在循环内调用异步函数

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?

Answer 1

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) }); 

Answer 2

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()