Call function when for loop finished in Node js

Question

function inboxUsers(){
    for (var i=0; i<uniqueArray.length; i++){
        var getUsername    = 'SELECT userName FROM users WHERE userId = ' + uniqueArray[i];
        db.query(getUsername, function(err, results) {
            if (err) {
                console.log('Error in database');
                throw err;
            }
            for(var i in results){
                console.log('single',results[i].userName);
                inboxUserList.push(results[i].userName);
            }
        });
    }
    sample();
}

function sample(){
    console.log('same function');
}

This is my console output.

same function
single user1
single user2
single user3

In this code I called the function sample() after for loop, but it called the sample() function before for loop ends.

I want to call the sample() function when for loop ends. I am a beginner for stackoverflow, if i have error please apologize me. Thank you

Answer 1

Your call to db.query is asynchronous. What that means is this:

1. The call to db.query(...) returns immediately, returning nothing.

2. Instead of assigning a return value to a variable (var results = db.query(...)), you pass in a callback function as an argument so that the db module can call when it's done fetching its results. It will hang onto the callback function until the database has your results, then it will call the function when it's ready.

3. Because the call to db.query(...) returns immediately, your for loop will complete and the call to sample() will fire before the callback functions you supplied to your queries are ever called by the db module.

---

To ensure that sample runs when all of your calls are done, you'll need to track the completion of each query and then fire the sample function when all of the queries have returned. In my opinion, the easiest way to do this without introducing you to complex topics like "promises", is with a module called async and its parallel method.

$ npm install async --save

var async = require('async');
var queries = [];

function inboxUsers(){
  uniqueArray.forEach(function (userId) {
    var getUsername = 'SELECT userName FROM users WHERE userId = ' + userId;
    queries.push(function (done) {
      db.query(getUsername, done);
    });
  });
  async.parallel(queries, function (err, allQueryResults) {
    if (err) { return console.error(err); }
    allQueryResults.forEach(function (queryResults) {
      queryResults.forEach(function (result) {
        console.log('single', result.userName);
        inboxUserList.push(result.userName);
      });
    });
    sample();
  });
}

function sample(){
  console.log('same function');
}

---

Here it is again but with fewer shortcuts taken and detailed comments.

var async = require('async');

// create an array to store a bunch of functions that the async library
// should fire and wait to finish.
var queries = [];

function inboxUsers(){
  uniqueArray.forEach(function (userId) {
    var getUsername = 'SELECT userName FROM users WHERE userId = ' + userId;
    var queryFunc = function (done) {
      db.query(getUsername, function(err, results) {
        // let the async lib know this query has finished.
        // the first argument is expected to be an error.
        // If the err is null or undefined then the async lib
        // will ignore it. The second argument should be our results.
        done(err, results);
      });

      // You could make the above even simpler by just passing
      // the done function as the callback to db.query. I just
      // didn't want to confuse you by doing that.
      // db.query(getUsername, done);
    };
    queries.push(queryFunc);
  });
  // Fire all async functions by passing in our queries array.
  // The async library will wait for them all to call "done()" 
  // before it invokes this final function below.
  async.parallel(queries, function (err, allQueryResults) {
    // If any of our queries pass an error to "done" then the async
    // lib will halt the rest of the queries and immediately invoke
    // this function, passing in the error.
    if (err) { return console.error(err); }

    // queryResults is an array containing the results of each query we made.
    allQueryResults.forEach(function (queryResults) {
      queryResults.forEach(function (result) {
        console.log('single', result.userName);
        inboxUserList.push(result.userName);
      });
    });

    // All your queries are complete and your inboxUserList array
    // is populated with the data you were after. Now we can call
    // "sample".
    sample();
  });
}

function sample(){
  console.log('same function');
}

The async lib knows how many functions you supplied to the array so it knows how many calls to done it should wait for before invoking the final function.

Answer 2

You are most likely experiencing this problem because your db.query() function is not synchronous. It expects a callback function that it can invoke when it's finished.

Your code inboxUserList.push(...) will not be called until your database library queries the database and gets a result. Meanwhile, your for loop will continue running, prepping all of your queries and continuing before they all finish. Then sample() is called, because the for loop is done, even if the callbacks you passed in have not been called yet.

There are many solutions, but perhaps the simplest with your current code would be something like this:

function inboxUsers(){
    var completed = 0;
    for (var i=0; i<uniqueArray.length; i++){
        var getUsername    = 'SELECT userName FROM users WHERE userId = ' + uniqueArray[i];
        db.query(getUsername, function(err, results) {
            if (err) {
                console.log('Error in database');
                throw err;
            }
            for(var i in results){
                console.log('single',results[i].userName);
                inboxUserList.push(results[i].userName);
            }

            completed++;
            if (completed == uniqueArray.length) {
                sample();
            }
        });
    }
}

function sample(){
    console.log('same function');
}