给定接口的特定键，以参数方式推断接口上的属性类型

Question

Given an heterogeneous interface, I would like to generate a type-safe function that operates on a given property of that interface. For example, something like this:

interface State {
  a: boolean;
  b: string;
  c: number;
}

// "enclose" the Interface type variable <I> so that I can generate several functions that
// operate on different properties of <I>
function createGenerator<I>() { 
  return function createOperationForProperty<K extends keyof I>(propertyName: K) {
    return function operation<T>(t: T): T {
      return calculationWith(t);
    }
  }
}

The returned function "createOperationForProperty" is constrained to K extends keyof I , so that I can only pass as propertyName a key that is on I . But T is unconstrained. I would like it to be the type of I[propertyName] . The type I[keyof I] would only restrict it to any type in I , whereas I would like it to only accept the specific type corresponding to "propertyName".

Just to finish the example, you could apply it like this:

const createOperation = createGenerator<State>();
const operationOnA = createOperation("a");

dispatch(operationOnA("wrongType")); 

The hope is that this last line would fail type checking, because State.a is a boolean .

Answer 1

You can require that T extends I[K] in operation , to ensure that the type of the argument matches the type of the property value on the object:

return function operation<T extends I[K]>(t: T): T {