NodeJS Synchronous Trouble

Question

Using NodeJS, I'm trying to get info from the database on a list of IDs and populate an array of objects. I need this to process synchronously. I'm having trouble figuring out how to do this so the next function call will wait for the previous one to finish before being called. Then all of the 'each' iterations to finish for the return to be called.

Example:

getInfo();

function getInfo(){
   var return_arr = Array();
   var ids = getListOfIDs();
   // loop through each id getting more info from db
   $.each( ids, function( i, v ){
       return_arr.push(getFullInfo( id ));
   });
   return return_arr;
}

function getListOfIDs(){
  // database call to get IDs
  // returns array
}

function getFullInfo(){
  // database call to get full info
  // returns object
}

This is a simplified example, so assume that a single query to get all info will not work as there are multiple joins going on and post processing that needs to take place in js. Also, assume I'm doing proper error handling, which I omitted in my example.

score 0 · Answer 1 · answered Sep 19 '17 at 18:28

0

Your database queries are asynchronous so you need to either use callbacks or promises to perform your work once the database returns with results.

function getListOfIDs(callback) {
  // database call to get IDs
  db.query('...', function(data) {
    // call the callback and pass it the array
    callback(data);
  });
}

function getInfo() {
  // pass an anonymous function as a callback
  getListofIDs(function(data) {
    // we now have access to the data passed into the callback
    console.log(data);
  });
}

answered Sep 19 '17 at 18:28

Soviut

88,194
49
192
260

Callbacks won't ensure that the calls are processed synchronously. Can you provide a code example with promises? – user6041966 Sep 19 '17 at 18:55
Callbacks ensures calls to be processed synchronously from my understanding. You just have to call the previous ones with later ones. – zhangjinzhou Sep 19 '17 at 19:16
Of course promise code is cleaner without the multiple levels embed. But callback should have not problem solving synchronous issue. – zhangjinzhou Sep 19 '17 at 19:17
@user6041966 Callbacks and promises work the same way; Both are async. Both wait for a long running operation to finish before calling a handler function. Both are guarenteed to fire when that operation completes. Neither is synchronous, nor is your database querying code. – Soviut Sep 20 '17 at 00:18

score -1 · Answer 2 · answered Sep 19 '17 at 19:57

Your current code example is synchronous, although I don't know how your db code works. The each loop is synchronous it just iterates over your ids and calls the getFullInfo function.

I'm not sure why you want synchronous code though as it doesn't utilise Nodes event loop architecture.

What I would do is use a good Promises framework such as Bluebird (http://bluebirdjs.com/docs/api/promise.each.html) with Promise.each ensuring each iteration occurs serially. Or you could also use a Callback library such as async (http://caolan.github.io/async/) with async.eachSeries. Either of these will ensure that you (a) get the benefit of asynchronous and (b) iterate in a serial fashion.

zhangjinzhou · Answer 3 · 2017-09-19T20:39:33.093

-2

Promise way to do it:

function getListOfIDs() {
  return new Promise((resolve, reject) => {
       // database call
       if (err) 
       {
           reject(your_db_err);
           return;
       }
       resolve(your_db_result);
  });
}

function getInfo() {
    getListOfIDs().then((result) => {
        //do whatever you want with result
    })
    .catch((err) => {
        //handle your err
    });
}

edited Sep 19 '17 at 20:39

answered Sep 19 '17 at 19:14

zhangjinzhou

2,461
5
21
46

Why do I get down vote? – zhangjinzhou Sep 19 '17 at 19:22
Honestly I don't think it is worthy of a down vote (and personally hate down votes without comments) but perhaps it was the lack of conditions around the resolve/reject handling? – akaphenom Sep 19 '17 at 20:00
Maybe this is the issue. I will add it. – zhangjinzhou Sep 19 '17 at 20:36
inconsistent code formatting, lack of explaining what was wrong and why this change is needed, etc – Kevin B Sep 19 '17 at 21:43
@KevinB Thanks for your comment. The author only asked for example of asynchronous handling. I glanced Soviut's post and posted this answer. That's why it lacks explanation. I should mention it. – zhangjinzhou Sep 19 '17 at 22:27

NodeJS Synchronous Trouble

3 Answers3