Object.assign not inferring

[dev-ardi]

🔎 Search Terms

typescript Object.assign

💻 Code

function foo() {
    const a = {a:1, b:2}
    const b = {}

    Object.assign(b,a)
    return b
}

🙁 Actual behavior

ReturnType<foo> is {} when

{
    a: number;
    b: number;
}

could be inferred

[MartinJohns]

Please remember to completely fill out the issue template.

This is working as intended. Object.assign() does not propagate type information to it's target property, so TypeScript is not aware of any changed type of b and the type remains {}.

[dev-ardi]

Shouldn't it?

[MartinJohns]

There's no mechanism in the language that allows this, and I believe analyzing such calls would be very error prone and hard on the performance. The only way to change the type of another variable like these are assertion functions, but these are not allowed to return anything (and Object.assign returns something).

A relevant issue for this would be #26792.

[fatcerberus]

The only way to change the type of another variable like these are assertion functions, but these are not allowed to return anything

I'm pretty sure there's a feature request for that, but I can't find it.

[whzx5byb]

I'm pretty sure there's a feature request for that, but I can't find it.

#40562

[typescript-bot]

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