Return types of Object.assign

Bug Report

🔎 Search Terms

Object.assign

🕗 Version & Regression Information

4.3.2

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

⏯ Playground Link

Playground link with relevant code

💻 Code

type Foo = { bar: number }

// expected return type: Foo
// actual return type:   Foo & { bar: number }
const createFoo = (obj: Foo) =>
    Object.assign(obj, {
        bar: 1,
    })

// expected return type: String & { __html: string }
// actual return type:   string & { __html: string }
const markSafe = (html: string) =>
    Object.assign(html, {
        __html: html,
    })

🙁 Actual behavior

This may be 2 separate bugs, but I suspect they may be related.

1. For createFoo above, the returned type specifies & { bar: number }, even though { bar: number } is already part of the Foo type.
2. For markSafe above, the returned type specifies string, even though what is actually returned is a String object with the additional property added, not a primitive string (so, for example, markSafe('') !== markSafe('')).

🙂 Expected behavior

1. createFoo return type is Foo.
2. markSafe return type is String & { __html: string }.

[fatcerberus]

HTMLCanvasElement & { width: number, height: number } should be functionally equivalent to HTMLCanvasElement in every way - if it’s not, then that sounds like a bug.

[sirian]

I think assign<T, U>(target: T, source: U): T & U; signature is wrong. Probably it should be assign<T, U>(a: T, b: U): Omit<T, keyof U> & U;

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhjAvDA3gDwFwwIwF8DcAUKJLAEZKqYwBEAZiCDboceNDMJQPJkBWAU2BQAdHAgQAlgHMwACjgAaGGQCU+GAHpNnGJIhUsYAQDcBAJxaE4ItCKggAylHOSw0uesJlb9py7cPL2BfB2dXd08iQgATIQAbOHMBGDoAVzBhSXB4CRkwAB4AFWUAVQA+BSwSlSxS1SxuAFtJKGLlAGsBAE8QOhgKmAAyAeiSDhjKcSlZBWU1DW0YSf1DGGgI6RYY0P9NqNjfeIF3KAALfCA