Incorrect Object.assign return type

[AndreyKuznetsovQS]

Bug Report

🔎 Search Terms

object.assign return type

🕗 Version & Regression Information

Faced this problem in the 4.3.2 version and checked on 4.3.5, 4.4.0-beta(problem still exists)

⏯ Playground Link

Playground link with relevant code

💻 Code

type SomeData = { a: number };

const getSomeData = (): SomeData | undefined => {
  return undefined;
}

// expected data type: SomeData | undefined
// actual data type: SomeData
const data = Object.assign({}, getSomeData())

🙁 Actual behavior

Incorrect type inference

🙂 Expected behavior

Correct type inference

[MartinJohns]

// expected data type: SomeData | undefined

Why would you expect undefined? That can never be the result. I'd rather expect something like { a?: number }.

[jcalz]

The return type of Object.assign() is modeled as the intersection of the types of the input values, which is a rough approximation of what actually happens. It's a simple heuristic and is good enough to be useful, but there are edge cases (see #31982, #44983, #45034, #43110). It is possible to get a better approximation, but at the expense of being a lot more complicated and so far nobody has decided it's worth it to do that.

In this example, the compiler sees Object.assign({}, undefined) as returning {} & undefined which is never, while what actually gets returned is of type {} (not undefined, as @MartinJohns points out). So the right return type for Object.assign({}, getSomeData()) is something like SomeData | {} (or possibly Partial<SomeData> since you know that the value {} has no a property).

In the likely case that this will be closed as a design limitation, you should probably just work around it by asserting it like Object.assign({}, getSomeData()) as Partial<SomeData>.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.