Microsoft/TypeScript

Declaration emit for private identifiers in class expressions is wrong

Open

#36,548 opened on Jan 31, 2020

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 (3 comments) (2 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: Declaration EmitHelp WantedRescheduled

Description

Code

const O = class { #g }

Expected behavior:

declare const O = class { #private }

Actual behavior:

declare const O: {
    new (): {
        "__#1@#g": any;
    };
};

Contributor guide

Tech stack
typescript
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
understanding of TypeScript's declaration emitfamiliarity with class expressions and private fields
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate the TypeScript compiler's declaration emitter, specifically how it handles private identifiers (#) in class expressions. The issue is in the emitter's logic for generating declare statements. Look at files like src/compiler/emitter.ts and related test cases for declaration emit. The expected output is `declare const O = class { #private }` but the actual output is a type literal with a mangled private field name. The fix may involve ensuring private identifiers are preserved in declaration emit for class expressions, similar to how they are handled in class declarations.