Distinguish between plain object types and class types

Search Terms

• plain object class type

Suggestion

Some way to distinguish an object type that only has Object in its prototype chain.

Use Cases

In this library, we use mapped types to convert immutable objects/arrays into their mutable representation. We never do this for class instances, so I need a way to skip the mapped type when an object type is actually a class type (in order to preserve readonly properties on class instances).

Examples

I have no suggestions for such syntax.

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. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

Related

https://stackoverflow.com/questions/53819550/distinguish-between-plain-objects-and-class-types#53819550

[aleclarson]

@DanielRosenwasser @ahejlsberg Can this be brought up in a design meeting soon? 🤞

[aleclarson]

Proposal B

Add a new Exact<T> type that forbids narrow class types.

Details

• When T is a literal type, the resolved type is T
• When T is Object, any object literal is allowed (but Object sub-classes are not)
• When T is Array, any array literal is allowed (but Array sub-classes are not)
• When T is Function, any function literal is allowed (but Function sub-classes are not)
• When T is any other class, only instances of T are allowed (no sub-classes or super-classes)

Example

class Foo extends Object {
  constructor(public a: number) {}
}

declare const testObject: (arg: Exact<Object>) => void
testObject(new Foo(1)) // 💥
testObject({ a: 1 }) // 👍
testObject(Object.create(null)) // 👍

declare const testArray: (arg: Exact<Array>) => void
class MyArray extends Array {}
testArray(new MyArray()) // 💥
testArray([]) // 👍

declare const testFunc: (arg: Exact<Function>) => void
class MyFunc extends Function {}
testFunc(new MyFunc()) // 💥
testFunc(() => 0) // 👍

declare const testFoo: (arg: Exact<Foo>) => void
class Cart extends Foo {}
testFoo(new Foo(1)) // 👍
testFoo(new Cart(2)) // 💥

Related: #12936