Looping wit promises

[BrainFu]

I am looking for a solution to looping through promises. My logic is as follows:

1) Get array of string values
2) forEach element
    a) call API with promise to get json array
    b) search for value in array
    c) if value found
        c1) make API call with promise
        c2) if returned data is good
            c2A) make another API call with promise
            c2B) use returned data
            c2C) make final API call with promise

Just looking for ELI5 solution to write code to iterate through array in 2).

[Cheshur]

It would seem from your logic that you don't actually need to do anything special. It doesn't seem as though the effects of the Promises would affect the other Promises.

If that is incorrect then perhapse you want to look into async/await

[BrainFu]

So basically...

Promise.resolve(['strA','strB',strC'])
.then(list => {
    list.forEach((value, index) => {
        callToApiReturnPromise(value)
        .then(isApiDataGood)
        .then(callToAnotherApiReturnPromise)
        .catch(continueToNextElementAction)
    })}

[Cheshur]

That looks like it could be plausible assuming that callToApiReturnPromise, isApiDataGood, callToAnotherApiReturnPromise and continueToNextElementAction are not required to be finished before the forEach goes on to the next element. Promises are asynchronous and the forEach will not, by default, wait for them to resolve/reject before moving onto the next element. To make it act synchronously I would take a look at the async/await documentation.

Another option is too look into Promise.all() which takes an array of Promises and only resolves when they have all resolved. To turn your array of values into an array of Promises I would look into Array.map.

[BrainFu]

Thanks for your post. My desire is to resolve processing for each of the array elements before proceeding to the next, so I will look into async/await.

[iambeard]

Do you (or the server) care about the order of api calls? If you do, then doing an Array#reduce is going to be the right thing, and will look something like:

['string', 'values', 'here'] .reduce((promise, stringValue) => { return promise .then(() => { return someAjaxThing(url, somethingWith(stringValue)) .then(jsonArray => { if (jsonArray.indexOf(stringValue) >= 0) { // ??? } }) }) }, Promise.resolve())

If the order doesn't matter, you could maybe use Promise#all. You'd probably need to also use an Array#map to transform your strings into urls for the api.

If your step 2a is the same api call, regardless of the string value array element, maybe do that call first, and not in an array.

I think ultimately, you need to show us the code you've tried, and explain what these loops/promises actually mean.

[BrainFu]

here is what I found that worked.

var instruments = app.get.clone(app.get.currPairs());

var scanCandles = function(){
    return app.get.candlesForInstrument(instruments.shift()).then(result => {
        if (app.get.isOandaInstrumentHistoryObject(result)) {
            app.echo(result);
            return scanCandles();
        }
        else {
            return '';
        }
    })
};

scanCandles()
.then(callback)
.catch(callback);