For example, I have an object:

const obj = {
  a: (valueA: string) => true;
  b: (valueB: string) => false;
  c: (valueC: string) => false;
};

You see all properties in this object have same type ((value: string) => boolean). Now I want it to have IntelliSense which makes follows possible:

1. Display a, b, c after I typed obj.;
2. Limit its type when I try to add a property into obj.

I tried indexer:

const obj: Record<string, ((value: string) => boolean)> = {
  a: (valueA: string) => true;
  b: (valueB: string) => false;
  c: (valueC: string) => false;
};

But now target 1 won't happen, if I don't use indexer, I cannot achieve target 2.

CodePudding user response：

Variable inference is either all or nothing, you either let typescript infer the type or you specify the type yourself.

You can only do this with a helper function:

function makeFunctionsMap<T extends Record<string, ((value: string) => boolean)>>(o: T) { return o; }
const obj = makeFunctionsMap({
  a: (valueA: string) => true,
  b: (valueB: string) => false,
  c: (valueC: string) => false,
})
obj.a;
obj.d // Error

Playground Link

You can also inline the function if you want to make it shorter and more unreadable (Playground Link)