使用把手迭代 javascript 对象

问题描述

我正在尝试使用 Handlebars 注册帮助程序以允许迭代 JSON 对象.这个要点 看起来是一个合适的解决方案.我将其转换为以下 CoffeeScript.当我使用任何一个帮助程序时，似乎什么都没有发生(对于 vanilla JavaScript 和 CoffeeScript 版本都是如此).有什么想法吗?

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

在模板中:

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

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

我目前在 Gemfile 中使用 gem 'handlebars-assets' 将把手添加到 Rails 应用程序.

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

推荐答案

你的 JavaScript 到 CoffeeScript 的音译坏了.你不使用 for ... in 来迭代 CoffeeScript 中的对象，你使用 for k，v of ...:

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 ...:

使用 of 表示理解对象的属性而不是数组中的值.

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

这个 CoffeeScript 循环:

This CoffeeScript loop:

for x in y
    ...

变成这个 JavaScript:

becomes this JavaScript:

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

所以如果 y 是一个没有 length 属性的对象，那么 _len 将是 undefined 并且 JavaScriptfor(;;) 循环根本不会迭代.

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.

您还应该使用 own 而不是 hasOwnProperty:

You should also be using own instead of hasOwnProperty:

如果您只想迭代对象本身上定义的键，通过添加 hasOwnProperty 检查以避免可能从原型中插入的属性，请使用 for own对象的键、值.

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.

但这更多是为了方便而不是正确.

but that's more for convenience than correctness.

此外，CoffeeScript 循环是表达式，因此您通常会说 array = expr for own k, v in o 或等效形式:

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

如果 expr 超过一行或太长而无法理解.

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

CoffeeScript 中帮助程序的正确且更惯用的版本看起来更像这样:

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('')

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

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