Node.js async array iteration

Question

I am trying to synchronously iterate over an array using async.each:

async.each(supplier_array, function(supplier) {
 console.log('looking at : ' + supplier);
 knex(host_name + '.order_item').where({
 supplier: supplier,
 order_id: order_id
 }).then(function(data) {
 console.log(data);
 knex(host_name + '.distributor').select()
 .then(function(data) {
 console.log(data);
 }).catch(function(error) {
 console.log('error: ' + error);
 });
 }).catch(function(error) {
 console.log('error: ' + error);
 });
});

My supplier_array has 3 elements. So what SHOULD happen is the app should (synchronously):

For supplier 1 / first arr / first array element:

• console.log(supplier)
• console.log(order_item)
• console.log(distributor)

For supplier 2 / second array element:

• console.log(supplier)
• console.log(order_item)
• console.log(distributor)

For supplier 3 / third array element:

• console.log(supplier)
• console.log(order_item)
• console.log(distributor)

HOWEVER, it acts asynchronously:

• console.log(supplier)
• console.log(supplier)

• console.log(supplier)

• console.log(order_item)

• console.log(order_item)

• console.log(order_item)

• console.log(distributor)

• console.log(distributor)
• console.log(distributor)

Can someone help me achieve the desired effect of running through the steps inside of async synchronously?

Thanks in advance!

• Handonam: You might have to do a "return" on each knex promise. That acknowledges the async to let it know that there's a promise to process. Try it, and let me know if it's the right answer. I ran into this issue with knex myself. Also, you can move the inside "then" on line 9 out a level, since you're returning a promise.

Answer 1

You should use async.eachSeries if you want to iterate them in serial order. Try something like this:

async.eachSeries(supplier_array, function(supplier, callback) {
 console.log('looking at : ' + supplier);
 knex(host_name + '.order_item').where({
 supplier: supplier,
 order_id: order_id
 }).then(function(data) {
 console.log(data);
 knex(host_name + '.distributor').select()
 .then(function(data) {
 console.log(data);
 callback(); // Advance to next iteration here
 }).catch(function(error) {
 console.log('error: ' + error);
 callback(error); // Also callback when error occurs
 });
 }).catch(function(error) {
 console.log('error: ' + error);
 callback(error); // Also callback when error occurs
 });
});

• Trung Tran: Thanks - to confirm - callback() will tell it to look at the next supplier, and apply all those processes to the next supplier?
• Donghua Li: It seems to work like that. Besides, you should also take care about a common pitfall documented in the README of async.js: github.com/caolan/async#synchronous-iteration-functions