How do I avoid deeply nested code in node.js?

Question

In node.js, it being event-driven, all I/O is done via callbacks. So I end up writing code that looks like this:

app.get('/test', function (req, res) {
  http.get('some/place', function (req1, res1) {
    if (res1.statusCode == 200) {
      res1.on('data', function (data) {
        http.get('other/place?q=' + data, function (req2, res2) {
          if (res2.statusCode == 200) {
            res2.on('data', function (data) {
              db.query(data).on('data', function (rows) {
                res.writeHead(200)
                res.end(JSON.stringify(rows))
              })
            })
          }
        })
      })
    }
  })
})

And that doesn't even include error handling.

What can I do to unwind this mess?

Possible duplicate: http://stackoverflow.com/questions/4234619/how-to-avoid-long-nesting-of-asynchronous-functions-in-node-js — Anderson Green, Sep 23 '12 at 04:59

mikerobi · Answer 1 · 2012-05-05T16:28:25.230

6

Don't use anonymous functions.

EDIT

Your code isn't even valid. You aren't closing most of your function calls.

If you switched to named functions it would look something like this: Updated to reflected comment about global namespace

(function () {
    app.get('/test', f0)

    function f0(req, res) {
      http.get('some/place', f1)
    }

    function f1(req1, res1) {
        if (res1.statusCode == 200) {
          res1.on('data', f2)
        }
     }
    function f2(data) {
        http.get('other/place?q=' + data, f3)
    }
    function f3(req2, res2) {
      if (res2.statusCode == 200) {
        res2.on('data', f4)
      }
    }

    function f4(data) {
          db.query(data).on('data', f5)
        }

    function f5(rows) {
        res.writeHead(200)
        res.end(JSON.stringify(rows))
    }
})()

edited May 05 '12 at 16:28

answered Feb 04 '11 at 21:38

mikerobi

20,527
5
46
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ah, that was why it wasn't syntax highlighting properly. Fixed. – nornagon Feb 04 '11 at 21:47
@nornagon, that doesn't change my answer. – mikerobi Feb 04 '11 at 21:49
1

@nornagon, there is a very simple solution for that, I have updated my example. – mikerobi Feb 04 '11 at 21:53
1

This code is invalid. Ironic. You cannot immediately invoke an unnamed function declaration. – tbranyen May 04 '11 at 22:36

score 6 · Answer 2 · answered Feb 05 '11 at 02:30

6

You could use async module to avoid this.

answered Feb 05 '11 at 02:30

Alfred

60,935
33
147
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nornagon · Accepted Answer · 2011-02-13T02:12:16.460

I wrote a library based on node-seq, which looks like this:

app.get('/test', function (req, res) {
  Seq()
    .seq(function () {
      http.get('some/place', this.next)
    })
    .seq(function (req1, res1) {
      if (res1.statusCode == 200) {
        res1.on('data', this.next)
      }
    })
    .seq(function (data) {
      http.get('other/place?q=' + data, this.next)
    })
    .seq(function (req2, res2) {
      if (res2.statusCode == 200) {
        res2.on('data', this.next)
      }
    })
    .seq(function (data) {
      db.query(data).on('data', this.next)
    })
    .seq(function (rows) {
      res.writeHead(200)
      res.end(JSON.stringify(rows))
    })
})

The code is here.

Also, there's a lengthy discussion on the nodejs mailing list about this issue.

Step is another library for doing this stuff.

score 1 · Answer 4 · answered Feb 04 '13 at 22:23

Another (and perhaps better) way to clean up this kind of thing is to use EventEmitters rather than callbacks. Here's an example showing several EventEmitters in use:

    var events = require('events'),
    util = require('util');

    var Search = function () {
    "use strict";

    var search,
        requestPageData,
        makeReturnObject,
        sendResults,

        emptyObj = {};

    events.EventEmitter.call(this);

    search = function (query) {
        this.emit("requestPageData", query);
    };

    requestPageData = function (query) {
        var resultsArray = [];

        // some logic

        if (.....some condition....) {
            this.emit("makeReturnObject", resultsArray);
        } else {
            this.emit("sendResults", emptyObj);
        }
    };

    makeReturnObject = function (resultsArray) {
        var resultsObj = {};

        if (magnetArray) {

            // some logic

            this.emit("sendResults", resultsObj);
        } else {
            this.emit("sendResults", emptyObj);
        }
    };

    sendResults = function (obj) {
        // finally, do whatever it is you want to do with obj
    };

    this.on("requestPageData", requestPageData);

    this.on("makeReturnObject", makeReturnObject);

    this.on("sendResults", sendResults);

    this.search = search;

};

util.inherits(Search, events.EventEmitter);
module.exports = new Search();

score 1 · Answer 5 · answered Jul 19 '11 at 20:38

1

Please have a look at Streamline; it is a JavaScript preprocessor that allows you to write simple 'streamlined' code and transforms it to callback-heavy code.

answered Jul 19 '11 at 20:38

George

11
1

score 0 · Answer 6 · answered Mar 19 '14 at 22:13

0

You can use promises. Check this out https://github.com/kriskowal/q

answered Mar 19 '14 at 22:13

phani

1,134
1
11
24

How do I avoid deeply nested code in node.js?

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