By saying testObj: ExampleType, you are saying that testObj will be an object in which the properties are either string or () => string. That's a broad, weak promise. However, you know exactly which properties will be of which kind.

Solution one

Tell the compiler everything you know. Include concrete property names in ExampleType.

interface ExampleType {

  firstName: string;

  lastName: string;

  gender: string;

  getFullName: () => string;

}



const testObj: ExampleType = {

  firstName: "Peter",

  lastName: "Parker",

  gender: "male",

  getFullName: () => "I am Peter Parker",

};



const { firstName, lastName, getFullName } = testObj;



console.log(getFullName()); // this works

Solution two

If you want to use ExampleType as a blueprint, but want the specific properties inferred for you, use a helper function like specific defined below.

interface ExampleType {

  [key: string]: string | (() => string);

}



const specific = <T>() => <U extends T>(argument: U) => argument;



const testObj = specific<ExampleType>()({

  firstName: "Peter",

  lastName: "Parker",

  gender: "male",

  getFullName: () => "I am Peter Parker",

});



const { firstName, lastName, getFullName } = testObj;



console.log(getFullName()); // this works