Optional Chaining Edge Case

[awjreynolds]

Bug Report

Optional chaining is allowing undefined to be assigned to a variable that does not allow undefined.

🔎 Search Terms

optional chaining

🕗 Version & Regression Information

3.8.3
4.2.3
4.3.0

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.3.0-beta#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

💻 Code

const test2: string[] = test[0]?.map(name => name);
console.log(test2);

🙁 Actual behavior

No errors reported.

🙂 Expected behavior

test2 assignment should be flagged as an error as optional chaining can return undefined.

[whzx5byb]

test[0]?.map(name => name) is equivalent to test[0].map(name => name) here, because test[0] is always string[].

To produce the expected behavior, set noUncheckedIndexedAccess to true.

[awjreynolds]

test is an empty array and test[0] is undefined. The optional chaining stops at that point and returns undefined. test2: string[] | undefined = test[0]?.map(name => name) is the only way this should not be an error.

[whzx5byb]

test is an empty array and test[0] is undefined. The optional chaining stops at that point and returns undefined. test2: string[] | undefined = test[0]?.map(name => name) is the only way this should not be an error.

The compiler doesn't know test is an empty array. It only knows test is of type string[][].
Also, any index access of T[] will return T without noUncheckedIndexedAccess.
So in your example, the type of test[0] is string[] instead of string[] | undefined.

[awjreynolds]

My issue is that Optional Chaining can return undefined. The following correctly identifies the error:

const test=  (): string[] | undefined => undefined;
const test2: string[] = test(); // error

What I'm saying is that if you use optional chaining for assignment then the result can be a type or undefined and this is not picked up by Typescript for this specific scenario.

[nmain]

Besides noUncheckedIndexedAccess, you might also be interested in #36672.

[awjreynolds]

I think that covers the issue and I can close this. I didn't search hard enough to find it.