Repository: TypeScript
Issue: Don't require to implement optional abstract properties
Labels: Suggestion,Awaiting More Feedback

## Search Terms
abstract optional override

## Suggestion

If in abstract class abstract property is marked as optional, allow child classes not to implement it.
So I suggest to remove an error for property `x` of class `D` in the following code:

https://www.typescriptlang.org/play?ts=4.0.2#code/IYIwzgLgTsDGEAJYBthjAgggg3gKAUIQAcoB7CAU3koBMFRIZ4EpLhayA7ZATwQAeAfgBcCLgFcAtiEpQCRUhWpV6jaHERsO3Pgl5jJMuQgA+CCV1qUAZgEsudPAsLEJIZHdgJOAZSkQABYAFACUuC5EhLDcYGTIlAB0yGQA5sFBdmCJAgA0CJnZvKGRAL545XgoaBgAIgiUAlRWGNj4UUpUNPTanDz8-AC8CACMAAwVzo4A7gi1YYl+ASGhQA

```ts
abstract class A {
    protected abstract readonly x?: number
    protected abstract readonly y: number | undefined

    public doSmth() {
        console.log(this.x, this.y)
    }
}

class D extends A {
    protected readonly y = 10
}

new D().doSmth()
```

But property `y` still must be implemented.

Note that we already can do some sort of it

https://www.typescriptlang.org/play?ts=4.0.2#code/JYOwLgpgTgZghgYwgAgJLIN4Chm+QBygHt9owBPAYThAGUBrYfAfgC5kQBXAWwCNoceQiTLkAspwDOYVN3wAbCNwjh2XPtGQAfZJxAATCDFAR9g3OeTKwACyL7qdRiwAUASnYA3IsDN4rELb2EtKyCkoqYO5ePmYAvlhYcLzSUIhgyAjycJKSyACCmJbCpFAUjgxMbBw8-FCWyanpBMSlFCEycorKqjUaUNq6BkYmfniNYGkIGSWiHWHdkQAqRETV6nWJ-tZ2DjSVrh7I3r4NKZPNO8FSneE9UUcnY7gTUxlX+vNdEeAra9HHWJYBJYLI5PKUQoQAAekAMeUK2AA9AAqSwAOSIIAAtK9mmDcsgAOSQonIfRECB5EBEDLAb73ZCgGzQYCQfTIPHTAL9YkfL53SJkmDEbiZbKEon5IkAOhcACYAKwARkVbgxWNx5ze4vBxNJ5Mp1NpTIZkSZIBZUDZpk52suSjqfMCuwFi1+q2Fot1kulcqVqvV-kxOK5GQJeRJ0sNVI4JvpgvAFqtNo5YZ5TqJszK4huCx+YC9RDFEeJfoVKrVGtD9u5pajZIpsZpdLNSeZrPZdqa3OUvKzrTmebbYD+RZLEsj5YDVZRSOBiVLlHQCfdYDy6GRaP8lEn+tQZNACCIUCgEGm8nIpsT64tkFgiBQRIPMssS3IpH3h7y3GAuVAADmyC2CgMBEPI8hEAA7oBLQiGUwCxiKxbAR+T4Huw2btMON4ADQBEEnw4WuCoACzygADOqc7AkAA

```ts
interface I {
    propertyCanSkip?: number
    propertyMustImplement: number | undefined
    
    methodCanSkip?(): void
    methodMustImplement(): void
}

abstract class A {
    propertyCanSkip?: number
    abstract propertyMustImplement: number | undefined
    abstract propertyMustImplementToo?: number

    methodCanSkip?(): void
    abstract methodMustImplement(): void
    abstract methodMustImplementToo?(): void
}

class CA extends A {
/*
    Non-abstract class 'CA' does not implement inherited abstract member 'methodMustImplement' from class 'A'.(2515)
    Non-abstract class 'CA' does not implement inherited abstract member 'methodMustImplementToo' from class 'A'.(2515)
    Non-abstract class 'CA' does not implement inherited abstract member 'propertyMustImplement' from class 'A'.(2515)
    Non-abstract class 'CA' does not implement inherited abstract member 'propertyMustImplementToo' from class 'A'.(2515)
*/
}

class CI implements I {
/*
    Class 'CI' incorrectly implements interface 'I'.
    Type 'CI' is missing the following properties from type 'I': propertyMustImplement, methodMustImplement(2420)
*/
}
```

## But there is a set of problems for properties

We have two ways to override property (via property declaration or via getter and setter). And now (in TS4) the limitation have changed. For nonabstract property base class always defines how it should be implemented in children.

Let's look what implementations are possible (don't forget about `useDefineForClassFields` compiler flag that makes it more important):

Code | Property can be omitted | Child can implement as property | Child can implement as get/set
------------ | ------------- | ------------- | -------------
`propertyCanSkip?: number` | Yes | Yes | No
`abstract propertyMustImplement: number \| undefined` | No | Yes | Yes (except https://github.com/microsoft/TypeScript/issues/40632)
`abstract propertyMustImplementToo?: number` | No | Yes | Yes (except https://github.com/microsoft/TypeScript/issues/40632)
`get getter?(): number` | N/A | N/A | N/A
`abstract get getterMustImplement(): number \| undefined` | No | No | Yes
`abstract getterToo?(): number` | N/A | N/A | N/A

It's easy to see, that if the property should really be optional, there is only one way to make it such which will not allow to implement it as getter and setter. But we have 2 absolutely identical lines with optional and nonoptional abstract property. I see no sense for them to be synonyms as `?` in the 3rd line definitely says that the property _is optional_, but doesn't give me ability to make so in further code.

So I propose to change this table in following way:

Code | Property can be omitted | Child can implement as property | Child can implement as get/set
------------ | ------------- | ------------- | -------------
`propertyCanSkip?: number` | Yes | Yes | No
`abstract propertyMustImplement: number \| undefined` | No | Yes | Yes
`abstract propertyCanSkipToo?: number` | **Yes** | Yes | Yes
`get getter?(): number` | N/A | N/A | N/A
`abstract get getterMustImplement(): number \| undefined` | No | No | Yes
`abstract get getterCanSkipToo?(): number` | **Yes** | **No** | **Yes**

Abstract getter is NOT a part of this feature request, just shown for consistency.

## Use Cases

Provide ability to list and use for reading an optional property in abstract class without limiting a way of its implementation in child classes. Such [problem occured](https://stackoverflow.com/q/63937846/4928642) in a real project because of migration from TS3 to TS4. Before that it was possible, but because of breaking changes of TS4 it's not anymore.

https://www.typescriptlang.org/play?ts=4.0.2#code/IYIwzgLgTsDGEAJYBthjAgggg3gKAUIQAcoB7CAU3koBMEpLhayA7ZATwTAFsIALAPwAuBKwCuPEJSh4CRYuJDIAlrAQAzABQBKXPKJFYbMGWSUAdMjIBzLQJVgLvAToMIAvni94UaDABCCJQAHlSstBjY+IYu-AgAvAgAjAAM3nJ+6AgAwsFhlBFR+oY2lIhxurgM5eJQrAgAssACFjARZDxVXj5ZGAAi+eGRWPo+rJQA7ggBuhbabhPTOXMLeEsI-au6QA

```ts
abstract class A {
    protected readonly smth?: number

    public f() {
        console.log(this.smth)
    }
}

class B extends A {
    smth = 10
}

class C extends A {
    get smth() { return Math.random() }
}

class D extends A {
}

new B().f()
new C().f()
new D().f()
```

## Examples

See above.

## Checklist

My suggestion meets these guidelines:

* [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
* [x] This wouldn't change the runtime behavior of existing JavaScript code
* [x] This could be implemented without emitting different JS based on the types of the expressions
* [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
* [x] This feature would agree with the rest of [TypeScript's Design Goals](https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals).

## Why it's not a breaking change?

It changes behavior of existing construction, but it's not a breaking change in terms of code.

If you had your code working and you have

    abstract propertyMustImplementToo?: number

in it, that means that you implemented this property in all child classes. So after its meaning changes all you code keeps being valid and compiles into absolutely the same javascript code as before. **Nothing changed.**

At the same time, for further development you have to decide whether you want to allow child classes to skip the property or not. If yes, or you don't care - keep it with `?` as it is. If no then update it to

    abstract propertyMustImplementToo: number | undefined

without any other changes needed.

## Related Issues:
https://github.com/microsoft/TypeScript/issues/6413
https://github.com/microsoft/TypeScript/issues/22939