type guard doesn't seem to work on interfaces

[donnut]

The type guard instanceof Array does not seem to recognize the type of the interface IMessage.

interface IMessage {
    name: string;
}

function itemFn(pipIn: IMessage[]) {
    return pipIn;
}

function baseFn(pipIn: IMessage) {
    return [pipIn];
}

function main(pipIn: IMessage|IMessage[]) {
    if (pipIn instanceof Array) {
        return itemFn(pipIn);
    } else {
        return baseFn(pipIn);
    }
}

The compiler outputs the error message:

unions6.ts(19,23): error TS2345: Argument of type 'IMessage | IMessage[]' is not assignable to parameter of type 'IMessage'.
  Type 'IMessage[]' is not assignable to type 'IMessage'.

I'm using master of today.

Thanks

[danquirk]

This is working as intended according to the latest spec.

From section 4.20 Type Guards:

• A type guard of the form x instanceof C, where x is not of type Any, C is of a subtype of the global type 'Function', and C has a property named 'prototype'
  • when true, narrows the type of x to the type of the 'prototype' property in C provided it is a subtype of the type of x, or, if the type of x is a union type, removes from the type of x all constituent types that aren't subtypes of the type of the 'prototype' property in C, or
  • when false, has no effect on the type of x.

That said, I'm not aware of the rationale here to do no narrowing in the else clause, it seems like this should work. @ahejlsberg can weigh in.

[NoelAbrahams]

Seems related to #1719

[danquirk]

Thought I had seen this before... don't know why I couldn't find it. Closing this as a dupe then.

[donnut]

Ok, this limits the usage of type guard to primitives only. Type guard

function(x: string|string[]) {
    if (x instanceof Array) {
       A(x);
    else {
       B(x);
    }
}

should be replaced by

function(x: string|string[]) {
    if (typeof x === 'string') {
       B(x);
    else {
       A(x);
    }
}

We are working with interfaces a lot, so I hope there will be a solution soon.
Thanks
(A TypeScript Fan)

[NoelAbrahams]

@danquirk, this may not be a duplicate. It's not about narrowing in the else block.

[NoelAbrahams]

This may be related to #1587, in which case it's fixed in 1.5.

@donnut, there's also a bug in there. once #1785 is fixed it should be possible to write

function(x: string|string[]) {
    if (x instanceof Array) {
       A(x);
    else {
       B(x);
    }
}

[donnut]

Thanks @NoelAbrahams. I just tried the example:

function(x: string|string[]) {
    if (x instanceof Array) {
       A(x);
    else {
       B(x);
    }
}

with the latest master, but the compiler still errors. Can you confirm this?

[donnut]

See https://gist.github.com/donnut/64ad9017ea7b78da379d

[danquirk]

That would be #1719, the else block isn't getting a narrowed type by design at the moment.