json.ts

type AsJSON<T> = 
    T extends Date ? string :
    T extends Function ? undefined :
    T extends Symbol ? undefined :
    T extends Array<any> ? AsJSONArray<T> :
    T extends object ? AsJSONObj<T> :
    T;

type AsJSONObj<T extends object> = {
    [K in keyof T]: AsJSON<T[K]>
};

type Inspect<T extends Array<any>> = T extends Array<infer X> ? X : never;
interface AsJSONArray<T extends Array<any>> extends Array<AsJSON<Inspect<T>>> {}

type InvalidKeys<T> = {[K in keyof T]: AsJSON<T[K]> extends undefined ? K : never}[keyof T];

type JSONable<T> = InvalidKeys<T> extends never ? T : ObjectContainsInvalidKeys<InvalidKeys<T>>;
type ObjectContainsInvalidKeys<T> = {_:void}

type User = {
    age: number,
    name: string,
    hi: ()=> void,
    createdAt: Date,
    friends: User[]
}

function userFromJSON(v: AsJSON<User>): User {
    return {
        age: v.age,
        name: v.name,
        createdAt: new Date(v.createdAt),
        hi: ()=>{},
        friends: v.friends.map(userFromJSON)
    }
}

function safeToJSON<T>(v: JSONable<T>): string {
    return JSON.stringify(v);
}

safeToJSON({hi: 1});
safeToJSON({good: 'string'});
safeToJSON({foo: ()=>{}, bar: 1});