How do I use the CoffeeScript existential operator to check some object properties for undefined?

Question

I would like to use the CoffeeScript existential operator to check some object properties for undefined. However, I encountered a little problem.

Code like this:

Answer 1

This JavaScript:

a.foo != null

actually does check if the foo property of a is neither undefined nor null. Note that a.foo? is translated to JavaScript that uses != null rather than !== null. The conversions that != does means that both of these are true:

null == null
undefined == null

A plain a? becomes this JavaScript:

typeof a !== "undefined" && a !== null

because there are three conditions to check:

1. Is there an a in scope anywhere?
2. Does a have a value of undefined?
3. Does a have a value of null?

The first condition is important as just saying a != null will trigger a ReferenceError if there is no a in scope but saying typeof a === 'undefined' won't. The typeof check also takes care of the a === undefined condition in 2. Then we can finish it off with a strict a !== null test as that takes care of 3 without the performance penalty of an unnecessary != (note: != and == are slower than !== and === due to the implicit conversions).

A little reading on what != and !== do might be fruitful:

MDN: Comparison Operators

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As far as your comment on the deleted answer is concerned, if(a.foo) is perfectly valid syntax if you complete the if statement:

if(a.foo)
    do_interesting_things()
# or
do_interesting_things() if(a.foo)

However, if(a.foo) and if(a.foo?) differ in how they handle 0, false, and ''.