# flow-parser

> JavaScript parser written in OCaml. Produces ESTree AST

Latest version **0.331.0** (published 2026-09-04) · MIT license · 0 weekly downloads

## Install

```sh
npm install flow-parser
pnpm add flow-parser
yarn add flow-parser
bun add flow-parser
```

## Health

**Score 60/100 (C)** — status: active.

Positive: no vulnerabilities; recently updated; high maintenance score; popular repo.

Warnings: low downloads; no types; no esm support; pre 1.0.

## Facts

| | |
|---|---|
| Version | 0.331.0 |
| Published | 2026-09-04 |
| First published | 2015-02-05 |
| Weekly downloads | 0 |
| License | MIT |
| TypeScript types | none |
| Module format | CommonJS |
| Node | >=0.4.0 |
| Dependencies | 1 |
| Unpacked size | 1.7 MB |
| Known vulnerabilities | 0 |
| Install scripts | no |
| GitHub stars | 22283 |
| Author | Flow Team |
| Maintainers | mroch, gabelevi, nmote, flowtype, samwgoldman, avikchaudhuri, marcoww |

## Links

- npm: https://www.npmjs.com/package/flow-parser
- Repository: https://github.com/facebook/flow
- Homepage: https://flow.org
- Issues: https://github.com/facebook/flow/issues
- npm.io page: https://npm.io/package/flow-parser

## Dependencies (1)

- [flow-estree](https://npm.io/package/flow-estree.md) 0.331.0

## Recent versions

- 0.331.0 (latest) — 2026-09-04
- 0.330.0 — 2026-08-31
- 0.329.0 — 2026-08-22
- 0.328.0 — 2026-08-14
- 0.327.0 — 2026-08-12
- 0.326.0 — 2026-08-05
- 0.325.0 — 2026-07-30
- 0.324.0 — 2026-07-24
- 0.323.0 — 2026-07-20
- 0.322.0 — 2026-07-08
- 0.321.0 — 2026-06-24
- 0.320.0 — 2026-06-21
- 0.319.0 — 2026-06-17
- 0.318.0 — 2026-06-10
- 0.317.0 — 2026-06-03
- … 417 more at https://npm.io/package/flow-parser/versions

## README

# flow-parser

A JavaScript parser built from the Flow's parser compiled to WebAssembly. Can parse ES6, Flow, and JSX syntax.

## What is Flow

See [flow.org](https://flow.org/). The code for the Flow parser [lives on GitHub](https://github.com/facebook/flow/tree/master/src/parser).

## What is the Flow Parser

The Flow Parser is a JavaScript parser for Flow syntax. It produces an AST that conforms to the [ESTree spec](https://github.com/estree/estree).

## API
The Flow parser exposes a single `parse(code, [options])` function, where `code` is the source code to parse as a string, and `options` is an optional object that may contain the following properties:
- **babel**: `boolean`, defaults to `false`. If `true`, output an AST conforming to Babel's AST format. If `false`, output an AST conforming to the ESTree AST format.
- **allowReturnOutsideFunction**: `boolean`, defaults to `false`. If `true`, do not error on return statements found outside functions.
- **assertOperator**: `boolean`, defaults to `false`. If `true`, enable Flow's postfix non-null assertion syntax (`value!`).
- **flow**: `"all"` or `"detect"`, defaults to `"detect"`. If `"detect"`, only parse syntax as Flow syntax where it is ambiguous whether it is a Flow feature or regular JavaScript when the `@flow` pragma is present in the file. Otherwise if `"all"`, always parse ambiguous syntax as Flow syntax regardless of the presence of an `@flow` pragma. For example `foo<T>(x)` in a file without an `@flow` pragma will be parsed as two comparisons if set to `"detect"`, otherwise if set to `"all"` or the `@flow` pragma is included it will be parsed as a call expression with a type argument.
- **enableExperimentalComponentSyntax**: `boolean`, defaults to `true`. Controls parsing of Flow component syntax.
- **enableExperimentalFlowMatchSyntax**: `boolean`, defaults to `true`. Controls parsing of Flow match syntax.
- **enableExperimentalFlowRecordSyntax**: `boolean`, defaults to `true`. Controls parsing of Flow record syntax.
- **reactRuntimeTarget**: `"18"` or `"19"`, defaults to `"18"`. Controls how component `ref` parameters are lowered when `babel` is `true`. React 18 treats `ref` as a separate parameter; React 19 treats it as a regular prop.
- **sourceFilename**: `string`, defaults to `null`. The filename corresponding to the code that is to be parsed. If non-null, the filename will be added to all source locations in the output AST.
- **sourceType**: `"module"`, `"script"`, or `"unambiguous"` (default). If `"unambiguous"`, source type will be automatically detected and set to `"module"` if any ES6 imports or exports are present in the code, otherwise source type will be set to `"script"`.
- **throwOnParseErrors**: `boolean`, defaults to `true`. If `true`, throw a `SyntaxError` for the first parse error. If `false`, return the partial AST with parse errors in the root node's `errors` property.
- **tokens**: `boolean`, defaults to `false`. If `true`, add all tokens to a `tokens` property on the root node.
- **transformOptions**: `object`. Options to be supplied to relevant transforms.
  - `TransformEnumSyntax`
    - `enable`: `boolean` - Whether to enable the transform. By default, `false`.
    - `getRuntime` (optional): `() => Expression` - The expression which should be a reference to the Flow Enums runtime. By default, is `require('flow-enums-runtime')`.

## Babel plugin

Flow parser plugin for [Babel](https://babeljs.io/). This plugin switches Babel to use `flow-parser` instead of the `@babel/parser`. The Flow parser uses Rust compiled to WASM for full Flow syntax support.

### Install

Using npm:

```sh
npm install --save-dev flow-parser
```

or using yarn:

```sh
yarn add flow-parser --dev
```

### Usage

The plugin can be enabled via:

```
// babel.config.json
{
  "plugins": ["flow-parser/babel-plugin"]
}
```

If parser options need to be provide you can do so via the `parserOpts` config:

```
// babel.config.json
{
  "plugins": ["flow-parser/babel-plugin"],
  "parserOpts": {
    "allowReturnOutsideFunction": true
  }
}
```

---
_Source: https://npm.io/package/flow-parser · Machine-readable twin of the npm.io package page. Health data is recomputed on every publish._