Comment by hex13

I've created library `transmutable` for performing automatic copy-on-write behavior in JS (you just write imperative code which is transformed behind scenes into immutable updates https://npmjs.com/package/transmutable

This can avoid boilerplate traditionally associated with immutable updates in JavaScript, because you just write

    const copy = transform(original, stage => {
       stage.someArray[index] = 42;
    });

and it returns immutable copy of original object with appropriate mutations.