Using Async waterfall in node.js

Question

I have 2 functions that I\'m running asynchronously. I\'d like to write them using waterfall model. The thing is, I don\'t know how..

Here is my code :

Answer 1

Basically nodejs (and more generally javascript) functions that require some time to execute (be it for I/O or cpu processing) are typically asynchronous, so the event loop (to make it simple is a loop that continuously checks for tasks to be executed) can invoke the function right below the first one, without getting blocked for a response. If you are familiar with other languages like C or Java, you can think an asynchronous function as a function that runs on another thread (it's not necessarily true in javascript, but the programmer shouldn't care about it) and when the execution terminates this thread notifies the main one (the event loop one) that the job is done and it has the results.

As said once the first function has ended its job it must be able to notify that its job is finished and it does so invoking the callback function you pass to it. to make an example:

var callback = function(data,err)
{
   if(!err)
   {
     do something with the received data
   }
   else
     something went wrong
}


asyncFunction1(someparams, callback);

asyncFunction2(someotherparams);

the execution flow would call: asyncFunction1, asyncFunction2 and every function below until asyncFunction1 ends, then the callback function which is passed as the last parameter to asyncFunction1 is called to do something with data if no errors occurred.

So, to make 2 or more asynchronous functions execute one after another only when they ended you have to call them inside their callback functions:

function asyncTask1(data, function(result1, err)
{
   if(!err)
     asyncTask2(data, function(result2, err2)
     {
           if(!err2)
        //call maybe a third async function
           else
             console.log(err2);
     });
    else
     console.log(err);
});

result1 is the return value from asyncTask1 and result2 is the return value for asyncTask2. You can this way nest how many asynchronous functions you want.

In your case if you want another function to be called after updateJson() you must call it after this line:

console.log("File successfully written");