Allow to add a call signature to the Mapped Type OR to remove all `Function.prototype` methods

Search Terms

add a call signature to the Mapped Type; remove all Function.prototype methods

Suggestion

Being able to create a callable type, but without Function.prototype methods.

Use Cases

Imagine one creates some API that exposes a function that is (because function is an object) also have some custom properties. This function is not supposed to be call-ed, apply-ed, bind-ed, etc, in fact all the Function.prototype methods are removed from it (e.g. with setPrototypeOf..null). However IDE will still suggest the Function.prototype methods.

Desired behavior:

let callableObject = () => 'foo'
Object.setPrototypeOf(callableObject, null)
callableObject.bar = 'baz'

callableObject() // 'foo'
callableObject // {bar: 'baz'}
callableObject.call // <---------- error

Current behavior:

let callableObject = () => 'foo'
Object.setPrototypeOf(callableObject, null)
callableObject.bar = 'baz'

callableObject() // 'foo'
callableObject // {bar: 'baz'}
callableObject.call // <---------- OK

What I tried:

type ExcludeFunctionPrototypeMethods<T extends () => any> = {
    [K in Exclude<keyof T, keyof Function>]: T[K]
}
// the type above "lacks a call signature" and I have no Idea how to add it there.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

Start with the callable-object examples and the ExcludeFunctionPrototypeMethods mapped type in the issue, then read the comment thread for prior discussion. Determine whether the type system can preserve a call signature while excluding Function.prototype members. Done means the proposed type can be expressed and produces an error for callableObject.call while retaining callableObject().