Enforce definite initialization of `static` members

[bcherny]

Is there some clever reason why TSC can't check this (maybe... related to semantics of JS inheritance?)?

TypeScript Version: 3.2.0-dev.20181011

Search Terms: strictPropertyInitialization, static, initialized, uninitialized, assigned, property

Code

class A {
  static s: number
  a() {
    A.s * 3 // Should be a compile error
  }
}
A.s * 7 // Should be an compile error

Expected behavior: Both lines should produce compile errors.

Actual behavior: Both lines result in runtime exceptions.

Playground Link

Related Issues:

• Suggestion/feature request - enforce static properties/fields  #21976

Is the suggestion that we should always error on a static field with no initializer?

[bcherny]

@Andy-MS Would it be possible to error when I use a static field that hasn't been definitely assigned? Or, if this isn't a common enough pattern to support, then your suggestion is spot on. Either one feels safer than the current behavior to me.

Eg.

class A {
  static s: number
}

if (x) {
  A.s = 3
  // OK to read A.s in this branch
}
// NOT OK to read A.s in this branch

[jcalz]

Related stackoverflow question

I can't think of a decent workaround either. Any ideas?

[SlurpTheo]

Why were static properties specifically excluded from --strictPropertyInitialization?

[Yogu]

Is the suggestion that we should always error on a static field with no initializer?

Same rules as for non-static members should apply - if the type does not allow undefined, the property needs to be initialized, otherwise it would have a value that's not allowed.

@weswigham What kind of feedback do you await?

I just discovered a very dumb bug that would have been caught by this check:

export class JSONPreprocessorType implements MessagePreprocessorType {
    static contentType: 'application/json';
}

(It should have been an = instead of :.)

[Skalman]

I would prefer to have the error early on - at the site of the declaration. Usually you can't figure out the usage anyway (since it's in another file).

I recently had the same problem as @Yogu, and was thoroughly surprised by strictNullChecks not catching this problem.