Object.assign unexpectedly returns never type

[VVatashi]

Bug Report

🔎 Search Terms

Object.assign, intersection type, never type, undefined

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about intersection type and never type

⏯ Playground Link

Playground link with relevant code

💻 Code

I commonly use this pattern to "remove" unnecessary fields from an object in JS:

const foo = { a: 1, b: 2 };
const bar = Object.assign({}, foo, { b: undefined }); // Making a copy of 'foo' without field 'b'
bar.a = 1; // 'bar' should have at least 'a' field here, but TS gives error

🙁 Actual behavior

However, in TS the result of Object.assign was inferred to be of never type. So I got this error:

Property 'a' does not exist on type 'never'.
 The intersection '{ a: number; b: number; } & { b: undefined; }' was reduced to 'never' because property 'b' has conflicting types in some constituents.(2339)

Isn't this an error in definition of Object.assign function or in intersection type logic?

🙂 Expected behavior

I expected the result type to be something like { a: number; b?: number; }, { a: number; b: undefined; } or { a: number; }.

[MartinJohns]

See the second and third point of @jcalz comment: #43110 (comment)

You can use a type assertion as a workaround: Object.assign({}, foo, { b: undefined as undefined | number });

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.