Doesn't work type checking for function arguments

[iKBAHT]

TypeScript Version: 1.8.0 / nightly (2.0.0-dev.201xxxxx)

Code

function fn(options: { a?: number, b?: boolean }) {
    return options.a || 1;  
}

fn('foo');

Expected behavior:
Must be an error in

fn('foo');

Actual behavior:
No errors, TS compiler say "OK".

[nippur72]

I guess what's happening here: since a and b are optional, the resulting type is similar to {}. Now {} in TypeScript is a type with no members whatsoever, so anything will cast to it.

Indeed it's possible to write:

'foo' as {}; // ok

[yortus]

As @nippur72 points out, this is a consequence of function argument bivariance, where if a parameter expects a type X, then you can pass a subtype or a supertype of X with no errors.

In this case, the type string satisfies the requirements of {a?: number, b?: boolean}, since no particular properties are required to do so. It is thus a subtype and so due to bivariance, it's valid to pass a string as the options argument.

What you seem to want in this example is parameter invariance, i.e. the exact type with no extra props. TS doesn't support that.

[mhegazy]

An interface with all optional arguments is just like one that is empty. an empty interface matches anything, including strings. see https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-all-types-assignable-to-empty-interfaces for details.

[holiber]

It is not intuitive behavior. Flowjs works as expected https://flowtype.org/try/#0GYVwdgxgLglg9mABMMAKOAHWCDOAuRAb0QEMB+AsEAWwCMBTAJwBpFaK244AbekpAL4BKIgChG9KCEZJM2MDgB0JRAB9ViAIwBuRKIGjRKVAHJgXE0O1A

[Igorbek]

@mhegazy you right, however at the same time TS disallows "extra" properties:

function fn(options: { a?: number, b?: boolean }) {
    return options.a || 1;  
}

fn('foo'); // ok
fn({ c: 1; }); // error even though { c: number; } is assignable to { a?: number; b?: boolean; }

the reason you do this is to catch common errors, right? so must be done here.