async for loop in node.js

Question

I am new to this node.js ..I am little bit confused about this callback..In my app inside a for loop i am calling a asynchronous function call,i think my problem is that before i am getting response of async call my for loop get looped.

My code:

async.forEach(Object.keys(config), function(key, next) {
        search(config[key].query, function(err, result) { // 
        console.log("fffffffffff="+ util.inspect(result))-------- >>>Getting undefined..
            if (err) return next(err) // 
            var json = JSON.stringify({
                "result": result
            });
            results[key] = {
                "result": result
            }
            console.log("rrrrrrrr="+util.inspect(results[key]))
            next() // <---- critical piece.  This is how the forEach knows to continue to the next loop.  Must be called inside search's callback so that it doesn't loop prematurely.                   
        })
    },
    function(err) {
        console.log('iterating done');
        
         res.writeHead(200, {
        'content-type': 'application/json'
    });
    res.end(JSON.stringify(results));  
    });
    
       
}

Search function code:

var matches = [];
    var qrySubString = query.substring(0, 4);
    client.query("select * from xxxxxxxxx where level4 ILIKE '%" + query + "%'", function(err, row1, fields) {
        for (var id in row1.rows) {                
            var match, name;                
            if (query == row1.rows[id].level4) {
                match = true;
                name = row1.rows[id].level4;
            }
            else {
                match = false;
                name = query;
            }
            matches.push({
                "id": id,
                "name": row1.rows[id].level4,
                "score": 100,
                "match": match,
                "type": [{
                    "id": "/people/presidents",
                    "name": "US President"
                }]
            })
        }           
        callback(matches);
    })

I want to execute for loop after successful execution of 1 search function, I think I have to use async for loop.

Answer 1

I've reduced your code sample to the following lines to make it easier to understand the explanation of the concept.

var results = [];
var config = JSON.parse(queries);
for (var key in config) {
    var query = config[key].query;
    search(query, function(result) {
        results.push(result);
    });
}
res.writeHead( ... );
res.end(results);

The problem with the previous code is that the search function is asynchronous, so when the loop has ended, none of the callback functions have been called. Consequently, the list of results is empty.

To fix the problem, you have to put the code after the loop in the callback function.

    search(query, function(result) {
        results.push(result);
        // Put res.writeHead( ... ) and res.end(results) here
    });

However, since the callback function is called multiple times (once for every iteration), you need to somehow know that all callbacks have been called. To do that, you need to count the number of callbacks, and check whether the number is equal to the number of asynchronous function calls.

To get a list of all keys, use Object.keys. Then, to iterate through this list, I use .forEach (you can also use for (var i = 0, key = keys[i]; i < keys.length; ++i) { .. }, but that could give problems, see JavaScript closure inside loops – simple practical example).

Here's a complete example:

var results = [];
var config = JSON.parse(queries);
var onComplete = function() {
    res.writeHead( ... );
    res.end(results);
};
var keys = Object.keys(config);
var tasksToGo = keys.length;
if (tasksToGo === 0) {
   onComplete();
} else {
    // There is at least one element, so the callback will be called.
    keys.forEach(function(key) {
        var query = config[key].query;
        search(query, function(result) {
            results.push(result);
            if (--tasksToGo === 0) {
                // No tasks left, good to go
                onComplete();
            }
        });
    });
}

Note: The asynchronous code in the previous example are executed in parallel. If the functions need to be called in a specific order, then you can use recursion to get the desired effect:

var results = [];
var config = JSON.parse(queries);
var keys = Object.keys(config);
(function next(index) {
    if (index === keys.length) { // No items left
        res.writeHead( ... );
        res.end(results);
        return;
    }
    var key = keys[index];
    var query = config[key].query;
    search(query, function(result) {
        results.push(result);
        next(index + 1);
    });
})(0);

What I've shown are the concepts, you could use one of the many (third-party) NodeJS modules in your implementation, such as async.

Answer 2

You've correctly diagnosed your problem, so good job. Once you call into your search code, the for loop just keeps right on going.

I'm a big fan of https://github.com/caolan/async, and it serves me well. Basically with it you'd end up with something like:

var async = require('async')
async.eachSeries(Object.keys(config), function (key, next){ 
  search(config[key].query, function(err, result) { // <----- I added an err here
    if (err) return next(err)  // <---- don't keep going if there was an error

    var json = JSON.stringify({
      "result": result
    });
    results[key] = {
      "result": result
    }
    next()    /* <---- critical piece.  This is how the forEach knows to continue to
                       the next loop.  Must be called inside search's callback so that
                       it doesn't loop prematurely.*/
  })
}, function(err) {
  console.log('iterating done');
}); 

I hope that helps!

Answer 3

I like to use the recursive pattern for this scenario. For example, something like this:

// If config is an array of queries
var config = JSON.parse(queries.queryArray);   

// Array of results
var results;

processQueries(config);

function processQueries(queries) {
    var searchQuery;

    if (queries.length == 0) {
        // All queries complete
        res.writeHead(200, {'content-type': 'application/json'});
        res.end(JSON.stringify({results: results}));
        return;
    }

    searchQuery = queries.pop();

    search(searchQuery, function(result) {
        results.push(JSON.stringify({result: result}); 
        processQueries();            
    });
}

processQueries is a recursive function that will pull a query element out of an array of queries to process. Then the callback function calls processQueries again when the query is complete. The processQueries knows to end when there are no queries left.

It is easiest to do this using arrays, but it could be modified to work with object key/values I imagine.

Answer 4

Node.js introduced async await in 7.6 so this makes Javascript more beautiful.

var results = [];
var config = JSON.parse(queries);
for (var key in config) {
  var query = config[key].query;
  results.push(await search(query));
}
res.writeHead( ... );
res.end(results);

For this to work search fucntion has to return a promise or it has to be async function

If it is not returning a Promise you can help it to return a Promise

function asyncSearch(query) {
  return new Promise((resolve, reject) => {
   search(query,(result)=>{
    resolve(result);
   })
  })
}

Then replace this line await search(query); by await asyncSearch(query);