Question

Example:

[42] == [42]

The result of the comparison gives different results:

CS/JS: false
Ruby:  true

On the other hand:

42 == 42

gives the result:

CS/JS: true
Ruby:  true

What is the reasoning behind this?

Answer 1

The other answerers have done a good job of explaining the JavaScript/CoffeeScript equality semantics. (CoffeeScript’s == compiles to JavaScript’s stricter ===, but that makes no difference in this case.)

The Ruby case is more complex: Everything in Ruby is an object, and so every object has a == method which, in principle, could do anything. In the case of arrays, it looks at the other array, checks if it has the same length, and then checks if x == y for each x in itself and y in the other array.

If you want to emulate the Ruby behavior, it’s quite simple to write a function to do so:

deepEquals = (arr1, arr2) ->
  return false unless arr1.length is arr2.length
  for i in [0...arr1.length]
    if arr1[i] instanceof Array and arr2[i] instanceof Array
      return false unless deepEquals(arr1[i], arr2[i])
    else
      return false if arr1[i] isnt arr2[i] 
  true