node.js bracket syntax

Question

I am trying to understand some code used to make a Node.js dispatcher but I can't understand one line. Maybe my JavaScript gap . . . I commented the code with my doubts.

var HttpDispatcher = function() {
  this.listeners = { get: [ ], post: [ ] };
}

HttpDispatcher.prototype.on = function(method, url, cb) {
  this.listeners[method].push({
    cb: cb,
    url: url
  });
}

HttpDispatcher.prototype.onGet = function(url, cb) {
  this.on('get', url, cb);
}

HttpDispatcher.prototype.onPost = function(url, cb) {
  this.on('post', url, cb);
}

HttpDispatcher.prototype.dispatch = function(req, res) {
  var parsedUrl = require('url').parse(req.url, true);
  var method = req.method.toLowerCase();
  this.listener[method][parsedUrl.pathname](req, res); // i don't understand this line
}

Why do we refer to this.listener as a bidimensional array? We defined listeners like an array of object! and why do we pass parameters?

Robbie · Accepted Answer · 2015-08-18 10:26:15Z

1

It's not a bidimensional array, its bracket notation being used to access a nested property of an object.

this.listener[method][parsedUrl.pathname](req, res)
|-------------------||------------------||--------|
 ^object property of  ^nested function    ^ invocation of the function 
  the listener object  of the listener
  where the property   object where the
  key is the method    property key is
                       the path name

Properties of nested objects can be accessed by chaining dot and/or bracket references together. The following are all equivalent:

object.baz.foo.bar;
object["baz"]["foo"]["bar"];
object["baz"].foo["bar"];

Check this for more details.

edited Aug 18, 2015 at 10:26

answered Aug 17, 2015 at 23:36

Robbie

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5 Comments

Stefano Maglione Over a year ago

I can understand that with this.listeners['get'] we can access get array but in get array we have two properties

Robbie Over a year ago

yes that's right, properties of nested objects can be accessed by chaining brackets together - if that's what you mean?

Stefano Maglione Over a year ago

So here this.listener[method][parsedUrl.pathname](req, res); we access only to one property?

Robbie Over a year ago

i've updated my explanation to make it clearer - it only accesses one function, so it can call it, but it uses 2 bracket references to reach it because its nested within the listener object

Stefano Maglione Over a year ago

are we accessing to "cb" property or "url" property, inside listeners object?

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