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In which we describe the module system and metadata input.

Metadata Format#

In order to provide a directory or zipfile to Hermes, it must contain a metadata.json file at the root.

The metadata.json file must be a JSON object, with the following fields:

  • segments
    • An object with string keys representing integers, which are the segments IDs. The IDs are generally sequential but there can be gaps (ex. having IDs "6" and "8" and no "7"). ID zero has special meaning, this is the "main segment" that gets loaded on startup (ex. when React Native starts).
    • Each value must be an array containing file paths of the files to place into that segment, expressed relative to the root of the ZIP file (ex. subdir/foo.js). File paths may include ./ at the start, but they are not required to.
    • The first element of segments["0"] is the first module required at run time.
  • resolutionTable (Optional)
    • An object for which the keys are relative file names (the same file names as in segments).
    • Values are objects which map from strings given to require() to their actual resolved file path relative to the directory or zip file root. These relative paths should be the same as in segments, with leading ./.

Example Metadata File#

"segments": {
"0": [
"resolutionTable": {
"./cjs-subdir-main.js": {
"foo": "./foo/cjs-subdir-foo.js"
"./foo/cjs-subdir-foo.js": {
"bar": "./bar/cjs-subdir-bar.js"

Require system#

Modules can require each other using the require function provided as a local variable by the runtime. The argument to require must be an absolute path which root is the root of the ZIP file or input directory. For example:

const Foo = require('/subdir/foo.js');

Source maps#

Each JavaScript file can optionally provide a corresponding source map, which is the name of the source file with the suffix .map. For example subdir/foo.js might provide a subdir/ file.

Complete design#

Module mode is currently activated via passing -commonjs to Hermes while compiling. Hermes can then be given a directory and a metadata.json file, or a simple list of files with the first being the entry point.

First, each file is parsed just like any other JS file. Then, a call to hermes::wrapCJSModule puts each file in its own "module function", which binds exports, require, module as parameters.

IR generation relies on this wrapping of the AST. Every CJS module file generates IR into a shared hermes::Module (it's called M in generateIRForSourcesAsCJSModules). Each of these generated CJS modules are now hermes::Function *s. These are registered in the cjsModules_ field using the hermes::Module::addCJSModule function, which stores information in the CJSModule struct regarding filename, id, and IR function. This allows lookup of the CJS modules either via hermes::Function * or by string literal (file path).

If the caller of the Hermes CLI passes -static-require, then we attempt to resolve all require calls at compilation time. This occurs in ResolveStaticRequire.cpp, which is able to resolve files if they were povided by the user at invocation time and if all require calls only take string literals as arguments. If every require call is able to be resolved, every one of these require calls is replaced with a call to HermesBuiltin_requireFast with an ID for the CJS module, and that function does no string work and is therefore very fast.

After all CJS modules have generated IR and all the requires have been resolved, we generate one or more bytecode files from our IR::Module M. This requires two special bits of logic in hbc::generateBytecodeModule.

  • We add a mapping from a CJS module to the HBC function ID. This allows us to actually run require when JS execution demands it. If requires have been resolved, we add to the cjsModulesStatic_ table in the HBC file, else we add to to the cjsModules_ table in the HBC file (which maps from strings instead of IDs).
  • To accommodate bundle splitting, we also pass a SegmentRange to the function. This allows us to only compile the functions which are needed by the CJS modules in the segment. We then set a cjsModuleOffset_ field in the HBC file, which allows us to know how far into the complete set of CJS modules this segment is. Because every HBC file contains a contiguous set of CJS modules, each of which are unique, it is not possible for multiple segments to contain the same CJS module.

Having built 1 or more HBC files in the compiler, we can now execute them. The vm::Domain data structure is used for keeping track of all HBC files which were compiled together. In particular, the Domain owns a CJS module table. This table is an array of CJS modules, indexed by the CJS module's ID. If requires were not resolved at compile time, there's also a mapping from file path to that ID.

The first HBC file to be loaded must have a cjsModuleOffset_ of 0. CJSModule 0 is the entry point, and will be executed first. Loading of other segments is then done via a loadSegment call in the runtime; the user can call it via the ConsoleHost loadSegment or more commonly via the Hermes API. loadSegment does need a requireContext, which allows us to determine which Domain to actually load the new CJS modules from the segment into. This is registered into require.context (recall that require was bound as a param in all CJS modules).

Finally, we can require. When require is called, one of two things happens:

  • If we DID resolve all static requires, then calls to require were turned into Hermes builtin calls, so the actual value of requires (the parameter) is not used. We simply call HermesBuiltin_requireFast via the CallBuiltin instruction.
  • If we DID NOT resolve all static requires, we call the require function. The runtime will have set up that function to also pass along the base path (to allow relative require), so require is now a BoundFunction (as if we'd done require = require.bind(currentPath)). Note that this is transparent to the application developer, and doesn't change the way require is actually called in JS source. In either case, we do runRequireCall as implemented in require.cpp. It performs any necessary checks, calls the target CJS module, and caches and reads exports back.

Hermes also partially supports ECMAScript modules. Currently, the interop of ESM and CJS modules is defined simply by turning import into require calls. This precludes full correctness of the ESM module system, which requires live bindings; when a value is changed in the imported module, the local binding to that value must also be updated. Tests for the current emitted code can be found in test/hermes/esm/.

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