Node js

Controlling the Callback Flow
Jason
NodeJs

Controlling the Callback Flow
● Understanding the boomerang effect
● Using async to control callback flow
Executing in Series
Executing in Parallel
Cascading
Queuing
Iterating
Mapping
Reducing

A demonstration of the boomerang effect.
var fs = require('fs');
function append_some_a_to_b(callback) {
fs.open(__dirname + '/a.txt', 'r', function(err, aFd) {
var buffer = new Buffer(10);
fs.read(aFd, buffer, 0, buffer.length, 0, function(err) {
fs.close(aFd, function(err) {
fs.open(__dirname + '/b.txt', 'a', function(err, bFd) {
fs.fstat(bFd, function(err, bStats) {
fs.write(bFd, buffer, 0, buffer.length, bStats.size,
function(err) {
fs.close(bFd, callback);
});
});
});
});
});
});
}

Using async to control callback flow
● To install async you include it in your package.json manifest or simply install
it
on your application root directory like this:
> npm install async
● The async module comes with a series of helper functions that allow you to
perform asynchronous iteration and flow control.

Executing in Series
series(tasks, [callback])
● Run the functions in the tasks array in series, each one running once the previous function has
completed.
● If any functions in the series pass an error to its callback, no more functions are run, and callback
is immediately called with the value of the error
● tasks - An array or object containing functions to run, each function is passed a callback(err,
result) it must call on completion with an error err (which can be null) and an optional result value
● callback - Callback receives an array of results when tasks have completed.

Executing in Series
var async = require('async');
async.series([
function(callback){
callback(null, 'one');
},
function(callback){
callback(null, 'two');
}
],
function(err, results){
if (err) { throw err; }
console.log('results: %j', results); // results is now equal to ['one', 'two']
});

Executing in Series
async.series({
one: function(callback){
setTimeout(function(){ callback(null, 1); }, 200);
},
two: function(callback){
}
},
function(err, results) {
console.log('results: %j', results); // results is now equal to: {one: 1, two: 2}
});

parallel(tasks, [callback])
● Run the tasks array of functions in parallel, without waiting until the previous function has
completed
● If any functions in the series pass an error to its callback, no more functions are run, and callback
is immediately called with the value of the error
● tasks - An array or object containing functions to run. Each function is passed a callback(err,
result) which it must call on completion with an error err (which can be null) and an optional result
value.
● callback(err, results) - An optional callback to run once all the functions have completed. This
function gets a results array (or object) containing all the result arguments passed to the task

async.parallel([
function(callback){
setTimeout(function(){ callback(null, 'one'); }, 200);
},
function(callback){
setTimeout(function(){ callback(null, 'two'); }, 100);
}
],
});

async.parallel({
one: function(callback){
},
two: function(callback){
}
},
});

Cascading
waterfall(tasks, [callback])
● Runs the tasks array of functions in series, each passing their results to the next in the array.
● If any of the tasks pass an error to their own callback, the next function is not executed, and the
main callback is immediately called with the error.
● tasks - An array of functions to run, each function is passed a callback(err, result1, result2, ...) it
must call on completion. The first argument is an error (which can be null) and any further
arguments will be passed as arguments in order to the next task.
● callback(err, [results]) - An optional callback to run once all the functions have completed. This will
be passed the results of the last task's callback.

Cascading
async.waterfall([
function(callback){
callback(null, 'one', 'two');
},
function(arg1, arg2, callback){
callback(null, 'three');
},
function(arg1, callback){
callback(null, 'done');
}
], function (err, result) {
console.log('result : ', result);
});

Queuing
queue(worker, concurrency)
● If you need to perform some repetitive asynchronous jobs and you want to control the
concurrency
● worker(task, callback) - An asynchronous function for processing a queued task, which must call
its callback(err) argument when finished, with an optional error as an argument.
● concurrency - An integer for determining how many worker functions should be run in parallel.

Queuing
var q = async.queue(function (task, callback) {
console.log('hello ' + task.name);
callback();
}, 2);
q.drain = function() { console.log('all items have been processed'); }
q.push( {name: 'foo'}, function (err) { console.log('finished processing foo'); });
q.push( {name: 'bar'}, function (err) { console.log('finished processing bar'); });
q.push( [{name: 'baz'},{name: 'bay'},{name: 'bax'}] , function (err) { console.log('finished processing item'); });
q.unshift( {name: 'bar'}, function (err) { console.log('finished processing bar'); });

Iterating
forEach(arr, iterator, callback)
● If you use this async.forEach function, you are invoking an iterator for each element in parallel
● iterator(item, callback) - A function to apply to each item in arr.
● callback(results) - A callback which is called when all iterator functions have finished, or an
error occurs.

Iterating
var results = {};
function iterator(value, callback) {
results[value] = Math.pow(value, 2);
callback();
}
async.forEach([1, 2, 3, 4], iterator, function(err){
console.log('results: %j', results);
});
Output :
results: {“1”: 1, “2” : 4, “3” : 9, “4” : 16}

Mapping
map(arr, iterator, callback)
● Produces a new array of values by mapping each value in arr through the iterator function
● iterator(item, callback) - A function to apply to each item in arr. The iterator is passed a
callback(err, transformed) which must be called once it has completed with an error (which can be
null) and a transformed item.
● callback(err, results) - A callback which is called when all iterator functions have finished, or an
error occurs. Results is an array of the transformed items from the arr.

Mapping
function iterator(value, callback) {
callback(null, Math.pow(value, 2));
}
async.map([1, 2, 3, 4], iterator, function(err, results) {
if (err) {
throw err;
}
console.log('results: %j', results);
});
Output : results : [1,4,9,16]

Reducing
reduce(arr, memo, iterator, callback)
● Reduces array into a single value using an async iterator to return each successive step.
● memo - The initial state of the reduction.
● iterator(memo, item, callback) - A function applied to each item in the array to produce the next
step in the reduction. The iterator is passed a callback(err, reduction) which accepts an optional
error as its first argument, and the state of the reduction as the second.
● callback(err, result) - A callback which is called after all the iterator functions have finished. Result
is the reduced value.

Reducing
async.reduce([1,2,3], 0, function(memo, item, callback){
console.log(‘memo:‘ + memo, ‘item:’ + item);
callback(null, memo + item);
}, function(err, result){
console.log('Result : %j', result);
});
Output :
memo : 0 item : 1
memo : 1 item : 2
memo : 3 item : 3
Result : 6

Filtering
filter(arr, iterator, callback)
● Returns a new array of all the values in array which pass an async truth test.
● The callback for each iterator call only accepts a single argument of true or false
● It does not accept an error argument first!

Filtering
function filter(item, callback) {
callback(Math.pow(item, 2) > 10);
}
async.filter([1, 2, 3, 4, 5], filter, function(results)
{
console.log('Square value is greater than 10: %j', results);
});
Output : Square value is greater than 10: [4,5]

Filtering
var fs = require(‘fs’);
async.filter(['file1','file2','file3'], fs.exists, function(results){
// results now equals an array of the existing files
});

Detecting
detect(arr, iterator, callback)
● Returns the first value in array that passes an async truth test.
● The iterator is applied in parallel, meaning the first iterator to return true will fire the detect callback
with that result

Detecting
function filter(item, callback) {
callback(Math.pow(item, 2) > 10);
}
async.detect([1, 2, 3, 4, 5], filter, function(result)
{
console.log('Square value is greater than 10: %j', result);
});
Output : Square value is greater than 10: 4

Detecting
var fs = require(‘fs’);
async.detect(['file1','file2','file3'], fs.exists, function(results){
console.log("exist : ", result);
});

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