Typescript does not identify when a property can be undefined due to a safe operator being used

[joel-thompson]

🔎 Search Terms

"safe navigation operator", "optional chaining"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about our issue.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgPICMBWyDeAoZZABzijgFsAuZAZzClAHMBuPAXzzzAE8iUNMqKADkA9mGQBeNFmQAfZAFcQAEwgxQEFazwAbCBJijR1AULFgdCUSBqj9AOl2jGACiOiA-A5JlyASmZkAHpg5B4+GgQGIgkAaxBRAHcacIALOAlfCmRrRV0VZHQUOgYQRmRRKCVVdU1CtOgIBwc8a1t7Zuc3D29s8gcaXWAkV39AkLCIiCiYiRo00XzC63IiXThQZEaoCAAaYlIc8jhuIpRlNQ0QLWQ4VWREiThaeiYHIA

💻 Code

interface Obj {
  param: string;
}

type ObjOrNot = Obj | undefined;

let foo: ObjOrNot;

console.log(foo?.param); // typescript knows that param could be string or undefined here.
console.log(foo?.param.slice()); // typescript should complain here, param may be undefined and not a string, but it says it is a string.

🙁 Actual behavior

typescript allows .slice() to be called on param

🙂 Expected behavior

typescript should give an error when calling .slice() on param because param may be undefined because of the safe navigation operator before it.

Additional information about the issue

Seems related to but still a bit different from this issue: #53872

[MartinJohns]

When foo is undefined, then the entire right side expression won't be called. That means param.slice() is never evaluated.

Try running your playground. It works just fine, even when foo is undefined.

[fatcerberus]

typescript knows that param could be string or undefined here.

Correction: It knows that the whole expression foo?.param can be string or undefined. However, ?. is short-circuiting so if foo is defined, foo.param is always a string, per the types. If foo is nullish then foo?.param.slice() doesn't even get evaluated.

edit: ninja'd 🥷

[RyanCavanaugh]

The behavior of ?. is admittedly counterintuitive; the expression a?.b.c is not the same as (a?.b).c

Regardless, TypeScript accurately models the runtime behavior here.

[joel-thompson]

interesting, apologies for mix up. I could have sworn I've run into issues around Cannot read properties of undefined similar to my example, but I can't reproduce it now so it must have been something else!