Function of return type void in interface can return anything

[sant123]

TypeScript Version: 1.8.10

Hi, I have this code and as you can see the function greet has a return type of void.

Code

interface Person {
    name : string;
    lastName : string;
    greet : (person : this) => void;
}

declare var some : Person;

var obj : Person = {
    name : "Santi",
    lastName : "Aguilar",
    greet : function(person) {
        return "some string";
    }
}

Expected behavior:

Type string is not assignable to type void.

Actual behavior:

No error.

However, if you explicitly specify the return type void in the implementation; it works!

var obj : Person = {
    name : "Santi",
    lastName : "Aguilar",
    greet : function(person) : void {
        return "some string"; //Type string is not assignable to type void.
    }
}

[zpdDG4gta8XKpMCd]

cpnsider your vote to fix: #8584

[zpdDG4gta8XKpMCd]

@RyanCavanaugh, this what rationale applies here? (doesn't seem to be the case of #8581 (comment))

[kitsonk]

There is no bottom type until TypeScript 2.0. Therefore void in an interface is essentially saying "you should ignore a return value from a function. But just like excess parameters, TypeScript evaluates if the resulting object can be conformed to the shape of the interface. So the below is valid (although a bit surprising):

interface Foo {
    foo(a: string, b: number): void;
}

const foo: Foo = {
    foo() {
        return 'bar';
    }
};

But then of course if you were to depend upon the return type TypeScript would get upset:

function bar(c: string) {
    console.log(c);
}

bar(foo.foo()); // error here

In TypeScript 2.0 we get the bottom type never, which is a hard "this will NEVER return a value" versus a soft "don't depend upon any returned value" and some more strictness around unused parameters which will allow us to enforce these contracts more specifically.