Javascript promise, nested functions

Question

my question is that im running node.js, and i have 2 functions that needs to be run in a specific order, however they are not returning a promise at this this. So i wonder how would i rewrite this to make sure that the main function would return a promise, and if i have nested functions, do i just run the 2nd function from the 1st functions resolve?

Here is the code:

handleMd5Convert = (file) => {
  fs.readFile(file, (err, buf) => {
    fs.rename(file, directoryPath + md5(buf) + '.mp3', (err) => {
      if (err) console.log('ERROR: ' + err);
    })
  })
})

HandleMd5Convert should be able to to .then()

/Alex

Mark · Accepted Answer · 2018-09-04 21:32:48Z

5

You should be able to wrap the whole thing in a new Promise() and use resolve() & reject() to handle success and errors:

handleMd5Convert = (file) => {
  return new Promise((resolve, reject) => {
    fs.readFile(file, (err, buf) => {
      if (err) return reject(err)
      fs.rename(file, directoryPath + md5(buf) + '.mp3', (err) => {
        if (err) return reject(err);
        resolve()
      })
    })
  })
}

handleMd5Convert('test.txt')
.then(() => console.log("done"))
.catch(err => console.log("error:", err))

answered Sep 4, 2018 at 21:32

Mark

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6 Comments

Alexander Munck Af Rosenschöld Over a year ago

The resolve() here, it shouldnt have anything inside of it? or just emtpy () is fine? if i were to console log this now, it would return undefined, but maybe it shouldnt return anything

Mark Over a year ago

@AlexanderMunckAfRosenschöld it's not required, but if you have a value you want to retrieve in the then() you can resolve that value. You would: resolve(someValue) then after the promise resolves you could get it in then(someValue => {/* use some value */}). It doesn't seem like you have anything to resolve in this case though, but maybe something like the final file name would make sense.

Alexander Munck Af Rosenschöld Over a year ago

Thanks alot for your help!

Arjun Over a year ago

@AlexanderMunckAfRosenschöld You should mark the answer as "accepted", if it worked for you. Do this by clicking the tick mark below vote buttons of this answer.

Arjun Over a year ago

Why did you write return reject(err) instead of reject(err)? (inside the callback for fs.readFile function)

|

Guy who types fast · Accepted Answer · 2018-09-04 21:32:19Z

1

You can create a promise using new Promise like this:

var myPromise = function(resolve) {
    someAsyncMethod(param1, callback(x){
        resolve (x);
    });
}

This way myPromise is able to .then(). the promise will be completed only after you call resolve()

myPromise.then(function(result){
    // Your code here...
});

answered Sep 4, 2018 at 21:32

Guy who types fast

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