Why can't you use WeakMap for caching results keyed by primitive values like strings or numbers?

WeakMap keys must be objects (or, more recently, Symbols) specifically because weak references only make sense for garbage-collectable heap objects — primitives are immutable values, not references, so there's no 'object' for the GC to reclaim and the weak-reference mechanism doesn't apply.

const cache = new WeakMap();
// cache.set('key', 'value'); // TypeError: Invalid value used as weak map key