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. The code for the Flow parser lives on GitHub.

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.

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. 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:

npm install --save-dev flow-parser

or using yarn:

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
  }
}