Existential Operator / Null Propagation Operator

Brendan Eich brendan at mozilla.org
Tue Jun 2 16:57:53 UTC 2015

Sander Deryckere wrote:
> 2015-06-02 17:49 GMT+02:00 Brendan Eich <brendan at mozilla.org 
> <mailto:brendan at mozilla.org>>:
>     Sander Deryckere wrote:
>         For the prefix operator, it's unclear to me how you would do
>         the following: Say you know `obj` is non-null, you want to
>         test if it has a key `k1`, but if `k1` exists, you know it
>         will also have a key `k2` a level deeper. With the suffix
>         operator, this would be `obj[k1]?[k2]`, but with the prefix
>         operator, it could be `obj?[k1][k2]`
>     You circled back to the incompatible syntax, `?[`, but the prefix
>     idea would have `?obj[k1][k2]`. The `?` goes in front at the start
>     of an operand, and is thus unambiguous with respect to the ternary
>     operator.
> The question is not about the existence of `obj`, but if `obj` has a 
> key `k1`. AFAICS, `?obj[k1][k2]` would test the existence of `obj`, 
> which I don't need in this example. To test the existence of a key 
> inside `obj`, a prefix operator should come somewhere before the key.

You might hope for that, but as we both noted, `?[` is not going to fly. 
Don't break the (minified) Web.

The prefix idea generalizes:


and if you are not using computed property names, rather literal ones:



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