How to avoid long nesting of asynchronous functions in Node.js

Question

I want to make a page that displays some data from a DB, so I have created some functions that get that data from my DB. I'm just a newbie in Node.js, so as far as I understand, if I want to use all of them in a single page (HTTP response) I'd have to nest them all:

http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/html'});
  var html = "<h1>Demo page</h1>";
  getSomeDate(client, function(someData) {
    html += "<p>"+ someData +"</p>";
    getSomeOtherDate(client, function(someOtherData) {
      html += "<p>"+ someOtherData +"</p>";
      getMoreData(client, function(moreData) {
        html += "<p>"+ moreData +"</p>";
        res.write(html);
        res.end();
      });
    });
  });

If there are many functions like that, then the nesting becomes a problem.

Is there a way to avoid this? I guess it has to do with how you combine multiple asynchronous functions, which seems to be something fundamental.

Answer 1

Interesting observation. Note that in JavaScript you can normally replace inline anonymous callback functions with named function variables.

The following:

http.createServer(function (req, res) {
   // inline callback function ...

   getSomeData(client, function (someData) {
      // another inline callback function ...

      getMoreData(client, function(moreData) {
         // one more inline callback function ...
      });
   });

   // etc ...
});

Could be rewritten to look something like this:

var moreDataParser = function (moreData) {
   // date parsing logic
};

var someDataParser = function (someData) {
   // some data parsing logic

   getMoreData(client, moreDataParser);
};

var createServerCallback = function (req, res) {
   // create server logic

   getSomeData(client, someDataParser);

   // etc ...
};

http.createServer(createServerCallback);

However unless you plan to reuse to callback logic in other places, it is often much easier to read inline anonymous functions, as in your example. It will also spare you from having to find a name for all the callbacks.

In addition note that as @pst noted in a comment below, if you are accessing closure variables within the inner functions, the above would not be a straightforward translation. In such cases, using inline anonymous functions is even more preferable.