dynamic type based on existing in typescript

Question

For a given object like:

class Product {
  constructor(
    public metadata: Metadata,
    public topic: Topic,
    public title = 'empty'
  ) {}
  ...
}

I want to create an interface:

interface State<T> {
}

such that TypeScript ensures that a product: State<Product> has the same structure as Product but its matching leaf-level properties are boolean. So product is supposed to have a title property of type boolean.

I had a look at the readonly example at mapped-types but in this case I probably need a composite:

interface State<T> {
  [p in keyof T]: typeof T[p] === 'object' ? State<T[p]> : boolean;
}

Any ideas of how to make this work?

Titian Cernicova-Dragomir · Accepted Answer · 2019-04-24 11:50:44Z

3

You are close, your syntax is off, you need to use a type not an interface, and typeof T[p] === 'object' is written as T[p] extends object

class Metadata {
  constructor(
    public key = 'empty',
    public value = 'empty'
  ) { }
}
class Topic {
  constructor(
    public name = 'empty'
  ) { }
}
class Product {
  constructor(
    public metadata: Metadata[],
    public topic: Topic,
    public title = 'empty'
  ) { }

}

type State<T> = {
  [P in keyof T]: T[P] extends object ? State<T[P]> : boolean
}
let o: State<Product> = {
  title: true,
  topic: {
    name: true
  },
  metadata: [
    { key: true, value: false }
  ]
}

Playground link

edited Apr 24, 2019 at 11:50

answered Apr 24, 2019 at 11:05

Titian Cernicova-Dragomir

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7 Comments

Andrei Roba Over a year ago

sounds promising, i'll have a look at it

Andrei Roba Over a year ago

for some reason compile breaks at extends keyword: TS1005: ';' expected.

Titian Cernicova-Dragomir Over a year ago

@RobyRodriguez what version of TS are you using ? Code was tested on 3.4 in the playground, I will add a link

Andrei Roba Over a year ago

"typescript": "~2.7.2" :(

Titian Cernicova-Dragomir Over a year ago

@RobyRodriguez yeah .. that will not work, you might get something working on 2.8 (arrays will be an issue, proper array handling in mapped types was added in 3.2 or 3.1 I belive) but 2.7 does not have conditional types

|

Roberto Zvjerković · Accepted Answer · 2019-04-24 11:05:57Z

0

Use type instead of an interface:

type State<T> = { [P in keyof T]: boolean };

answered Apr 24, 2019 at 11:05

Roberto Zvjerković

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2 Comments

Titian Cernicova-Dragomir Over a year ago

While not 100% clear, I think OP wants to map a nested structure with primitive properties being boolean

Roberto Zvjerković Over a year ago

Oh, I have misunderstood then. "...but its matching leaf-level properties are boolean.", you're right. Well then he has your answer. :)

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