erasablefy

erasablefy

Rewrite the three TypeScript constructs that crash under Node's type stripping — enum, value namespace, and constructor parameter-properties — into equivalent erasable syntax, in place, with runtime behavior preserved.

npx erasablefy --write "src/**/*.ts"

The problem this solves

Node 24 LTS runs .ts files directly by stripping the types out. Node 26 went further and removed --experimental-transform-types (nodejs/node#61803) — the flag that used to compile enums and parameter-properties. There is no replacement and none is planned.

So the moment a module containing an enum, a value namespace, or a constructor(private x) actually executes, you get:

SyntaxError [ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX]: Enum is not supported in strip-only mode

It passes tsc. It passes install. It blows up at runtime, on the one code path that touches the construct. tsc --erasableSyntaxOnly (TS 5.8) will flag every offender, but the fix has been manual up to now. erasablefy is the fix.

Before / after

// before — throws ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX under node --strip-types
enum Dir { Up, Down, Left, Right }

class Point {
  constructor(public readonly x: number, private y = 5) {}
}

// after — plain runtime code, strips cleanly, same behavior
const Dir = {
  "Up": 0, "Down": 1, "Left": 2, "Right": 3,
  "0": "Up", "1": "Down", "2": "Left", "3": "Right",
} as const;
type Dir = 0 | 1 | 2 | 3;

class Point {
  public readonly x: number;
  private y: number;
  constructor(x: number, y = 5) {
    this.x = x;
    this.y = y;
  }
}

Note the reverse-mapping keys ("2": "Left"). Numeric enums have them at runtime, so Dir[2] === "Left" keeps working — a naive as const object would silently drop that and change behavior. erasablefy keeps it.

What it rewrites

Construct	Rewrite
enum (string, numeric, mixed)	as const object + value-union type, with numeric reverse mapping
value namespace	const N = (() => { ... return {...} })() — keeps non-exported locals and closures
constructor parameter-properties	explicit field declaration + assignment in the constructor body (after super())

What it refuses to touch (and tells you why)

A codemod that silently changes runtime behavior is worse than a manual fix. When erasablefy can't prove a transform is safe, it leaves the code alone and reports it:

• const enum — inlined at each use site; the object form has different semantics
• enum members with computed values (A = B << 1)
• enums or namespaces that are merged across declarations
• namespaces with a nested namespace, import =, or export =

Fix those by hand, or restructure and re-run.

Usage

# preview (default glob: src/**/*.ts)
npx erasablefy "src/**/*.ts"

# apply in place
npx erasablefy --write "src/**/*.ts"

# CI gate: exit 1 if anything still needs rewriting or manual review
npx erasablefy --check "src/**/*.ts"

Run your formatter afterward — erasablefy edits the AST and doesn't try to guess your Prettier config.

GitHub Action

Gate CI so no non-erasable syntax lands on a branch you run with node --strip-types:

# .github/workflows/erasable.yml
name: erasable-check
on: [push, pull_request]
jobs:
  check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: fernforge/erasablefy@v1
        with:
          globs: "src/**/*.ts"

Programmatic API

import { transformText, transformFiles } from "erasablefy";

const { code, result } = transformText(`enum E { A, B }`);
// result.changes  -> [{ kind: "enum", name: "E", line: 1 }]
// result.skips    -> [{ kind, name, line, reason }]

Scope

This handles the deterministic, mechanical rewrites. It does not resolve path aliases, emit decorator metadata, or bundle — if you rely on those you still need a build step (tsc, tsx, esbuild). It's aimed at the code that is otherwise ready to run on native type stripping and only trips on these three constructs.

License

MIT

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