node.js - strange behavior of coffeescript compiler

I noticed an unexplainable behavior of the coffeescript compiler for me :)

For example:

getImage: (req, res) =>
    realty_id = req.query.id

    if (realty_id?)

Result

ImageController.prototype.getImage = function(req, res) {
    var realty_id,
        _this = this;
      realty_id = req.query.id;
      if ((realty_id != null)

But actually the last line should be: if ((typeof realty_id !== "undefined" && realty_id !== null))

When I comment out "realty_id = req.query.id" it works well. Has anyone a explanation for that?

tldr; typeof x !== "undefined" is not needed with local JavaScript variables.

The SO question CoffeeScript Existential Operator and this has information on why CoffeeScript will make this optimization.

Now, to see why it is a valid code generation in the presented case:

-> x         x != null   typeof x !== "undefined" && x !== null
----------   ---------   -------------------------------------
ANY_TRUE     true        true
0            true        true
null         false       false
undefined    false       false

So, according to the logic tables, they are semantically equivalent. It is the "non strict" nature of the == operator that determines the - perhaps surprising - result of this comparison: SO questions on the topic abound.

However, here is the important difference of when/why typeof x !== "undefined" is sometimes used: it will not result in a ReferenceError. If x is known to be a local variable then there is no such consideration and the shorter (!=) JavaScript expression can be safely used.

In the case when the assignment in CoffeeScript is commented out, there is no local variable/binding with the name reality_id - note that the var statement is also different - and CoffeeScript inserts the additional guard as appropriate.