fast-flow-transform

0.0.3 verified Mon Apr 27 auth: no javascript

A native Hermes/FFT-based Flow type stripper built on napi-rs, providing programmatic transform, CLI, and bundler adapters for webpack, rspack, rsbuild, Parcel, Vite, Rollup, Rolldown, and esbuild. Current version 0.0.3 (pre-1.0, rapid iteration). Differentiator: uses the battle-tested Hermes parser (same as Metro/React Native) for speed and correctness, supports source map merging, removes empty type-only value imports automatically, and offers a 'preserve' format mode (experimental) that keeps whitespace structure. Compared to alternatives like babel-plugin-flow, it is purpose-built for stripping Flow types without transforming JSX or other syntax.

Common errors

error Error: Cannot find module 'flow-enums-runtime' ↓

cause Generated code imports flow-enums-runtime when Flow enums are lowered, but it is not installed.

fix

npm install flow-enums-runtime or add to dependencies.

error TypeError: (0 , fast_flow_transform_1.default) is not a function ↓

cause Named import { transform } used instead of default import.

fix

Use import transform from 'fast-flow-transform' or const transform = require('fast-flow-transform').

error SyntaxError: Unexpected token (1:16) - Flow syntax not stripped ↓

cause transform was called without waiting for promise (missing await) or source was not passed correctly.

fix

Ensure transform is awaited: const result = await transform({...}).

error Error: Could not resolve 'fast-flow-transform' ↓

cause Package not installed or mismatched version in package.json.

fix

npm install fast-flow-transform@latest

Warnings

breaking Source map merge does not preserve upstream sourcesContent or custom metadata fields. ↓

fix If you need full source map chain fidelity, work with babel-plugin-flow or wait for a future release.

gotcha Flow enums lower to flow-enums-runtime package. Generated code imports it at runtime. ↓

fix Install flow-enums-runtime as a production dependency if your project uses Flow enums.

gotcha Empty type-only value imports are removed. This may break code expecting side-effect imports that are actually type-only. ↓

fix If you need to retain an import that appears type-only but has runtime side effects, mark it with a comment or restructure imports.

deprecated reactRuntimeTarget option only affects Flow component lowering. It does not change JSX handling. ↓

fix Use reactRuntimeTarget only when using Flow component syntax with React 18 vs 19. For pure type stripping, omit it.

gotcha Preserve format mode is experimental. Output may differ from input whitespace. ↓

fix Stick to 'compact' or 'pretty' for production. Test preserve mode thoroughly.

Install

npm install fast-flow-transform

yarn add fast-flow-transform

pnpm add fast-flow-transform

Imports

transform

wrong

import { transform } from 'fast-flow-transform';

correct

import transform from 'fast-flow-transform';

Default export only. Named export does not exist.

transform (CJS)
wrong
```
const { transform } = require('fast-flow-transform');
```
correct
```
const transform = require('fast-flow-transform');
```
CommonJS require returns the default export directly, not an object with a transform property.
TypeScript types
wrong
```
import { TransformOptions } from 'fast-flow-transform';
```
correct
```
import type { TransformOptions } from 'fast-flow-transform';
```
Types are exported as value-like symbols; use `import type` for type-only imports to avoid runtime side-effects.

Quickstart

Shows basic programmatic usage: importing the default transform function, passing a Flow source string with filename and sourcemap enabled, and logging the stripped output.

import transform from 'fast-flow-transform';

const { code, map } = await transform({
  filename: '/path/to/input.js',
  source: `
    // @flow
    function greet(name: string): string {
      return 'Hello, ' + name;
    }
  `,
  sourcemap: true,
  comments: true,
});

console.log(code);
// Output (stripped): function greet(name) { return 'Hello, ' + name; }
console.log(map); // Source map object