Common Gotchas

This page lists common gotchas you may encounter when working with CoffeeScript. You can refer to this page for general answers on "why does CoffeeScript act that way"

Q: Why does CoffeeScript require "foo" to be defined when doing foo ?= value or foo ||= value

A: Otherwise, it'd create a global, which is not what we want (if that is what you want, use window.foo ?= value) If you're declaring the variable in the current scope, you know for sure it doesn't exist - Javascript has no mechanic like PHP's static keyword.

Note that it works perfectly when used with classes :

class Foo
 getCache: ->
  @cache ?= "value"

If you want an alternative to PHP's static keyword, you can use a closure :

do (staticVariable = theValueYouWant) ->
 (args...) ->
  #you now have "static"-like access to "staticVariable"

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Q: Why is CoffeeScript sometimes using ["bar"] notation over .bar ?

A: CoffeeScript detects reserved keywords (as the auto-quoting of keywords in array notation) and prefer to use the array-access syntax ([]), because in ES3, reserved keywords (throw, class, ...) were not allowed as the right operand in a member access. See this comment for more information.

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Q: Why is the existential "?" operator only checking this.foo != null, shouldn't it also check for typeof === 'undefined' ?

A: X == null tests that either X is null or undefined, assuming it is in scope. If we can't make that assumption, we need to do a typeof test to avoid ReferenceErrors. See the Abstract Equality Comparaison Algorithm (section 11.9.3) for more information (especially steps 2 and 3).

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Q: Why is foo +a different from foo + a ?

A: CoffeeScript is a whitespace-significant language. foo<space> starts an implicit call, you must either dually-space your operator or dually-unspace it.