`() => void` is assignable to `Record<string, any>`

[aaditmshah]

Bug Report

🔎 Search Terms

1. function
2. record
3. any
4. mapped type

🕗 Version & Regression Information

I see the bug in all the versions of TypeScript, from v3.3.3 to v5.0.2, and even in the Nightly version.

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

⏯ Playground Link

Playground link with relevant code

💻 Code

const x: Record<string, any> = () => {};

🙁 Actual behavior

I do not get any type error.

🙂 Expected behavior

I expected to get the following type error.

Type '() => void' is not assignable to type 'Record<string, any>'.
  Index signature for type 'string' is missing in type '() => void'.

[ahejlsberg]

This is working as intended. Any object type (which includes function types) is assignable to Record<string, any> by design. You can get the stricter checking you suggest by using Record<string, unknown>.

[aaditmshah]

Could you provide the rationale for this? It unclear to me why this would be the desired behavior.

[DanielRosenwasser]

It's because functions can have properties on them the same way that any other object can. Similarly, we (kind of unfortunately?) also allow functions to be assignable to the type { length: number }.

[typescript-bot]

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