Class static block does not allow forward reference to static method

[fwienber]

Bug Report

🔎 Search Terms

class static blocks, forward reference, incorrect compiler error

🕗 Version & Regression Information

4.4.2 + nightly (4.5.0-dev20210910)

• I was unable to test this on prior versions because the feature is new in 4.4

⏯ Playground Link

Playground link with relevant code

💻 Code

class A {
    static {
        A.doSomething();
    }

    static doSomething() {
       console.log("gotcha!");
    }
}

🙁 Actual behavior

Compiler error in line 3: "Property 'doSomething' is used before its initialization.(2729)"

The code works as-is in native JavaScript, tested in Firefox Developer Edition.

The rewrites for class static blocks in Babel as well as in TypeScript itself also work. Both execute the static block's code at a time where static methods of the class are already available.

🙂 Expected behavior

No compiler error, because everything's fine.

A forward reference to a static field, however, should be an error (I guess).

Workaround

Fall back to the way class static blocks could be emulated before they were supported:
Define an unused #-private static constant whose initializer is an immediate-evaluating function.

class A {
    static #_ = (() => {
        A.doSomething();
    })();

    static doSomething() {
       console.log("gotcha!");
    }
}

Playground link for workaround

Looks ugly, but works in the desired execution order and produces no compiler error.

Possibly related issues

• Execution order of static field initialization and static initialization blocks is not kept under target: esnext and useDefineForClassFields: false Execution order of static field initialization and static initialization blocks is not kept under target: esnext and useDefineForClassFields: false #45574
• Statements in class static blocks do not narrow types Statements in class static blocks do not narrow types #44949

[fwienber]

Any first evaluation / labels for this one? Maybe @rbuckton or @Zzzen?
I hope the example code makes the case quite obvious.

[fwienber]

Thank you for adding this to 4.6.0 milestone.
I was just wondering why this still has the "needs investigation" label. Do you doubt this is a bug? Do you need further elaboration?

[andrewbranch]

Issues are currently coming in faster than I can triage them. It looked like a probable bug to me but I know very little about class static blocks. If I were to take the time to be 100% confident that this is a bug, I would not have time to do the things that need to happen to release TypeScript 4.5. On the other hand, Ron will know if this is a bug within about 30 seconds. “Needs investigation” ~= “Looks suspicious but I defer to the assignee’s judgment, because they have more expertise than me, can devote more time to studying it than me, or both.” Sadly that doesn’t fit in a label.

[rbuckton]

Methods should be accessible. Static fields and static blocks are deferred until the end of ClassDefinitionEvaluation after static and prototype methods are defined (including private static elements).

[fwienber]

To motivate that this should really be included in the next milestone: It's even worse!
I just found out that from a static code block, you cannot even reference a constant defined before the class:

const FOO = "FOO";
class Bar {
    static {
        console.log(FOO); // Variable 'FOO' is used before being assigned.(2454)
    }
}

Run in TypeScript Playground

This results in the unjustified compile error "Variable 'FOO' is used before being assigned.(2454)", but if you still run it, works as it should, logging "FOO".
For this additional case, I can't even think of a work-around, which makes this bug far more serious.

[Zzzen]

You are probably the first person in the world who use class static block with TypeScript.