typescript elegantly enforce a constraint while preserving types

Question

so originally to enforce a constraint, i\'d just apply an interface like this

// this is the constraint
interface Topic {
  [key: string]: (...args: any[]) =>          


        

Answer 1

One option is to use a no-op function that has the relevant generics in the argument so it constrains in place:

(playground)

// this is the constraint
interface Topic {
  [key: string]: (...args: any[]) => Promise
}
function ensureTopic(topic: T){
    return topic;
}

// object that must pass the constraint
const topic = ensureTopic({

  // GOOD: topic methods must conform
  async actuate(a: boolean) {}
})

related: Use function interface to ensure parameters but infer more specific return type