Create array of promises

Question

I have an array of coins, which I get from a database query. I pass this array to a function that needs to do an API call for each coin to get the value.

The FOR loop like this below will result in: (

)

// for (let i =0; i <= coins.length-1; i++){
//             console.log(i);
//              await (this.coinRepository.getActiveCoinAsync(i.coinName)
//                 .then(response => {
//                     // console.log((response[0].price_eur))
//                     // console.log((result[i].amount))
//                     // console.log( value +(response[0].price_eur) * (result[i].amount))
//
//                     value = value +((response[0].price_eur) * (i.amount))
//                     console.log(value)
//
//                 })
//                 .catch(error => reject(error))
//             )
//         }

Result:

console.log(i)
console.log(i)
console.log(i)
value
value
value

I tried this below but it doesn't seem to execute any promises:

return new Promise((resolve, reject) => {
let value = 0;
let promises =[];
for (let i =0; i <= coins.length-1; i++){
    let promise =  (this.coinRepository.getActiveCoin(coins[i].coinName))
    promises.push(promise);
    console.log(promise);
}
console.log(promises);
Promise.all(promises)
   .then(results =>{
        console.log(results);
    })
   .catch(error => reject(error))

coinRepository.getActiveCoin

    getActiveCoin(coin) {
    return new Promise((resolve, reject) => {
        console.log(coin)
        request('https://api.coinmarketcap.com/v1/ticker/'+coin+'/?convert=EUR', function (error, response, body) {
            if (!error && response.statusCode == 200) {
                resolve (JSON.parse(body));
            }else {
                reject(error);
            }
        })

    });
}

Looks alright to me. What is the out put of your console.log — AbhinavD
– AbhinavD, Commented May 9, 2018 at 0:11
Bitcoin Promise { <pending> } Ethereum Promise { <pending> } Stellar Promise { <pending> } Monero Promise { <pending> } NEM Promise { <pending> } Vechain Promise { <pending> } Waltonchain Promise { <pending> } [ Promise { <pending> }, Promise { <pending> }, Promise { <pending> }, Promise { <pending> }, Promise { <pending> }, Promise { <pending> }, Promise { <pending> } ] — Achiel Volckaert
– Achiel Volckaert, Commented May 9, 2018 at 0:12
Your code looks fine. Could you edit your answer with the this.coinRepository.getActiveCoin method? My guess is that this method does not resolve. — James
– James, Commented May 9, 2018 at 0:19
Is the if (!error && response.statusCode == 200) { conditional truthy? (Put a console.log(..);within it to check) — James
– James, Commented May 9, 2018 at 0:24

mpen · Accepted Answer · 2018-05-09 00:32:09Z

1

Your code looks fine. Everything will be resolved unless there's a bug in the code you haven't shown us.

In particular, .catch(error => reject(error)) looks fishy because reject isn't defined anywhere.

I've simplified your code below, it runs as expected.

let i = 0;

function getActiveCoin(coin) {
  console.log(`getActiveCoin("${coin}")`);
  return new Promise((resolve, reject) => {
    resolve(`${coin} ${++i}`);
  });
}

let coins = [{
  coinName: "Bitcoin"
}, {
  coinName: "Stellar"
}];
let value = 0;
let promises = [];
for (let i = 0; i <= coins.length - 1; i++) {
  let promise = getActiveCoin(coins[i].coinName)
  promises.push(promise);
}
console.log('Promise.all');
Promise.all(promises)
  .then(results => {
    console.log(results);
  })

edited May 9, 2018 at 0:32

answered May 9, 2018 at 0:27

mpen

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3 Comments

Achiel Volckaert Over a year ago

my bad, the code that executes the promises is a promise itself. wil edit in OP

mpen Over a year ago

@AchielVolckaert Promise.all returns a new Promise already. Just return that instead of wrapping it with another new Promise(...). If any of the sub-promises reject, Promise.all will reject.

Achiel Volckaert Over a year ago

somehow it works with a function, but not from my repo class. code 100% same, except that getActiveCoins is in another class

Marc Trudel · Accepted Answer · 2018-05-09 00:26:25Z

0

My guess is that when you call reject reject does not exist, thus throwing an error that never gets called. Perhaps you meant:

let value = 0;
let promises = [];
for (let i =0; i <= coins.length-1; i++){
  let promise = this.coinRepository.getActiveCoin(coins[i].coinName)
  promises.push(promise);
}

Promise.all(promises)
    .then(results => console.log(results))
    .catch(error => console.error(error))

Edit 1: you do not explicitly reject on error in getActiveCoin(coin): this means that if you get an error, your promise will never resolve.

 return new Promise((resolve, reject) => {
    request('https://api.coinmarketcap.com/v1/ticker/'+coin+'/?convert=EUR', function (error, response, body) {
        if (error || response.statusCode != 200) {
            return reject()
        }

        resolve (JSON.parse(body));
    })
  })

edited May 9, 2018 at 0:26

answered May 9, 2018 at 0:12

Marc Trudel

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6 Comments

Achiel Volckaert Over a year ago

this is a step in the right direction, but it still does not execute the array of promises

mpen Over a year ago

@AchielVolckaert Promises aren't "executed". Does getActiveCoin really return a Promise or does it return a function that returns a Promise?

Achiel Volckaert Over a year ago

@mpen added it to OP

Marc Trudel Over a year ago

Edited the answer. Added another clue as to why your code is probably stalling the way it is stalling now.

Achiel Volckaert Over a year ago

added it, should be there in the first place, but not the issue. the console.log doesn't even execute

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