The following would assign to the address foo in obj if and only if it has not already been assigned.
var obj = {}
var foo = Math.random()
obj[foo] ?= 1
The same can be said for:
var undef
undef ?= 1
Which could both be transpired to
type obj[foo] === 'undefined' && obj[foo] = 1
or
type a === 'undefined' && undef = 1
respectively.
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