"Object is possibly null" After Non-Null Assertion

[ChayimFriedman2]

TypeScript Version: 3.7.x-dev.201xxxxx

Search Terms:

typescript object is possibly null after non null assertion
typescript object is possibly null after exclamation mark check

Code

const obj: { a: number } | null = null;

if (obj!.a) {
    console.log(obj.a);
}

Expected behavior:

Successful compilation, since we notify the compiler with non-null assertion at line 3 that obj is not null, then it's not null at line 4 too.

Actual behavior:

Error "Object is possibly null" at line 4 (console.log(obj.a)).

Playground Link: https://www.typescriptlang.org/play?ts=3.9.0-dev.20200328#code/MYewdgzgLgBCBGArAXDA3jAhqsBXAtvAKYBOMAvjAD4x4A2dMAvLbgwNwBQnAlgGYwAFAkQBCAHSYAlOk4x5MUJBB0i4uiADmwpJKldycmEA

Related Issues:

[nicojs]

I guess you meant to use the optional chaining here? https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#optional-chaining

const obj: { a: number } | null = Math.random() > .5? null : { a: 42 };

if (obj?.a) {
    console.log(obj.a);
}  

https://www.typescriptlang.org/play?ts=3.9.0-dev.20200328#code/MYewdgzgLgBCBGArAXDA3jAhqsBXAtvAKYBOMAvjAD4x4A2dMAvDALKZQAWAdCZmABMQ+ABQBKGAD4Y3AKwB+WrgYxUGbDAAsAJgoBuAFAGAlgDMYIhInndMEtAZhOYoSCDpFudEAHNLSWzFDckcYIA

[ChayimFriedman2]

@nicojs No. I mean non-null assertion.

I know that works with optional chaining operator, but that's not the case. It should produce no errors even with non-null assertion, since we let the compiler know (at line 3) that obj, not matter what you (= the compiler) knows about it, is NOT null. And since we don't assign null to obj between line 3 and 4, if in line 3 it can't be null - it can't be at line 4 too.

[nicojs]

I'm pretty sure the non-null assertion operator works as intended here. With non-null assertion you're effectively saying to TypeScript "I know it might be nullish, but I know better, please let me use it this one time". It doesn't interfere with control flow analysis.

Although this is not explicitly found in the handbook: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html#non-null-assertion-operator. Maybe adding this small example could help with that?

[ChayimFriedman2]

@nicojs What's the point? If obj is null, it'll crash. If not, why do we emit an error in the next statement?

[RyanCavanaugh]

See #9640

In general we don't do "if it didn't crash" reasoning since it's very difficult to fit into an efficient control flow analysis, which is really what you're asking for since the same reasoning would apply to this example:

const obj: { a: number } | null = null;

if (obj!.a) {
}
console.log(obj.a);