non-standard TypeScript interface/class fields

Question

I'm working on JSON-Schema-faker project where we use following TypeScript interface:

interface IGeneratorSchema { faker?: any; chance?: any; } interface IGeneratorSchema { faker?: any; chance?: any; } for objects such as: { "faker": "name.findName" } or: { "chance": { "bool": { "likelihood": 100 } } }

Now we need to introduce support for x-faker and x-chance fields that would semantically mean the same as faker and chance , eg:

{ "x-faker": "name.findName" } or: { "x-chance": { "bool": { "likelihood": 100 } } }

I know I can create neither x-faker nor x-chance field in TypeScript interface. The question is - how can I overcome that? I want TypeScript to strictly allow only those four fields: faker , chance , x-faker , x-chance .

Answer 1

I want TypeScript to strictly allow only those four fields: faker, chance, x-faker, x-chance

You can declare string properties:

interface IGeneratorSchema {
    faker?: any;
    chance?: any;
    "x-faker"?: any;
    "x-chance"?: any;
}

const foo:IGeneratorSchema = { "x-faker": {} } // Okay
const bar:IGeneratorSchema = { bad: {} } // Error: unknown property

Caveat

Extra member are only prevented in fresh object literal scenarios : https://basarat.gitbooks.io/typescript/content/docs/types/freshness.html