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

See original GitHub issue

Search Terms

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


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) = 'baz'

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

Current behavior:

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

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


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)

Issue Analytics

  • State:open
  • Created 5 years ago
  • Reactions:11
  • Comments:7 (2 by maintainers)

github_iconTop GitHub Comments

fwh1990commented, Jul 10, 2020

I agree with @anurbol . Sometimes we have custom property on function like = 123, and we want user see this property from IDE tips, however, so many unexpected properties like apply | call | bind | arguments ... are shown, in other words is hidden from them, user is very hard to find custom things in a second.

@RyanCavanaugh Maybe this issue should not be closed.

nth-commitcommented, Sep 15, 2020


Trying to create a concise function builder, and having the interesting methods polluted by the function prototype.

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