unless variable? compiles to two different things.

[Zegnat]

Setting any variable depending on its own existence (checked with the existential operator ?) results in different JavaScript code depending on whether the variable has been assigned a value somewhere prior to the check. See code examples below.

I would expect the JavaScript output to be the same in both cases. Because both cases add the var statement and therefor neither has to check via typeof.

---

Input

a = {} unless a?

Output

var a;
if (typeof a === "undefined" || a === null) {
  a = {};
}

Input

a = a # Setting a to itself, the most pointless of variable assignments.
a = {} unless a?

Output

var a;
a = a;
if (a == null) {
  a = {};
}

[paulmillr]

coffee checks if the variable was already declared, so this to be a proper way to me.

why the heck would someone assign var to itself?

[Zegnat]

why the heck would someone assign var to itself?

You wouldn’t, that is just by example. Doing a = a does, as far as I know, nothing. Yet CoffeeScript seems to think it does something because it changes the output if considerably.

When I use input number 1 CoffeeScript adds var a;. So it knows a exists. So why does it check the typeof? I would expect the output from example two with the input of example 1. That’s what this is about.

[michaelficarra]

In the first example, the test is made before the assignment, but the compiler should know that the declaration was hoisted above the test. Marking as bug.

[satyr]

Related: #1500

[matrinox]

Still broken here
I think it just makes sense to keep the ? as an existential check only. None of this smart "this variable once defined can never be undefined" logic.

This code:

if a?.b?.c
  a.b.c

Compiles to:

var ref;

if (typeof a !== "undefined" && a !== null ? (ref = a.b) != null ? ref.c : void 0 : void 0) {
  a.b.c;
}

It's like it gives up after the first try...
Safe navigation is really great syntax that provides safety to developers. Would be nice to get this to check for undefined as well

[jashkenas]

That's not broken.

If you really think it's broken, submit a failing test that proves it.

[vendethiel]

Why was this issue reopened, by the way?

[jashkenas]

It looks like the comments on that commit are why — but I'm not sure if they're still relevant or not.

[matrinox]

Just curious if the purpose of the ? operator is to always check for null/undefined or just null. The website says the former but code output often produces the latter

[vendethiel]

That's how != works -- differently from !==. It's in the FAQ and in a dozen other issues.

[matrinox]

@vendethiel In Coffeescript, you can't do !== and they automatically convert != and isnt to !==. Ditto with ==/is becoming ===

[vendethiel]

We're talking about generated code here, not coffeescript code.

[matrinox]

I see, you're right @vendethiel. Thanks for the help! != null is false for undefined and void 0, as well as null