Object.assign does not understand union types

[prettydiff]

🔎 Search Terms

Object.assign, union

🕗 Version & Regression Information

• I have only tried this in version 6.0.3

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBKkPrAgZ2A80C8UBEATA9gLbZQA+OYATvgB4jYDcAUAJYB2ilAZgIYDG0RCjgAbFsIKEoEGoja4kUAIKVKPEAB4UldgHMA2gF0AfFADeTKFahsehCAC4o2vc2sx0T2BATJU35gBfJiY+fDYUKB5VKCwAeQAjACsIPmAAOh4kJBZdNgAKS2sjABoiqwt3d1t7J2wkImgXNl1sMqrrbzqqWnpyqGCASgYgA

💻 Code

type type_test_type = "dom" | "proxy";
interface test_list_dom extends Array<string[]> {
    name: string;
    type: type_test_type;
}

const arr = Object.assign(
    [],
    {
        name: "some string",
        type: "proxy"
    }
);

🙁 Actual behavior

Type 'string' is not assignable to type 'type_test_type'.

🙂 Expected behavior

No issue.

Additional information about the issue

Yes, I can easily suppress the error by using: type: "proxy" as type_test_type but in all other cases of union types a matching value is recognized as a member of the union.

The provided code sample is simplified from the actual code in my repository generating the issue. I cannot generate this issue from either code in the TypeScript Playground.

[RyanCavanaugh]

TypeScript doesn't have hardcoded semantics for Object.assign, it's just a regular function like one you could write in usercode.

I think you couldn't repro in the Playground because you were missing a type annotation on arr. The correct workaround in this case would be to add as const:

const arr: test_list_dom = Object.assign(
    [],
    {
        name: "some string",
        type: "proxy"
    } as const
);