gleam-lang/gleam

Invalid TypeScript Declarations for property named `constructor` and other javascript keywords

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#5613 opened on Apr 16, 2026

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good first issuehelp wantedhigh priority

Description

Given this gleam code:

pub type Box {
  Box(constructor: Int)
}

And this gleam.toml:

name = "app"
version = "1.0.0"

target = "javascript"

[dependencies]
gleam_stdlib = ">= 0.44.0 and < 2.0.0"

[dev_dependencies]
gleeunit = ">= 1.0.0 and < 2.0.0"

[javascript]
typescript_declarations = true

The generated typescript declarations are invalid with the following error:

Similarly when you use other property names such as prototype or then, the generated typescript declarations are valid, however do not match the generated JavaScript classes.

For example:

pub type Box {
  Box(prototype: Int)
}

export class Box extends $CustomType {
  constructor(prototype) {
    super();
    this.prototype$ = prototype;
  }
}

export class Box extends _.CustomType {
  /** @deprecated */
  constructor(prototype: number);
  /** @deprecated */
  prototype: number;
}

Here the typescript autocompletion would suggest box.prototype even though it should be box.prototype$.

Maybe this is not important, because they are deprecated, but wanted to bring it up anyways.

This is on gleam 1.15.4

Contributor guide

Tech stack
rusttypescriptjavascript
Domain
backend
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.
1-3 hours
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
Rust programmingGleam compiler basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Investigate the TypeScript declaration generation code in the Gleam compiler. Look for where custom types and their fields are serialized into TypeScript declarations. Specifically, check how property names are emitted to avoid JavaScript reserved words like `constructor` and `prototype`. The generated JavaScript uses a `$` suffix for such names (e.g., `prototype$`), but the TypeScript declarations do not, causing mismatch. Ensure that the TypeScript declarations use the same mangled names as the JavaScript output.