Question

I have the following CoffeeScript code example:

class TestClass
    constructor: () ->
        @list = new Object()

    addToList: (key, value) ->
        @list[key] = value

    printList: () ->
        console.log("This is printed from printList:", @list)

    startHttp: () ->
        http = require("http")
        http.createServer(@printList).listen(8080)

test = new TestClass()
test.addToList("key", "value")
test.printList()
test.startHttp()

When I run the code, and make a HTTP request to 127.0.0.1:8080, I expect to get the following output:

This is printed from printList: { key: ‘value’ }
This is printed from printList: { key: ‘value’ }

But I get the following instead:

This is printed from printList: { key: ‘value’ }
This is printed from printList: undefined

Why is it the printList function can’t access the list variable when it is called from the HTTP server?

I am using Node.js v0.6.1 and CoffeeScript v1.1.3.

Answer 1

printList: () =>
    console.log("This is printed from printList:", @list)

Use => to bind the value of this to the function so it “works” as you expect.

Disclaimer: instances might break. Coffeescript is black magic for all I care.

What you really want to do is invoke the method on the correct object

that = this
http.createServer(->
  that.printList()
).listen 8080

Or in plain javascript.

var that = this;
http.createServer(function () {
  that.printList();
}).listen(8080);