
      }
    });
  });
}

getCircuitAndFuse(folderPath).then((getInfo) => {
  // do something with `getInfo`
});

answered Sep 21, 2021 at 16:45

Faruk

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Dan Zuzevich · Accepted Answer · 2021-09-21 16:58:05Z

1

Another alternative to Faruk's answer would be to just use fs.readFileSync instead of wrapping your function in a promise and requiring some of that extra ceremony. Using fs.readFileSync will ensure that your function doesn't return prematurely.

Here is your code rewritten with that in mind:

function getCircuitAndFuse(folderPath) {
  try {
    let temporaryList = [];
    const data = fs.readFileSync(__dirname + folderPath);
    const $ = cheerio.load(data);

    $("img").each(function (index, element) {
      let getClassAtr = element.attribs.class;
      temporaryList.push(getClassAtr.slice(0, getClassAtr.lastIndexOf(" ")));
    });

    return [...new Set(temporaryList)];
  } catch (error) {
    console.log(error);
  }
}

edited Sep 21, 2021 at 16:58

answered Sep 21, 2021 at 16:47

Dan Zuzevich

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