Yagiz Nizipli
7 min readPerformance

Improving Node.js loader performance

This article is originally available at Sentry.engineering.

Node.js supports 2 different modules. EcmaScript and CommonJS modules. ES modules are the official standard for modules in JavaScript and they are supported by all modern browsers. CommonJS modules are the modules that Node.js uses by default. They are not supported by browsers and they are not the official standard. However, they are still widely used.

How does Node.js load the entry point?

In order to differentiate which loader to use, Node.js depends on several factors. The most important one is the file extension. If the file extension is .mjs, Node.js will use the ES module loader. If the file extension is .cjs, Node.js will use the CommonJS module loader. If the file extension is .js, Node.js will use the CommonJS module loader if the package.json file has "type": "commonjs" field (or simply doesn't have the type field). If the package.json file has "type": "module" field, Node.js will use the ES module loader.

This decision is made in lib/internal/modules/run_main.js file. You can see a simplified version of the code below:

const { readPackageScope } = require('internal/modules/package_json_reader')
function shouldUseESMLoader(mainPath) {
  // Determine the module format of the entry point.
  if (mainPath && mainPath.endsWith('.mjs')) {
    return true
  if (!mainPath || mainPath.endsWith('.cjs')) {
    return false
  const pkg = readPackageScope(mainPath)
  switch (pkg.data?.type) {
    case 'module':
      return true
    case 'commonjs':
      return false
    default: {
      // No package.json or no `type` field.
      return false

readPackageScope traverses the directory tree upwards until it finds a package.json file. Prior to the optimizations done on this post, readPackageScope calls an internal version of fs.readFileSync until it finds a package.json file. This synchronous call makes a filesystem operation and communicates with Node.js C++ layer. This operation has performance bottlenecks depending on the value/type it returns because of the cost of serialization/deserialization of data. This is why we want to avoid calling readPackage a.k.a. fs.readFileSync inside readPackageScope as much as possible.

How Node.js parses package.json?

By default, readPackage calls an internal version fs.readFileSync to read the package.json file. This synchronous call returns a string from Node.js C++ layer, which later gets parsed using V8's JSON.parse() method. Depending on the validity of this JSON, Node.js checks and creates an object that's required for the remaining of the loaders to perform. These fields are pkg.name, pkg.main, pkg.exports, pkg.imports and pkg.type. If the JSON has faulty syntax, Node.js will throw an error and exit the process.

The output of this function is later cached at an internal Map to avoid calling readPackageScope again for the same path. This cache is stored for the rest of the process lifetime.

Usage of package.json fields and the reader

Before we dive into what optimizations we can do, let's see how Node.js uses these fields. The common use cases in Node.js codebase for parsing and re-using package.json fields are:

  • pkg.exports and pkg.imports are used to resolve different modules according to your input.
  • pkg.main is used to resolve the entry point of the application.
  • pkg.type is used to resolve the module format of the file.
  • pkg.name is used if there is a self referencing require/import.

Additionally, Node.js supports an experimental version of Subresource Integrity checkwhich uses the result of this package.json to validate the integrity of the file.

The most important usage is that, for every require/import call, Node.js needs to know the module format of the file. For example, if the user require's a NPM module that uses ESM on a CommonJS (CJS) application, Node.js will need to parse the package.json file of that module and throw an error if the NPM package is ESM.

Because of all of these calls and usages across ESM and CJS loaders, package.json reader is one of the most important parts of the Node.js loader implementation.


Optimizing caching layer

In order to optimize the package.json reader performance, I first moved the caching layer to the C++ side to make the implementation be closer to the filesystem call as much as possible. This decision forced to parse the JSON file in C++. At this point, I had 2 options:

  • Use V8's v8::JSON::Parse() method which takes a v8::String as an input and returns a v8::Value as an output.
  • Use simdjson library to parse the JSON file.

Since the filesystem returns a string, converting that string into a v8::String just to retrieve the keys and values as a std::string didn't make sense. Therefore, I added simdjson as a dependency to Node.js and used it to parse the JSON file. This change enabled us to parse the JSON file in C++ and extract and return only the necessary fields to the JavaScript side, reducing the size of the input that needs to be serialized/deserialized.

Avoiding serialization cost

In order to avoid returning unnecessary large objects, I changed the signature of the readPackage function to return only the necessary fields. This change simplified the shouldUseESMLoader as follows:

function shouldUseESMLoader(mainPath) {
  // Determine the module format of the entry point.
  if (mainPath && mainPath.endsWith('.mjs')) {
    return true
  if (!mainPath || mainPath.endsWith('.cjs')) {
    return false
  const response = getNearestParentPackageJSONType(mainPath)
  // No package.json or no `type` field.
  if (response === undefined || response[0] === 'none') {
    return false
  const { 0: type, 1: filePath, 2: rawContent } = response
  checkPackageJSONIntegrity(filePath, rawContent)
  return type === 'module'

Moving the caching layer to C++ enabled us to expose micro-functions that returns enums (integers) instead of strings to get a type of a package.json file.

Reducing C++ calls to 1 to 1

On CommonJS, readPackageConfig is implemented on the ESM loader under getPackageScopeConfig function. This function made a lot of C++ calls in order to resolve and retrieve the applicable package.json file. The implementation was as follows:

function getPackageScopeConfig(resolved) {
  let packageJSONUrl = new URL('./package.json', resolved)
  while (true) {
    const packageJSONPath = packageJSONUrl.pathname
    if (packageJSONPath.endsWith('node_modules/package.json')) {
    const packageConfig = packageJsonReader.read(fileURLToPath(packageJSONUrl), {
      __proto__: null,
      specifier: resolved,
      isESM: true,
    if (packageConfig.exists) {
      return packageConfig
    const lastPackageJSONUrl = packageJSONUrl
    packageJSONUrl = new URL('../package.json', packageJSONUrl)
    // Terminates at root where ../package.json equals ../../package.json
    // (can't just check "/package.json" for Windows support).
    if (packageJSONUrl.pathname === lastPackageJSONUrl.pathname) {
  const packageJSONPath = fileURLToPath(packageJSONUrl)
  return {
    __proto__: null,
    pjsonPath: packageJSONPath,
    exists: false,
    main: undefined,
    name: undefined,
    type: 'none',
    exports: undefined,
    imports: undefined,

To summarize, getPackageScopeConfig function calls C++ 3 times from the following functions:

  • new URL(...) calls internalBinding('url').parse() C++ method
  • path.fileURLToPath() calls new URL() if the input is a string
  • packageJsonReader.read() calls fs.readFileSync() C++ method

Moving this whole function to C++ enabled us to reduce the number of C++ calls to 1 to 1. This conversion also forced us to implement url.fileURLToPath() in C++.


The PR that contains these changes can be found on Github.

On a real-world Svelte application, the results showed 5% faster ESM execution. It also reduced the size of the cache stored by the loader by avoiding unnecessary fields.

❯ hyperfine 'node ../sveltejs-realworld/node_modules/vite/dist/node/cli.js --version' 'out/Release/node ../sveltejs-realworld/node_modules/vite/dist/node/cli.js --version' -w 10
Benchmark 1: node ../sveltejs-realworld/node_modules/vite/dist/node/cli.js --version
  Time (mean ± σ):     101.4 ms ±   0.6 ms    [User: 96.6 ms, System: 10.8 ms]
  Range (min … max):   100.3 ms … 102.5 ms    28 runs

Benchmark 2: out/Release/node ../sveltejs-realworld/node_modules/vite/dist/node/cli.js --version
  Time (mean ± σ):      96.3 ms ±   0.5 ms    [User: 90.9 ms, System: 10.1 ms]
  Range (min … max):    95.6 ms …  98.1 ms    30 runs

  out/Release/node ../sveltejs-realworld/node_modules/vite/dist/node/cli.js --version ran
    1.05 ± 0.01 times faster than node ../sveltejs-realworld/node_modules/vite/dist/node/cli.js --version

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