Typescript conditional mapped type with multiple conditions

Question

I am working with a normalized data inside redux store (all complex types are just ids referring to real object) and I would like to create a denormalized types using typescript's conditional mapped types feature.

I keep the actual type of the object the id is referring to as it's generic argument and from this I was able to create denormalized type correctly if only non array id attributes are used.

type Id<T extends HandlerBase<any>> = string & { __type: T };

class HandlerBase<T extends HandlerBase<T>> {};

class HandlerA extends HandlerBase<HandlerA> {
    str: string;
    b: Id<HandlerB>;
    bArr: Id<HandlerB>[];
};

class HandlerB extends HandlerBase<HandlerA> {};

type DenormalizedHandler<T> = {
    [P in keyof T]: T[P] extends Id<infer U> ? DenormalizedHandler<U> : T[P];
}

const handler: DenormalizedHandler<HandlerA> = undefined;
handler.str; // Is string
handler.b; // Is DenormalizedHandler<HandlerB>
handler.bArr; // Should be DenormalizedHandler<HandlerB>[]

Now I need to figure out how to possibly add second condition to DenormalizedHandler so it can correctly map Id<U> to DenormalizedHandler<U> and Id<U>[] to DenormalizedHandler<U>[].

Titian Cernicova-Dragomir · Accepted Answer · 2019-04-23 13:58:05Z

5

You just need to add another condition, conditional types can nest just like ternary expressions could:

type Id<T extends HandlerBase<any>> = string & { __type: T };

class HandlerBase<T extends HandlerBase<T>> {};

class HandlerA extends HandlerBase<HandlerA> {
    str: string;
    b: Id<HandlerB>;
    bArr: Id<HandlerB>[];
};

class HandlerB extends HandlerBase<HandlerA> {};

type DenormalizedHandler<T> = {
    [P in keyof T]: 
        T[P] extends Id<infer U> ? DenormalizedHandler<U> : 
        T[P] extends Array<Id<infer U>> ? Array<DenormalizedHandler<U>> : 
        T[P];
}

const handler: DenormalizedHandler<HandlerA> = undefined;
handler.str; // Is string
handler.b; // Is DenormalizedHandler<HandlerB>
handler.bArr; // Is DenormalizedHandler<HandlerB>[]

answered Apr 23, 2019 at 13:58

Titian Cernicova-Dragomir

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1 Comment

Maroš Beťko Over a year ago

I've tried that in my code and it wasn't working but for completely different reason. After fixing the other mistake it works great.

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