Microsoft/TypeScript

static block on unnamed class produces invalid javascript

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#60,879 opened on Dec 30, 2024

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 (0 comments) (1 reaction) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: JS EmitHelp Wanted

Description

🔎 Search Terms

  • class
  • invalid
  • codegen
  • static

🕗 Version & Regression Information

  • This changed after version 4.4
  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about classes, static members of classes
  • I was unable to test this on prior versions because static blocks where introduced in typescript 4.4

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20241229#code/KYDwDg9gTgLgBAE2AMwIYFcA28DGnUDOBA3gFBwVwEyowCWOiwAthAFwB26zARsFAG5ylarQZwylKXBgALOgQB0SVnAC8cAIxCpAX1K6gA

💻 Code

export default class{
    static demo:number;
    static {
        this.demo = 1;
    }
}

🙁 Actual behavior

TypeScript produces an invalid output where the code references an undefined default_1 I'm assuming the empty class is meant to be assigned to this.

example output:

var _a;
class {
}
_a = default_1;
(() => {
    _a.demo = 1;
})();
export default default_1;

🙂 Expected behavior

I would expect typescript to assign the class to the generated default_1

Additional information about the issue

TypeScript has been exhibiting this behavior since support for static blocks was introduced as far as I can tell, prior to version 4.4.4 in the playground produces a different set of invalid code where it just moves the block outside of the class with no modification.

this still technically happens without the export default but typescript does have a warning.

Contributor guide

Tech stack
typescriptjavascript
Domain
backendtooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript compiler internalsstatic block emissionJavaScript code generation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate the TypeScript emitter code (likely src/compiler/emitter.ts or checker.ts) to understand how static blocks in unnamed classes are transformed. The bug produces invalid JavaScript referencing an undefined default 1. Look for how the compiler assigns a name to default exports and whether static block code accesses the correct variable. Check the git history for changes around version 4.4 that introduced static block support, as this bug has been present since then.