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by mck- 4515 days ago
In CoffeeScript I discovered the other day that:

  0 <= "" <= 0   # True
  "" is 0        # False
2 comments
shoo 4515 days ago
I guess that is because coffeescript's <= operator is just javascript's <= operator, which does implicit type conversion.

but coffeescript's "is", aka ==, aka === in javascript, doesn't.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

having "==" be semantically quite different to "<=" doesn't seem a particularly nice choice of notation.

arguably coffeescript has made things worse in this case compared to plain javascript, where you're probably not going to expect === to behave similarly to <=.

not obvious how you'd improve this to make it consistent, without say redefining how "==" and "<=" work to make them raise type errors or evaluate to something undefined if the types of the arguments don't match.

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runlevel1 4515 days ago
It's because equality comparisons in CoffeeScript (==, !=, is, isnt) are always compiled into strict (non-type-converting) comparisons (===, !==). [1]

JavaScript doesn't have strict relational operators (>, <, >=, <=), so all relational comparisons undergo type-conversion. [2]

  CoffeeScript: 0 <= "" <= 0           #  True
  JavaScript:   (0 <= "" && "" <= 0);  // True

  CoffeeScript: "" is 0    #  False
  JavaScript:   "" === 0;  // False
[1]: http://coffeescript.org/#operators

[2]: http://www.ecma-international.org/ecma-262/5.1/#sec-11.8

EDIT: Forgot to include 'is' and 'isnt' operators.

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