Let’s say you want to associate a property with an object, but you don’t want to write to the object directly, maybe for design reasons or logistical reasons (what if the object is frozen and immutable?) You can use WeakMap for this, and the garbage collector is free to collect keys of the map:

object = []
map = ObjectSpace::WeakMap.new
map[object] = [object]

p map.member?(object) # => true

There is an issue with this demo, however. Currently, Ruby’s WeakMap doesn’t keep the values of the map alive, so the map evicts the pair in the demo after a garbage collection run, even though the key survives:

object = []
map = ObjectSpace::WeakMap.new
map[object] = [object]

p map.member?(object) # => true
GC.start
p map.member?(object) # => false, as of Ruby 3.4

So, you can have the property, but it may go away when the collector decides to run. A bit unruly, but alright for some situations.

ECMAScript also offers WeakMap, and they come with ephemerons. I like them because they sound like mystical creatures. Practically, they deliver a reliable way to achieve my contrived stated purpose. Nothing in the word “weak” and “map” indicate that ephemerons are involved, but the spec links to the 1997 paper by Barry Hayes that introduced the mechanism.

Here’s a program you can run in your browser to get a taste of their semantics:

// Make an object, and log when it becomes garbage
const registry = new FinalizationRegistry((value) => {
  console.log(value, "is garbage")
})
let object = []
registry.register(object, "object")

// Put the object into a WeakMap and have the value reference itself
let map = new WeakMap
map.set(object, object)

// Stop referring to the object through the local.
// FireFox collects after a few seconds.
object = undefined

From my admittedly imperfect understanding, they offer a sort of one-way conditional liveness, where in a pair the value is live if the key is. Further, values kept alive solely due to this mechanism cannot “upgrade” their liveness to stay alive forever by way of reference cycles. Implementing ephemerons in practice seems to require discovering them to a fixed-point after the usual object tracing – their special kind of liveness seems to call for tracing under a different mode.