Is there a Typescript equivalent of typeof for method signatures?

I'm interested in finding a way to determine a method's signature. The following code represents the question I have:

class MyClass {
    constructor(public foo: any){}
}

const object1 = new MyClass((): void => {
    console.log('My function is to say hi. Hello!');
});
const object2 = new MyClass((n: number): void => {
    console.log('My function is echo a number. Here it is: ' + n);
});

object1.foo();      // My function is to say hi. Hello!
object2.foo(15);    // My function is echo a number. Here it is: 15

console.log(typeof object1.foo); // prints 'function'. 
                                 // Is there a way to print '(): void' instead? 

console.log(typeof object2.foo); // prints 'function'. 
                                 // Can we print '(number): void' instead?

Answer №1

Consider this concept: static types are only accessible during the design phase (TypeScript), not during execution (JavaScript):

class MyClass<T> {
    constructor(public foo: T){}
}

const sayHello = (): void => {
    console.log('My function is to greet. Hello!');
};

const sayANumber = (n: number): void => {
    console.log('My function is to repeat a number. Here it is: ' + n);
};

const object1 = new MyClass(sayHello);

const object2 = new MyClass(sayANumber);

object1.foo();      // My function is to greet. Hello!
object2.foo(15);    // My function is to repeat a number. Here it is: 15

// Accessing design-time type information
type designtimeTypeOfFoo1 = typeof object1.foo; // () => void
type designtimeTypeOfFoo2 = typeof object2.foo; // (n: number) => number

// At runtime, static type details disappear
const runtimeTypeOfFoo1 = typeof object1.foo; // "Function"
const runtimeTypeOfFoo2 = typeof object2.foo; // "Function"

// Error: design-time types are not available at runtime
console.log(designtimeTypeOfFoo1, designtimeTypeOfFoo2);

// Success: runtime types are accessible during runtime
console.log(runtimeTypeOfFoo1, runtimeTypeOfFoo2);

Answer №2

Unsure of the solution, but you might want to give this a shot:

console.log(typeof var foo1 = object1.foo());
console.log(typeof var foo2 = object2.foo(15));

UPDATE: The previous code did not work as expected, however, I believe storing it in a variable is the way to go:

var foo1 = object1.foo();
var foo2 = object2.foo(15);

console.log(typeof foo1);
console.log(typeof foo2);

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