Calling async function inside a loop

Question

I have the following function:

function ipfsRetrieve( ipfsHash ){
  return new Promise( function( resolve, reject ) {
    ipfs.catJSON( ipfsHash, (err, result) => {
        if (err){                
            reject(err);
        }
        resolve( result);           
    });
});

}

Now, when I call this function inside a loop as below:

var hashArray   = [ "QmTgsbm...nqswTvS7Db",
                "QmR6Eum...uZuUckegjt",
                "QmdG1F8...znnuNJDAsd6",                    
              ]

var dataArray   = []; 
hashArry.forEach(function(hash){
    ipfsRetrieve( hash ).then(function(data){
        dataArray.push(data);
    });
});  

return dataArray

The "return dataArray' line returns an empty array. How should I change this code to have the "dataArray" filled with the data retrived from IPFS?

Possible duplicate of How do I return the response from an asynchronous call? — ASDFGerte
– ASDFGerte, Commented Aug 7, 2018 at 16:33
The question is about dealing with multiple async requests in a loop. — Steven Spungin
– Steven Spungin, Commented Aug 7, 2018 at 16:38

Luca Kiebel · Accepted Answer · 2022-09-26 07:25:53Z

4

You should use Promise.all.

Construct an Array of Promises and then use the method to wait for all promises to fulfill, after that you can use the array in the correct order:

let hashArray = ["QmTgsbm...nqswTvS7Db",
  "QmR6Eum...uZuUckegjt",
  "QmdG1F8...znnuNJDAsd6",
]

// construct Array of promises
let hashes = hashArray.map(hash => ipfsRetrieve(hash));

Promise.all(hashes).then(dataArray => {
  // work with the data
  console.log(dataArray) 
});

edited Sep 26, 2022 at 7:25

answered Aug 7, 2018 at 16:33

Luca Kiebel

10.1k7 gold badges34 silver badges47 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Steven Spungin · Accepted Answer · 2018-08-07 19:15:18Z

For starters, you need to return after rejecting, or else your resolve will get called too.

function ipfsRetrieve( ipfsHash ){
  return new Promise( function( resolve, reject ) {
    ipfs.catJSON( ipfsHash, (err, result) => {
        if (err){                
           reject(err);
           return;
        }
        resolve( result);           
    });
});

Now for the loop, use map instead of forEach, and return the promise. Then wait on the promises.

let promises = hashArry.map(hash=> return new Promise(resolve,reject) { // your code here handling hash, updating theData, and then resolving })
return Promise.all(promises).then( ()=> return theData)

In your case, the promise is provided by ipfsRetrieve, so you would call

let promises = hashArry.map(ipfsRetrieve)
return Promise.all(promises)

The caller of your functions will do this:

ipfsRetrieve().then(data=>{ // process data here } )

If you are cool with async await, do this. (marking containing function as async)

let data = await ipfsRetrieve()

"code here updating theData and then resolving" - no no no. Call ipfsRetrieve and use then, don't use new Promise
I think it's pretty clear what the OP wanted to achieve with ipfsRetrieve( hash ).then(function(data){ dataArray.push(data); }). When using Promise.all properly, you don't need any "data updating" at all.
Someone is going to want the data when the promises all resolve.
Yes that's true, Promise.all returns an array of the resolved values.

Collectives™ on Stack Overflow

Calling async function inside a loop

2 Answers 2

Comments

7 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

7 Comments

Linked

Related