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!

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. — Handonam
– Handonam, Commented Mar 23, 2016 at 2:30

Donghua Li · Accepted Answer · 2016-03-23 02:36:48Z

3

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
    });
});

edited Mar 23, 2016 at 2:36

answered Mar 23, 2016 at 2:32

Donghua Li

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2 Comments

Trung Tran Over a year ago

Thanks - to confirm - callback() will tell it to look at the next supplier, and apply all those processes to the next supplier?

Donghua Li Over a year ago

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

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