node.js括号语法

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?

Answer 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.