Understanding async.auto in node js

The best way for running the functions in tasks is determined as auto(tasks, [callback]) ,which is requirement based for the functions. Each function are dependent on other , and each and every function run when their requirements are fullfilled.

If error is passed in any function to its  callback , it will not complete and if any other functions dependent on the same function then it will not run, and the main callback will immediately called with the error. Functions also get an object containing the results of those functions which have completed till now.

Note, results  Objects are counted as a second argument when all functions are called, so it is unsafe to pass functions in the  tasks  object that may handle the extra argument.

For example, this snippet of code:

async.auto({
  readData: async.apply(fs.readFile, 'data.txt', 'utf-8')
}, callback);

will have the effect of calling  readFile  with the results object as the last argument, which will fail:

fs.readFile('data.txt', 'utf-8', cb, {});

Instead, wrap the call to  readFile  in a function which does not forward the  results  object:

async.auto({
  readData: function(cb, results){
    fs.readFile('data.txt', 'utf-8', cb);
  }
}, callback);

Arguments

• tasks -Tasks is basically An object which is a last item in the array with the function itself with all its properties wheather a function or an array of requirements. That property defined the name of the task which is servered by the object's key, i.e. may be used when specifying requirements for other tasks. The functions receive two arguments: (1) a  callback(err, result)  which must be called when finished, passing an error (which can be  null ) and the result of the function's execution, and (2) a  results  object which is the previously executed functions containing the results .
• callback(err, results) - An optional callback is called when all the tasks completed .If any  tasks  passed an error to their callback It receives the  err  argument . if an error occurs, no further  tasks  will be performed, and the results object will only contain partial results as Results are always returned.

Example

async.auto({
    get_data: function(callback){
        console.log('in get_data');
        // async code to get some data
        callback(null, 'data', 'converted to array');
    },
    make_folder: function(callback){
        console.log('in make_folder');
        // async code to create a directory to store a file in
        // this is run at the same time as getting the data
        callback(null, 'folder');
    },
    write_file: ['get_data', 'make_folder', function(callback, results){
        console.log('in write_file', JSON.stringify(results));
        // once there is some data and the directory exists,
        // write the data to a file in the directory
        callback(null, 'filename');
    }],
    email_link: ['write_file', function(callback, results){
        console.log('in email_link', JSON.stringify(results));
        // once the file is written let's email a link to it...
        // results.write_file contains the filename returned by write_file.
        callback(null, {'file':results.write_file, 'email':'[email protected]'});
    }]
}, function(err, results) {
    console.log('err = ', err);
    console.log('results = ', results);
});

Hope you find it helpful.