Using "instanceOf" seems to change the type of something to "never"

[dzearing]

TypeScript Version: 2.0.2

Code

This works:

let foo: boolean[] = [];
return !!(foo.splice);

The following fails on referring to "splice" because after the instanceof evaluation, the "foo" object changes from type boolean[] to type never, which doesn't have splice defined:

let foo: boolean[] = [];
return (foo instanceof Array) || !!(foo.splice);

Strangely, if I change the condition to AND instead of OR, we don't have any issues:

let foo: boolean[] = [];
return (foo instanceof Array) && !!(foo.splice);

Or, if I change the order of evaluation, that works as well:

let foo: boolean[] = [];
return !!(foo.splice) || (foo instanceof Array);

Note, this isn't something specific to arrays. You can change the boolean[] to HTMLElement, same sort of problem.

Expected behavior:
No errors in this case.

Actual behavior:
Property 'splice' does not exist on type 'never'.

[dzearing]

This seems very related to #10934

However I'm not sure completely. This seems very buggy and unexpected because I do not expect foo to change typings after a comparison. Code that was previously compiling fine is no longer compiling.

[ahejlsberg]

@dzearing This is effectively the type checker telling you that the particular expression will never execute.

let foo: boolean[] = [];
return (foo instanceof Array) || !!(foo.splice);

In the foo instanceof Array check, the type checker narrows the type of foo to only the subset that are arrays. That turns out to be all possible values of foo, so foo has type boolean[] in the true branch. But in the false branch the type checker narrows to the subset that aren't arrays. That turns out to be nothing, so foo has type never in the false branch.

[dzearing]

Oh man, awesome! I see it now! It really is "never". :) Thanks Anders.