Hi I'm new to Js and Coffeescript, here's a situation I feel difficult to refer the property of parent object which is App in the following example
App =
init: ->
this.foo = 'bar'
this.bindEvent()
bindEvent: ->
$('#test').click(this.show)
show: ->
alert this.foo
App.init()
I think the fat arrow may do the trick but once I've changed to show: =>, this in the context of show method refers to the window object, instead of App object that I want to
. Anybody can tell me how to do it right?
http://jsfiddle.net/kZpHX/
When you define your show function, # (AKA this) actually is window so
show: => console.log(#)
will bind show to window. The problem is that you're simply defining an object so there isn't anything to bind to: you aren't defining a class so this is window. You could refer to App explicitly like this:
App =
#...
show: -> alert(App.foo)
Demo: http://jsfiddle.net/ambiguous/3sRVh/
The this.foo in init will do the right thing because saying App.init() sets up the expected this.
You could also hook up the desired this manually:
bindEvent: ->
$('#test').click => #show()
# or
bindEvent: ->
_this = # # JavaScript style
$('#test').click -> _this.show()
Demos: http://jsfiddle.net/ambiguous/byL45/, http://jsfiddle.net/ambiguous/MT8fG/
Or you could create a class for your App instead:
class App
constructor: ->
#foo = 'bar'
#bindEvent()
bindEvent: ->
$('#test').click(#show)
show: =>
console.log(#foo)
new App
That way your show: => will behave the way you expect it to.
Demo: http://jsfiddle.net/ambiguous/byatH/
Related
While learning coffeescript, I'm trying to create an instance of class A inside a method of class B, this is the code:
class #A
constructor: (#x) ->
show: ->
alert #x
class #B
constructor: (#y) ->
show: ->
a = new #A("eric")
alert a.x
alert #y
b = new #B("daniel")
b.show()
the error is TypeError: undefined is not a function.
Any help is appreciated.
Thanks
You have two problems:
# is just another way of saying this in CoffeeScript. That's all it means.
Classes are (more or less) just variables or properties like any other in CoffeeScript.
So when you say #A, you're just looking for the A property of this and your show is really saying:
a = new this.A("eric")
In that context, # will be an instance of B and Bs don't have A properties. Instead you should just say:
a = new A('eric')
Using # when defining a class:
class #A
#...
is just a way to make a class globally available. At the top level, # will (almost always) be window in a browser so you're really saying:
class window.A
#...
and window properties are globals. Keep in mind that each CoffeeScript file is wrapped in a function when it is converted to JavaScript:
Although suppressed within this documentation for clarity, all CoffeeScript output is wrapped in an anonymous function: (function(){ ... })(); This safety wrapper, combined with the automatic generation of the var keyword, make it exceedingly difficult to pollute the global namespace by accident.
So if you just said:
class A
then A would only be available to other code in that file. Saying:
class #A
makes A global.
If you're only working with one file then you don't need the #s on your classes:
class A
constructor: (#x) ->
show: ->
alert #x
class B
constructor: (#y) ->
show: ->
a = new A("eric")
alert a.x
alert #y
b = new B("daniel")
b.show()
Don't get in the habit of prefixing everything with #, only use it on classes when you need it and you know exactly what it will do. Even when you need it, there are better ways: use require.js to manage your dependencies, use an global application-specific object to manage scopes, ...
I have a "class" in coffee script whose instance variables I want to initialize with instance methods that return a value via a callback, but it doesn't work as I had hoped:
EventEmitter = require('events').EventEmitter
class MyClass extends EventEmitter
constructor: ->
#datamember: setDatamember()
setDatamember: ->
someFunction (response) ->
#datamember = response
#emit 'init'
getDatamember: ->
return #datamember
----
myObj = new MyClass
myObj.on 'init', ->
console.log myObj.getDatamember
The result I get suggests that "this" in setDatamember is referring to something different from what "this" refers to in the object instance. If I explicitly call myObj.setDatamember, I get the expected result, but is there any way to call on a set method -- specifically one that sets the data member via a callback -- in the constructor? I've looked through the docs, as well as various other sources of coffeescript info (e.g. this one), and I haven't found anything that touches upon this.
Try using a fat arrow for the anonymous function:
setDatamember: ->
someFunction (response) =>
#datamember = response
#emit 'init'
Also, you'll need to call the correct function in the constructor:
constructor: ->
#setDatamember()
In general, avoid fat arrows on methods - the way Coffee-Script implements this does some bad things to memory usage. Also, it will rarely be necessary.
However, anonymous functions that refer to this will almost always need fat arrows. this is not held in closure like normal variables, and will only be set by binding (Function.prototype.bind) or by calling it as an object method (obj.myMethod() will set this to obj in myMethod).
Try using fat arrows on everything except the constructor:
class MyClass
constructor: ->
#setDatamember()
setDatamember: =>
someFunction (response) =>
#datamember = response
getDatamember: =>
return #datamember
However, you also look to have someFunction in there as an asynchronous function, so you'll never be able to just do
mc = new MyClass
console.log mc.datamember
Because that doesn't wait for someFunction to return before accessing mc.datamember.
I found this example of mixins in coffeescript faq but it seems like it doesn't work.
Am I missing something here?
extend = (obj, mixin) ->
for name, method of mixin
obj[name] = method
include = (klass, mixin) ->
extend klass.prototype, mixin
class Button
onClick: -> alert "click"
class Events
include Button, Events
(new Events).onClick()
# => Uncaught TypeError: Object #<Events> has no method 'onClick'
fiddle
You are missing the fact that onClick is defined on the prototype of Button,
and that you did not set the arguments with the right order in the include function
extend = (obj, mixin) ->
for name, method of mixin
obj[name] = method
include = (klass, mixin) ->
extend klass.prototype, mixin
class Button
onClick: -> alert "click"
class Events
include Events,Button.prototype
(new Events).onClick()
see the "fiddle"
So the mixin snippet works pretty well.
If I have this javascript:
function I_did_something(){
this.test.assertExists('#selector', 'exists');
}
casper.then(I_did_something);
The problem is that casper is using call to call the then method meaning that I cannot do something like this:
#I_did_something = ->
#assertExists('#selector', 'exists')
casper.then #I_did_something
Because this does not refer to the global object.
Can anyone suggest how I would translate this into coffeescript without using the window object preferably?
You can use a fat arrow (=>) to bind the function to the current this:
#I_did_something = =>
#assertExists('#selector', 'exists')
That has a similar effect to:
that = #
#I_did_something = ->
that.assertExists('#selector', 'exists')
and I think that's what you're after.
I come from a C#/Java background which use a class based OO system and I don't get the JavaScript/CoffeeScript prototype OO system yet. I've written a CoffeeScript class below which allows me to display names for contacts according to a system-side preference. I can only get the class to work by making the joinNonEmpty(stringList, joinText) method belong to the prototype and calling it the way I would call a static method in Java/C# land.
Is there a way I can make this method call using this.joinNonEmpty(...)?
Can you shed some light on why I can reference the firstLastRender, lastFirstRender and firstOrNickThenLast methods in the constructor with this. but it doesn't work from those methods when calling the joinNonEmpty helper?
Does this have something to do with how I'm locating the appropriate method via the preference map?
prefs = displayNameFormat: "FirstOrNickThenLast"
class DisplayNameRenderer
constructor: ->
#prefToRenderMap =
FirstLast: this.firstLastRender
LastFirst: this.lastFirstRender
FirstOrNickThenLast: this.firstOrNickThenLast
# Why does this method have to be static (a class method)?
#joinNonEmpty: (stringList, joinText) ->
nonEmptyStrings = []
for s in stringList
nonEmptyStrings.push(s) if s isnt null and s isnt ""
nonEmptyStrings.join(joinText)
firstLastRender: (contact) ->
# TypeError: Object expected.
joinNonEmpty([contact.firstName, contact.lastName], ' ')
lastFirstRender: (contact) ->
# TypeError: Object doesn't support this method or property
this.joinNonEmpty([contact.lastName, contact.firstName], ', ')
firstOrNickThenLast: (contact) ->
# Works correctly.
DisplayNameRenderer.joinNonEmpty([(if contact.nickname isnt null and contact.nickname isnt "" then contact.nickname else contact.firstName), contact.lastName], ' ')
render: (contact) ->
#prefToRenderMap[prefs.displayNameFormat](contact)
contact = firstName: "Jonathan", nickname: "Jonny", lastName: "Appleseed"
dnr = new DisplayNameRenderer()
# => "Jonny Appleseed"
console.log dnr.render(contact)
Thanks for taking the time to answer.
this (AKA #) is determined when the function is called (with exceptions as below). So when you do this:
#prefToRenderMap =
FirstLast: this.firstLastRender
LastFirst: this.lastFirstRender
FirstOrNickThenLast: this.firstOrNickThenLast
You're storing unbound references to the three functions in the #prefToRenderMap instance variable and #prefToRenderMap is itself an object. Then you try to call the methods in your DisplayNameRenderer instance like this:
#prefToRenderMap[prefs.displayNameFormat](contact)
and everything falls apart because the methods are called in the wrong context and # isn't what they're expecting it to be. If prefs is 'FirstOrNickThenLast' then you're effectively doing this:
#prefToRenderMap.FirstOrNickThenLast(contact)
and # (AKA this) will be #prefToRenderMap inside the firstOrNickThenLast method. But, of course, #prefToRenderMap doesn't have any of the methods that you're trying to call so you get various errors.
One solution is to use the fat arrow (=>) to define the methods:
The fat arrow => can be used to both define a function, and to bind it to the current value of this, right on the spot.
So you could have things like this:
joinNonEmpty: (stringList, joinText) ->
#...
firstLastRender: (contact) =>
#joinNonEmpty([contact.firstName, contact.lastName], ' ')
and everything will work out. Here's a stripped down demo that will also show you your this problem:
http://jsfiddle.net/ambiguous/RAPJw/1/
You could also avoid this problem by referring to the methods by their names. Given a method name in a string, m = 'some_method', you can call that method like this o[m]() in both JavaScript and CoffeeScript and the result will be the same as if you said o.some_method(). A better structure would look more like this:
class DisplayNameRenderer
constructor: ->
#prefToRenderMap =
FirstOrNickThenLast: 'firstOrNickThenLast'
joinNonEmpty: (stringList, joinText) ->
#...
firstOrNickThenLast: (contact) ->
#joinNonEmpty([(if contact.nickname isnt null and contact.nickname isnt "" then contact.nickname else contact.firstName), contact.lastName], ' ')
render: (contact) ->
#[#prefToRenderMap['FirstOrNickThenLast']](contact)
Note the change to the structure of #prefToRenderMap and how it is used in render. And a demo of this approach: http://jsfiddle.net/ambiguous/DFYwu/
As an aside, instead of saying ClassName.class_method() inside an instance method, you can use the constructor property instead: #constructor.class_method(). Also, you usually say #method() or #property rather than this.method() and this.property in CoffeeScript.