Small correction to localStorage example in documentation

[jlapointe]

Hi Adam! This is a small thing but it tripped me up for a while.

In the S readme file, the example code provided for integrating localStorage is as follows:

if (localStorage.todos) // load stored todos on start
    todos(JSON.parse(localStorage.todos).map(Todo));
S(() =>                 // store todos whenever they change
    localStorage.todos = JSON.stringify(todos()));

However, JSON.stringify(todos()) results in an array of empty objects.

The correct code taken from the functioning CodePen example is:

if (localStorage.todos) // load stored todos on start
    todos(JSON.parse(localStorage.todos).map(Todo));
S(() =>                 // store todos whenever they change
    localStorage.todos = JSON.stringify(todos().map(t => 
        ({ title: t.title(), done: t.done() }))
    ));

It would be super handy if the first syntax just worked automagically, but I don't know whether that's possible/feasible.

[adamhaile]

Thanks for the report, and thanks @shimaore for the PRs!

Yeah, the example used to work, because it used to be the case that all data signals got a .toJSON() method so that they could be JSONized transparently. That was removed a version back, though, because it turned out that it caused a noticeable performance regression in some cases. It made a lot of codepaths polymorphic: sometimes they got a plain function, sometimes a function (a data signal) that had a toJSON() method added.

Instead, in my own code, I now explicitly specify signals that I want to be jsonable by using a little jsonable(<signal>) utility which adds the .toJSON() method just to those signals:

// add a .toJSON() to a signal so that it will serialize its value to JSON
export function jsonable<T extends () => any>(obs : T) : T {
    (obs as any).toJSON = toJSON;
    return obs;
}

function toJSON(this : () => any) {
    var json = this();
    // if we shadowed the value's own toJSON, call it now
    if (json && json.toJSON) json = json.toJSON();
    return json;
}

So, for instance, the old code would work if we did:

var Todo = t => ({               // our Todo constructor
       title: jsonable(S.data(t.title)),   // properties are data signals
       done: jsonable(S.data(t.done))
    }),
    todos = jsonable(SArray([])),          // our array of todos

Sorry I hadn't updated the README and for the trouble that gave you, but thanks for bringing it to my attention!

[jlapointe]

Ahh I see, thanks for the solution!