How to create a loop of promises

Question

so i have a promise that collects data from a server but only collects 50 responses at a time. i have 250 responses to collect.

i could just concate promises together like below

new Promise((resolve, reject) => {
    resolve(getResults.get())
    })
    .then((results) => {
    totalResults.concat(results)
    return getResults.get()
    })
    .then((results) => {
    totalResults.concat(results)
    return getResults.get()
    }).then((results) => {
     totalResults.concat(results)
    return getResults.get()
    })

In this instance i only need 250 results so this seems a managable solution but is there a way of concating promises in a loop. so i run a loop 5 times and each time run the next promise.

Sorry i am new to promises and if this were callbacks this is what i would do.

Answer 1

If you want to loop and serialise the promises, not executing any other get calls once one fails, then try this loop:

async function getAllResults() {  // returns a promise for 250 results
    let totalResults = [];
    try {
        for (let i = 0; i < 5; i++) {
            totalResults.push(...await getResults.get());
        }
    } catch(e) {};
    return totalResults;
}

This uses the EcmaScript2017 async and await syntax. When not available, chain the promises with then:

function getAllResults() {
    let totalResults = [];
    let prom = Promise.resolve([]);
    for (let i = 0; i < 5; i++) {
        prom = prom.then(results => {
            totalResults = totalResults.concat(results);
            return getResults.get();
        });
    }
    return prom.then(results => totalResults.concat(results));
}

Note that you should avoid the promise construction anti-pattern. It is not necessary to use new Promise here.

Also consider adding a .catch() call on the promise returned by the above function, to deal with error conditions.

Finally, be aware that concat does not modify the array you call it on. It returns the concatenated array, so you need to assign that return value. In your code you don't assign the return value, so the call has no effect.

See also JavaScript ES6 promise for loop.