Cannot add up results of a function in a loop

Question

I have this function getSize() (from npm module: get-folder-size) that's calculating the total size of all files in a folder (directory).

const getSize = require('get-folder-size')

let folders = ["C:\\test folder", "C:\\test folder 2\\sub folder"]

funciton totalFilesizeOfAllFolders () {
  let totalSizeOfAllFolders = 0
  this.folders.forEach(folder => {
    getSize(folder, (err, size) => {
      if (err) { throw err }
      // ADD UP THE "SIZE" TO THE TOTAL SOMEHOW
      // Just doing the following returns 0: totalSizeOfAllFolders += size
    })
  })
  return totalSizeOfAllFolders 
}

Question

How do I loop through the array folders properly and add up the calculated sizes of all the folders in the array? I'm not sure how to return the size out of the function

That's an asynchronous API, so you won't be able to return a result. You'll have to structure your own API as something with a callback or use Promises. — Pointy
– Pointy, Commented Aug 31, 2018 at 10:48
@Pointy so I won't be able to use it in a loop, since loops are synchronous? — Un1
– Un1, Commented Aug 31, 2018 at 10:50
Well you certainly can call the function in a loop; that's not the crux of the problem. Loop or no loop, that getSize() function is asynchronous. — Pointy
– Pointy, Commented Aug 31, 2018 at 10:54

Luca Kiebel · Accepted Answer · 2018-08-31 11:01:30Z

2

You can use Promise.all here, first construct an array of Promises, and then await all of them calculating the total size:

//const getSize = require('get-folder-size')

let folders = ["C:\\test folder", "C:\\test folder 2\\sub folder"];


function totalFilesizeOfAllFolders(callback) {
  let folderPromises = folders.map(folder => { // use map to create a new array of promises
    return new Promise((resolve, reject) => getSize(folder, (err, size) => {
      if (err) {reject(err)}
      resolve(size);
    }));
  })
  Promise.all(folderPromises) // promise.all waits for all promises in the array to resolve
    .then(sizes => callback(sizes.reduce((a, b) => a + b, 0))); // reduce the array of sizes to a size
}


totalFilesizeOfAllFolders(s => console.log(s)); // => 127



// getsize stub. remove me once you have access to the npm package
function getSize(name, callback) {let sizes = {"C:\\test folder":112, "C:\\test folder 2\\sub folder":15}; callback(null,sizes[name])}

answered Aug 31, 2018 at 11:01

Luca Kiebel

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1 Comment

Un1 Over a year ago

Thanks for a very detailed answer, I get it now

Cookie · Accepted Answer · 2018-08-31 13:49:47Z

1

You could use a library like Async to help you iterate over the calls asynchronously then pass a callback to your function to return the totalSizeOfAllFolders.

function totalFilesizeOfAllFolders (done) {
  let totalSizeOfAllFolders = 0;
  async.each(folders, (folder, callback) => {
    getSize(folder, (err, size) => {
      if (err) { throw err }
      totalSizeOfAllFolders++;
      callback();
    });
  }, (err) => {
    done(totalSizeOfAllFolders);
  });
}

edited Aug 31, 2018 at 13:49

answered Aug 31, 2018 at 10:56

Cookie

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1 Comment

Un1 Over a year ago

Thanks for the answer!

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