Type returned not being checked in a function returning an union type

[thitemple]

TypeScript Version: 2.2.1

Code

interface ISomeType { 
    value: string;
}

interface ISomeOtherType { 
    value2?: string;
}

interface IStillSomeOtherType { 
    value3?: string;
}

type TypeOptions = ISomeType | ISomeOtherType | IStillSomeOtherType;

function fn(x: number): TypeOptions {
    if (x % 2 === 0) {
        return {
            value: "even"
        };
    } else if(x % 2 === 1) {
        return "odd";
    }

    return 123;
}

Expected behavior:
TypeScript should check that the returned value is of the three types defined in the type alias TypeOptions

Actual behavior:
Values of a type different from the specified can be returned, such as strings and numbers.

Link to playground

[krryan]

Unfortunately, this isn't a bug: you are returning valid values for TypeOptions here. Because ISomeOtherType and IStillSomeOtherType have only optional parameters, the empty object {} is a valid value for those interfaces. And since any extension of those interfaces is also valid as a value for those interfaces, that means any extension of {} is valid here. Which is to say, literally anything at all.

TypeScript does not have any way of specifying that you want objects that only have the indicated properties. You are always allowed to use bigger objects.

This is functionally the same problem as in this issue, and @ahejlsberg's comment there may prove useful to you

You might consider giving those interface some unique string literal identifier property. On our project, we call this property kind. So for example you could have:

interface ISomeType { 
    kind: 'some-type';
    value: string;
}

interface ISomeOtherType { 
    kind: 'some-other-type';
    value2?: string;
}

interface IStillSomeOtherType { 
    kind: 'still-some-other-type';
    value3?: string;
}

type TypeOptions = ISomeType | ISomeOtherType | IStillSomeOtherType;

function fn(x: number): TypeOptions {
    if (x % 2 === 0) {
        return {
            kind: 'some-type',
            value: "even"
        };
    } else if(x % 2 === 1) {
        return "odd";
    }

    return 123;
}

Now "odd" and 123 are not valid return values for the function, since they lack a valid kind property.

[thitemple]

I understand, I thought it was possible to check that at least an empty object was being returned. But I'll use your recommendation with the kind pattern.
Thanks