Typescript interface function member

Question

I am new to Typescript. I couldn't understand the following use of interface:

Say, there is a function :

function test(arg1: ITest):number {
  return 20;
}

If the ITest interface is like below :

interface ITest {
    (props: string):number;
    a1?: number;
}

Then, the below call is valid :

const obj = {

   func: (p: string): number => (10+2),
};

test(obj.func); //Should not I pass obj instead of obj.func ?

Q: Should not I pass obj instead of obj.func?

But, if the interface is like below :

interface ITest {
        (props: string):number;
        a1: number; //Removed the optional sign "?"

}

Q: Then what argument should I pass to the test function ?

Q: what is difference between above interface and the following changed declaration of the interface?

interface ITest {
        func: (props: string)=>number; //changed it from (props: string):number;
        a1: number;
 }

Thank you for clearing out my doubts.

Answer 1

Q: Should not I pass obj instead of obj.func ?

ITest is a function type. It describes a function itself, not an object with a function property or method.

Q: Then what argument should I pass to the test function ?

Now you have declared ITest as a function type with required property a1. The property declaration on a function works in TS by using a function declaration or const expression. Example:

const func = (p: string): number => 10 + 2 // or function declaration func(p: string) {...}
func.a1 = 42
const obj = { func };
test(obj.func) // works now

Q: what is difference between above interface and the following changed declaration of the interface ?

ITest now has become an interface declaration for an object with func and a1 properties.

const o1: ITest = {
    func: s => 137,
    a1: 42
}