hermes-parser

What is hermes-parser?

The hermes-parser npm package is a JavaScript parser that generates an Abstract Syntax Tree (AST) from JavaScript code. It is designed to be fast and efficient, making it suitable for use in development tools, linters, and other applications that need to analyze or transform JavaScript code.

What are hermes-parser's main functionalities?

Parsing JavaScript Code

This feature allows you to parse JavaScript code into an Abstract Syntax Tree (AST). The code sample demonstrates how to use the hermes-parser to parse a simple JavaScript statement and output the resulting AST.

const hermesParser = require('hermes-parser');
const code = 'const x = 42;';
const ast = hermesParser.parse(code);
console.log(JSON.stringify(ast, null, 2));

Customizing Parser Options

This feature allows you to customize the parser options, such as specifying the source type (script or module). The code sample shows how to parse JavaScript code as a module.

const hermesParser = require('hermes-parser');
const code = 'const x = 42;';
const options = { sourceType: 'module' };
const ast = hermesParser.parse(code, options);
console.log(JSON.stringify(ast, null, 2));

Handling Syntax Errors

This feature demonstrates how to handle syntax errors during parsing. The code sample shows how to catch and handle a syntax error when parsing invalid JavaScript code.

const hermesParser = require('hermes-parser');
const code = 'const x = ;';
try {
  const ast = hermesParser.parse(code);
} catch (error) {
  console.error('Syntax error:', error.message);
}

Other packages similar to hermes-parser

acorn

Acorn is a small, fast, JavaScript-based parser that generates an AST. It is highly modular and can be extended with plugins. Compared to hermes-parser, Acorn is more widely used and has a larger ecosystem of plugins and tools.

esprima

Esprima is a high-performance, standard-compliant ECMAScript parser. It is known for its accuracy and reliability in parsing JavaScript code. Esprima is similar to hermes-parser in terms of functionality but is more established and has been used in many popular projects.

README

hermes-parser

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

API

The Hermes 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.
• 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.
• 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".
• tokens: boolean, defaults to false. If true, add all tokens to a tokens property on the root node.