How to call New variable Class Name in coffeescript without eval

Question

So, I have a standard class that is extended by child classes. In some cases, I'm passing what child class to use through jQuery as a option instead of using the default standard class.

Example: jQuery('img').myPlugin();

Calls

New mainClass(arg1, arg2)

While

jQuery('img').myPlugin({classToUse : 'myExtendedClass'});

Calls

New myExtendedClass(arg1, arg2)

Here's the code that works, but it is ugly in so many ways. How can we do this properly? Here @settings is a simple object, and @settings.classToUse is a string that's passed.

@NewClass = eval("new #{@settings.classToUse}(this.$element, this.settings)")

The following does not work and returns a string error:

@Framer = new @settings['classToUse'](this.$element, this.settings)

A clipped version of the full source code is below. Any errors in the logic are a result of clipping the code as the code in its current state is 100% functional.

You can also see the code compiled here.

class mainClass
  constructor: (@framedObject, @options) ->
  doSomething: ->
    alert('something')

class myExtendedClass extends mainClass
  doSomething: ->
    alert('somethingDifferent')

class anotherExtendedClass extends mainClass
  doSomething: ->
    super
    alert('Woah!')

$ ->
  $.plugin = (element, options) ->
    @settings = {classToUse : 'myExtendedClass'} #built by merging defaults and options
    @init = ->      
      @mainClass = eval("new #{@settings.classToUse}(this.$element, this.settings)")
    @init()
    this
  $.fn.mediaFrame = (options) ->
    return this.each ->
      plugin = new $.plugin this, options

phenomnomnominal · Accepted Answer · 2013-05-21 14:23:50Z

3

How about something like:

class Border
  constructor: (@options) ->
class RedBorder extends Border
    constructor: (@options) ->
class BlueBorder extends Border
    constructor: (@options) ->

borders = 
  Default: Border
  Red: RedBorder
  Blue: BlueBorder

$.plugin = (element, options) ->
  @mainClass = new borders[@settings.class || 'Default'](@settings)

edited May 21, 2013 at 14:23

answered May 21, 2013 at 13:47

phenomnomnominal

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5 Comments

Aaron Butacov Over a year ago

Mostly, I just want to call "new Border". Using the child classes are only for exceptions in certain circumstances. I've updated my question to include this. And sticking everything in an object, or using an if/else isn't much cleaner than using eval().

Florian Margaine Over a year ago

@AaronHarun how is using an object not cleaner than eval()? Am I not seeing the big picture there?

robkuz Over a year ago

1) eval is strongly discouraged as you open up for code injections. 2) by explictly putting your classes into an hash your building a visible dependency. eval will simply confuse anybody including you in 3 months time

mu is too short Over a year ago

@AaronHarun: Using an object also makes error handling a lot cleaner and easier to trap yourself.

Aaron Butacov Over a year ago

With the changes, it's better than eval. Though, coffeescript should eally have a better way to handle variable classes.

robkuz · Accepted Answer · 2013-05-21 13:49:55Z

0

your second approach will fail as you cant instantiate a "String"

 @Framer = new @settings['classToUse'](this.$element, this.settings)

also there seems to be an error as you are using "type" instead of "classToUse"

The easiest way to would seem to simply put the class-object into the hash that you give to jQuery

 jQuery('img').myPlugin({classToUse : redBorder});

In this example assuming that redBorder is the class name.

answered May 21, 2013 at 13:49

robkuz

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2 Comments

Aaron Butacov Over a year ago

The 'type' typo was just from quickly rewriting all of the code to make more sense outside my plugin. Passing the class object directly doesn't work, since it isn't a global (and I wont't make it one).

robkuz Over a year ago

well, if you dont make those classes global how will they ever eval correctly? btw. what do you mean by "global"? you could define those classes in a local "namespace" easily

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