Don't require to implement optional abstract properties

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

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

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 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

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:

• 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.

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

Start by reproducing the linked TypeScript Playground examples and compare the current behavior for optional and non-optional abstract properties. Review related issues 6413 and 22939 for design context. Done means optional abstract properties can be omitted by subclasses while still supporting the implementation forms described in the issue, with existing required-property behavior unchanged.