CoffeeScript generating dynamic variable names for objects (named classes)

Question

I'm pretty new to the world of JavaScript and even newer to CoffeeScript. I have a problem and I'm not sure if it's even possible.

Say I have a CoffeeScript class like this

class @Model
  constructor: (@name) ->

how could I pass the names of models to be created to a function to instantiate these objects whilst appending the name of the variable [in this case] with _model? Something like:

makeModel = (name) ->
  "#{name}_model" = new Model(name)

My rails app tells me that:

unexpected =
      "#{name}_model" = new Model(
                ^

I'm assuming that this is because of the string. Is there some sort of method to convert a string to a variable name? I took a quick look at the .eval() method but the little book of CoffeeScript warns against it's use.

Thanks

instead of a named variable, use an array or object to collect instances... — dandavis
– dandavis, Commented Apr 8, 2014 at 16:24
That did not occur to me. As a last resort this would be possible but as these elements are all stored in a database it would be far more convenient being able to interact with them by name than by maintaining an array — DazBaldwin
– DazBaldwin, Commented Apr 8, 2014 at 16:28
how is all.fred_model any eaiser to use than window.fred_model or even just fred_model? plus, if they are in an object, you8 can iterate all instances, which is messy with variables. i would also consider attaching them to the constructor itself as a static object property. — dandavis
– dandavis, Commented Apr 8, 2014 at 16:29
Again, new to JS and CS, would you mind expanding with some sort of code examples? the point is that I probably wouldn't need to iterate through all models but I would likely need to directly target say current_model = fred_model Sorry, complete noob here! — DazBaldwin
– DazBaldwin, Commented Apr 8, 2014 at 16:40
i don't know how it would look in CoffeeScript, but thanks to leaky assignments, you can just prepend an additional assignment after "var xxx=" in normal JS. something like "var xxx= self[name]= ... " — dandavis
– dandavis, Commented Apr 8, 2014 at 16:43

kstev · Accepted Answer · 2015-10-19 19:28:58Z

2

You could do something like this:

ModelFactory = do -> 
  class @Model
    constructor: (@name) ->

  models = {}

  makeModel = (name) ->
    model = new Model(name)
    models["#{name}Model"] = model
    model

  getModel = (name) ->
    models["#{name}Model"] or makeModel(name)

  { getModel }

Secret Magic:

do -> - The do keyword creates an IIFE, a function that is called as soon as it is created. It can be used for creating modules like the following:

var myModule = (function () {
    var privateFunc = function () {};
    var publicFunc = function () { privateFunc(); };
    return {
        publicFunc: publicFunc
    }
})();

accessing properties with strings - In JavaScript, you can access a property of an object as a string, by using the square-bracket ([]) notation, plus a string, or a variable containing a string. Here we are combining this with the CoffeeScript string interpolation sugar ("#{}") to dynamically create property names and assign them.

var object = {
    prop: 'value',
    otherProp: 'otherValue'
};

var propertyName = 'prop';
var value = object[propertyName]; // 'value';

var something = 'Something'
object['my' + something] = 'a new property';
object.mySomething === 'a new property'; // true

edited Oct 19, 2015 at 19:28

kstev

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answered Apr 8, 2014 at 20:32

phenomnomnominal

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2 Comments

DazBaldwin Over a year ago

This looks like a very good solution, thanks. two little things though , I really don't like using solutions without knowing what the code is doing. I don't suppose you know where I could read up on the ModelFactory = do -> business and this whole using strings as indices in arrays (models["#{name}Model"])

DazBaldwin Over a year ago

Thanks, this is great! Nothing like being schooled before breakfast

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