const wm = new WeakMap([[{}, 'a'], [{}, 'b']]) // initialized with entries
You can initialize a WeakMap with an iterable of [key, value] pairs.
After wm.set(key, 42), wm.get(key) === 42.
wm.set(key, 42)
wm.get(key) === 42
Challenge: Verify basic set/get usage.