Result of a promise inside of forEach

Question

I'm trying to rewrite the code of one my project with the promise (Q). I don't know how to take the result of a chain of promises inside a foreach loop:

var resProducts = [];
products.forEach(currProduct) {
     Stock.saveNewStock(currProduct)
        .then(function(res1) {
            Product.saveNewProduct(currProduct)
               .then(function(res2) {
                  resProducts.push(res2);
                  Product.addStockToProduct(res1,res2)
               })
         })
         .catch(function(err) {
             console.log(err)
         })
});
console.log(resProducts);

I see that every time the log stamps an empty array, so how I can resolve it?

As a side note: javascript is synchronous and your function call to print the array value is outside the async block which makes it run almost instantly. Could be worth to reflect upon! :) — Daniel B, May 20 '16 at 15:48

score 7 · Answer 1 · edited May 23 '17 at 11:45

You should always return all promises, and you're forgetting to in several places here.

If you can use => arrow functions, then these implicitly return, which avoids forgetting.

Instead of forEach which ignores return values, use map and return results into an array. This does the work in parallel as well:

Promise.all(products.map(product => Stock.saveNewStock(product)
  .then(res1 => Product.saveNewProduct(product)
    .then(res2 => Product.addStockToProduct(res1, res2)
      .then(() => res2))))
.then(resProducts => console.log(resProducts))
.catch(err => console.error(err));

The nesting here is intentional, to access res1 and res2 together.

score 1 · Answer 2 · answered May 20 '16 at 08:34

How about this?

// Define iterator function
function doThingsWithProducts(productsLeft, resProducts) {
    if (productsLeft.length === 0) { return resProducts }
    else {
        var currProduct = productsLeft.shift(), res1, res2;
        return Stock.saveNewStock(currProduct)
        .then(function(res) {
            res1 = res;
            return Product.saveNewProduct(currProduct)
        })
        .then(function(res) {
            res2 = res;
            resProducts.push(res2);
            return Product.addStockToProduct(res1, res2)
        })
        .then(function() {
            return doThingsWithProducts(productsLeft, resProducts)
        })
        .catch(function(err) {
            console.log(err);
        })
    }
}

// resProducts is available in the result of the function
doThingsWithProducts(products, [])
.then(function(resProducts) {
    console.log(resProducts);
})

score 0 · Answer 3 · answered May 20 '16 at 08:28

Something like this is that you need?

var defer = $q.defer();
            var promises = [];
            var resProducts = [];
            var errorCallback = function (e) {

              defer.reject();
            };

           products.forEach(currProduct) {

                promises.push(function () {
                    Stock.saveNewStock(currProduct)
                        .then(function(res1) {
                            return Product.saveNewProduct(currProduct)
                        .then(function(res2) {
                            resProducts.push(res2);
                            Product.addStockToProduct(res1,res2)
                   })
             })
             .catch(function(err) {
                 console.log(err)
             })
                }


                );
              }

            });
            return $q.all(promises);

score 0 · Answer 4 · answered May 20 '16 at 09:34

Use Array.map() in combination with Q.all()

var resProducts = [];

Q.all(products.map(function (currProduct) {
        // Return promise that Q.all() can resolve.
        return Stock.saveNewStock(currProduct)
            .then(function (res1) {
                // Make sure you always return promises when they
                // are in a chain so  the chain will work correctly.
                return Product.saveNewProduct(currProduct)
                    .then(function (res2) {
                        resProducts.push(res2);
                        return Product.addStockToProduct(res1, res2)
                    })
            })
    }))
    .then(function (products) {
        // Products here is the array of resolved promises.

        // and here your array will be full.
        console.log(resProducts);
    })
    .catch(function (err) {
        console.log(err)
    });

Result of a promise inside of forEach

4 Answers4