Typescript: The Date constructor can't be passed string | Date, but takes either individually.

[johnfn]

TypeScript Version: 2.7.0-dev.20171209

Code

How come these are both error-free individually:

new Date(new Date())
new Date("123")

But this is a type error?

  new Date(Math.random() ? new Date() : "123");

Expected behavior:

I would expect the second code sample to not give a type error.

Actual behavior:

I get the following error:

Argument of type 'Date | "123"' is not assignable to parameter of type 'VarDate'.
  Type 'Date' is not assignable to type 'VarDate'.
    Property 'VarDate_typekey' is missing in type 'Date'.

I don't understand what VarDate is, so this is a little confusing.

[Jessidhia]

VarDate is a type that's only supposed to be present when you use --lib scripthost 🤔

Anyway, it seems that src/lib/es5.d.ts is missing a version of the constructor that accepts string | number, and src/lib/es2015.core.d.ts is missing a version that accepts Date | string | number. (new Date appears to not officially accept Date objects before ES2015; they do work because they go through ToPrimitive, which converts them to number, which is then processed, so maybe that should be unified with es5.d.ts?)

https://www.ecma-international.org/ecma-262/5.1/#sec-15.9.3.2

[chancancode]

I agree VarDate should not show up normally, but that issue seems relatively old and is unrelated to this bug.

Here is a repro.

This seems to be a regression in 2.6. You can see that:

1. It works in 2.4: https://travis-ci.org/tildeio/ts-repro-date/builds/326640424
2. It works in 2.5: https://travis-ci.org/tildeio/ts-repro-date/builds/326641134
3. It does not work in 2.6: https://travis-ci.org/tildeio/ts-repro-date/builds/326641480

Travis is having some issues currently so the actual tests doesn't run – but you can see that the third build fails at a much earlier step than the other two.

[Jessidhia]

Yep, that is because of an intentional change to make function signatures stricter in 2.6. It's not considered a regression in the compiler; the bug is that the signatures in lib.d.ts were not updated.

[mhegazy]

thanks @alan-agius4!

[DavidKDeutsch]

@Kovensky, could you post a link to where the motivation for this stricter behavior is discussed? I can't seem to find anything about it (it doesn't seem to be a bivariance thing, and I do not have --strictFunctionTypes enabled anyway), and I'd be interested to know what the motivation was.

[qJake]

I've got this issue as well, just updated VS to 15.6 which prompted to update TS to 2.6 and my build broke.

Is there a workaround or should I just revert to 2.5 until this is fixed?

EDIT: It seems that if you have something like:

var date = new Date(myOtherDate);

You can temporarily convert this to:

var date = new Date(myOtherDate.getVarDate());

And it will compile.

[mhegazy]

There are two sets of overloads for Date constructor. one in the ES6/S2015 lib, this was fixed by #21757. #21757 missed the set of overloads in ES5 though. this should be fixed by https://github.com/Microsoft/TypeScript/pull/23203.