Iterating over javascript objects with handlebars

Question

I am trying to register helpers with Handlebars to allow iterating over JSON objects. This gist looked like an appropriate solution. I converted that into the following CoffeeScript. Nothing seems to happen when I use either of the helpers (that holds true for both vanilla JavaScript and the CoffeeScript version). Any ideas?

$ ->
  Handlebars.registerHelper "key_value", (obj, fn)->
    buffer = ""
    key 
    for key in obj 
      if obj.hasOwnProperty(key)
        buffer += fn({key: key, value: obj[key]})
    buffer

  Handlebars.registerHelper "each_with_key", (obj, fn)->
    context
    buffer = ""
    key 
    keyName = fn.hash.key
    for key in obj 
      if obj.hasOwnProperty(key)
        context = obj[key]
        if keyName
          context[keyName] = key 
          buffer += fn(context)
    buffer

In the template:

{{#key_value categories}}
I'M ALIVE!!
{{/key_value}}

{{#each_with_key categories key="category_id"}}
I'M ALIVE!!
{{/each_with_key}}

I am currently using gem 'handlebars-assets' in the Gemfile to add handlebars to a rails app.

Answer 1

Your JavaScript to CoffeeScript transliteration is broken. You don't use for ... in to iterate over an object in CoffeeScript, you use for k, v of ... :

Use of to signal comprehension over the properties of an object instead of the values in an array.

This CoffeeScript loop:

for x in y
    ...

becomes this JavaScript:

for (_i = 0, _len = y.length; _i < _len; _i++) {
  x = a[_i];
  ...
}

So if y is an object without a length property, then _len will be undefined and the JavaScript for(;;) loop won't iterate at all.

You should also be using own instead of hasOwnProperty :

If you would like to iterate over just the keys that are defined on the object itself, by adding a hasOwnProperty check to avoid properties that may be interited from the prototype, use for own key, value of object .

but that's more for convenience than correctness.

Also, CoffeeScript loops are expressions so you'd usually say array = expr for own k, v in o or the equivalent form:

array = for own k, v in o
    expr

if expr is more than one line or too long to allow for a readable comprehension.

A correct and more idiomatic version of your helpers in CoffeeScript would look more like this:

Handlebars.registerHelper "key_value", (obj, fn)->
    (fn(key: key, value: value) for own key, value of obj).join('')

Handlebars.registerHelper "each_with_key", (obj, fn)->
    key_name = fn.hash.key
    buffer   = for own key, value of obj
        value[key_name] = key
        fn(value)
    buffer.join('')

Demo: http://jsfiddle.net/ambiguous/LWTPv/