One common issue arises when dealing with circular structures, known as the "tying the knot" problem. During deserialization, the flatted library needs to begin somewhere and pass in the original object that has not yet been revived. While it is possible to use a proxy or objects with getters to parse the involved objects in the correct order on-demand, this approach can lead to a stack overflow if all objects within the circle need to be revived simultaneously. JavaScript lacks lazy evaluation, making it challenging to reference the result of a call before evaluating it.
To address this challenge, a reviver function supporting a lazy approach that delays accessing the passed object until after deserialization is required:
const cache = new WeakMap();
const ret = parse(str, (k, v) => {
if (cache.has(v)) return cache.get(v);
if (v && v.className && (<any>models)[v.className]) {
const instance = new (<any>models)[v.className]();
cache.set(v, instance);
for (const p in instance) {
Object.defineProperty(instance, p, {
set(x) { v[p] = x; },
get() { return v[p]; },
enumerable: true,
});
}
return instance;
}
return v;
});
This approach essentially creates a proxy instance over `v`, retrieving its values from there. When flattened ties the knot by assigning revived values to `v` properties, they become available on `instance` as well. Only the `.className` property of `v` is accessed during the reviver call - other properties are deferred until accessed through `ret.something` to contain the revived objects.
A drawback to this method is that all models must declare and initialize their properties upfront. Additionally, model properties are replaced with accessors, potentially conflicting with the internal implementation.
An alternative approach involves forwarding property assignments made by flatted on the original object to the newly created instance post-reviver call:
const cache = new WeakMap();
const ret = parse(str, (k, v) => {
if (cache.has(v)) return cache.get(v);
if (v && v.className && (<any>models)[v.className]) {
const instance = new (<any>models)[v.className]();
cache.set(v, instance);
Object.assign(instance, v);
for (const p in v) {
Object.defineProperty(v, p, {
set(x) { instance[p] = x; },
get() { return instance[p]; },
});
}
return instance;
}
return v;
});
This approach mimics how circular references were originally constructed using direct property assignment instead of the model's method. It is essential to document that models must support constructor calls without arguments and direct property assignment to work effectively. If setters are needed, employ accessor properties rather than `set...()` methods.
The simplest and most efficient strategy may involve maintaining the identity of the object returned from the reviver:
const ret = parse(str, (k, v) => {
if (v && v.className) {
const model = (<any>models)[v.className];
if (model && Object.getPrototypeOf(v) != model.prototype) {
Object.setPrototypeOf(v, model.prototype);
}
}
return v;
});
This solution restores prototype methods by replacing the prototype of the revived object with the expected one. However, models are instantiated without a constructor call, limiting the usage of enumerable getters and private states even if exposed correctly through `.toJSON`.