Refactoring promise to return from function

Question

I am using JSZip to unzip a directory, which runs async. I want to write a function that unzips a file and returns the files associated with it, like this:

function unzipFile(filename){
  const zipper = new jsZip()
  return fs.readFile(filename, function(err, data) {
    return jsZip.loadAsync(data).then(function(zip) {
      return zip.files
    })
  })
}

but this is just return undefined. How can I make this async so that it returns the unzipped directory in a single function, and if I wanted to set a variable equal to its output would await its completion?

because returning in a regular callback (which you don't do either) has no effect — Jaromanda X, Mar 24 '17 at 02:50
you're not even using `zipper` - so, fundamentals are wrong anyway — Jaromanda X, Mar 24 '17 at 02:53
It's not possible. Learn to get comfortable with callbacks. You can use async/await keywords as syntax sugar for promises to make it LOOK like you're returning values but at some level an async function still return a promise so at some point you need to deal with callbacks anyway. I personally **strongly** recommend that people don't use async/await until they understand callbacks and promises. — slebetman, Mar 24 '17 at 02:56

score 5 · Accepted Answer · answered Mar 24 '17 at 02:55

5

You'll need to promisify fs.readFile first - then just a regular Promise chain should do the trick

e.g.

function unzipFile(filename){
    return new Promise((resolve, reject) => fs.readFile(filename, function(err, data) {
        if(err) {
            return reject(err);
        }
        resolve(data);
    }))
    .then(data => new jsZip().loadAsync(data))
    .then(zip => zip.files);
}

answered Mar 24 '17 at 02:55

Jaromanda X

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if the ES2015 "arrow functions" trip you up, you can always run the code through a transpiler like babel :p – Jaromanda X Mar 24 '17 at 02:58
nope, that's pretty much what my linter tells me to do – Rob Mar 24 '17 at 03:25

score 1 · Answer 2 · edited May 23 '17 at 11:54

You will need to promisify fs.readFile:

function readFileAsync(filename) {
    return new Promise((resolve, reject) => {
        fs.readFile(filename, function(err, data) {
            if (err) reject(err);
            else resolve(data);
        });
    });
}

Then you can chain onto that:

async function unzipFile(filename) {
    const zipper = new jsZip();
    const data = await readFileAsync(filename);
    const zip = await zipper.loadAsync(data);
    return zip.files;
}

Refactoring promise to return from function

2 Answers2