strictPropertyInitialization does not require initializer

[arv]

Bug Report

If I have strictPropertyInitialization set to true in tsconfig.json I expect that it would be an error if there is no initializer for a property.

🔎 Search Terms

strictPropertyInitialization
undefined
initializer

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

class C {
  // @ts-expect-error Missing initializer
  x: number | undefined;
}

🙁 Actual behavior

No compile error

🙂 Expected behavior

I expect there to be a compile error because instances of this class will not have an x property.

Always initializing properties in a fixed order is important to ensure all instances end up with the sam hidden class in a lot of JS VMs.

[whzx5byb]

Per #20075:

Strict property initialization checking verifies that each instance property declared in a class either (a) has a type that includes undefined or (b) has an explicit initializer or an assignment to the property in the constructor.

So it seems like the intended behavior.

[fatcerberus]

Also worth noting that in ES-compliant code (i.e. useDefineForClassFields: true, or any target that supports native classfields), the x property will be created with the value undefined on all instances.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.