I imagine careless use of such a structure would be an easy way to create a memory leak.
Is it possible to create persistent collections in js, that will free data no longer directly referenced?
The careless usage of shallow copies of objects (with JS spreading) presents the same issue. Properly scoping assigned constants and variables is still important.
Persistent collections are available today via immutable.js, so it can be done. The catch is that you have to use a library for it, and transform them back into plain JS when something expects an object or an array. The language itself could make this transparent and lower-cost by implementing it at the engine level.
Persistent collections are primarily useful in functional programming paradigms. JS is a multi-paradigmatic language, so it doesn’t make sense to use them as the default. It would sure be nice to be able to opt into using them in FP-aligned frameworks like the current React ecosystem though.
