0.15.1 • Public • Published


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


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.




DownloadsWeekly Downloads






Unpacked Size

962 kB

Total Files


Last publish


  • fb
  • flowtype
  • hermes-team