import-in-the-middle

import-in-the-middle is a module loading interceptor inspired by require-in-the-middle, but specifically for ESM modules. In fact, it can even modify modules after loading time.

Usage

The API for require-in-the-middle is followed as closely as possible as the default export. There are lower-level addHook and removeHook exports available which don't do any filtering of modules, and present the full file URL as a parameter to the hook. See the Typescript definition file for detailed API docs.

You can modify anything exported from any given ESM or CJS module that's imported in ESM files, regardless of whether they're imported statically or dynamically.

import { Hook } from 'import-in-the-middle'
import { foo } from 'package-i-want-to-modify'

console.log(foo) // whatever that module exported

Hook(['package-i-want-to-modify'], (exported, name, baseDir) => {
  // `exported` is effectively `import * as exported from ${url}`
  exported.foo += 1
})

console.log(foo) // 1 more than whatever that module exported

This requires the use of an ESM loader hook, which can be added with the following command-line option.

node --loader=import-in-the-middle/hook.mjs my-app.mjs

Since --loader has been deprecated you can also register the loader hook programmatically via the Node module.register() API. However, for this to be able to hook non-dynamic imports, it needs to be registered before your app code is evaluated via the --import command-line option.

my-loader.mjs

import * as module from 'module'

module.register('import-in-the-middle/hook.mjs', import.meta.url)

node --import=./my-loader.mjs ./my-code.mjs

When registering the loader hook programmatically, it's possible to pass a list of modules, file URLs or regular expressions to either exclude or specifically include which modules are intercepted. This is useful if a module is not compatible with the loader hook.

Note: This feature is incompatible with the {internals: true} Hook option

import * as module from 'module'

// Exclude intercepting a specific module by name
module.register('import-in-the-middle/hook.mjs', import.meta.url, {
  data: { exclude: ['package-i-want-to-exclude'] }
})

// Only intercept a specific module by name
module.register('import-in-the-middle/hook.mjs', import.meta.url, {
  data: { include: ['package-i-want-to-include'] }
})

Only Intercepting Hooked modules

Note: This feature is experimental and is incompatible with the {internals: true} Hook option

If you are Hook'ing all modules before they are imported, for example in a module loaded via the Node.js --import CLI argument, you can configure the loader to intercept only modules that were specifically hooked.

instrument.mjs

import { register } from 'module'
import { Hook, createAddHookMessageChannel } from 'import-in-the-middle'

const { registerOptions, waitForAllMessagesAcknowledged } = createAddHookMessageChannel()

register('import-in-the-middle/hook.mjs', import.meta.url, registerOptions)

Hook(['fs'], (exported, name, baseDir) => {
  // Instrument the fs module
})

// Ensure that the loader has acknowledged all the modules
// before we allow execution to continue
await waitForAllMessagesAcknowledged()

my-app.mjs

import * as fs from 'fs'
// fs will be instrumented!
fs.readFileSync('file.txt')

node --import=./instrument.mjs ./my-app.mjs

Development benchmarks

See benchmark/README.md for the PR benchmark method and how to check measurement noise locally.

Synchronous loader hooks

On Node.js versions that support module.registerHooks() the loader can run synchronously, on the application thread, instead of on the separate thread that module.register() uses. Running in-thread removes the message channel: Hook() registrations are visible to the loader directly, so the createAddHookMessageChannel / waitForAllMessagesAcknowledged step shown above is unnecessary.

module.registerHooks() was added in 22.15.0 / 24.0.0, but its synchronous load hook rejected the nullish CommonJS source the loader returns for require()s pulled into the ESM graph until nodejs/node#59929. The fix shipped in 22.22.3, 24.11.1, 25.1.0 and 26.0.0; earlier versions ship module.registerHooks() but cannot run the synchronous loader. Use supportsSyncHooks() to branch on this rather than a hand-written version check:

import { register, supportsSyncHooks } from 'import-in-the-middle/register-hooks.mjs'

if (supportsSyncHooks()) {
  register({ include: ['package-i-want-to-include'] })
} else {
  // Fall back to the asynchronous loader, e.g. module.register('import-in-the-middle/hook.mjs').
}

instrument.mjs

import { register } from 'import-in-the-middle/register-hooks.mjs'
import { Hook } from 'import-in-the-middle'

register({ include: ['package-i-want-to-include'] })

Hook(['package-i-want-to-include'], (exported, name, baseDir) => {
  // Instrument the module
})

node --import=./instrument.mjs ./my-app.mjs

register() accepts the same include / exclude options as the asynchronous loader and throws on a Node.js version where supportsSyncHooks() is false.

Custom matching with shouldInclude

Instead of include / exclude lists, you can pass a shouldInclude(url, specifier) predicate to decide which modules are intercepted. It is called for every resolved module with the resolved URL and the import specifier; return a truthy value to intercept the module. When a predicate is provided it takes over the decision and the include / exclude options are ignored.

This is useful when matching doesn't map cleanly onto bare specifiers, file URLs and regular expressions — for example a matcher built from your own configuration, or a decision that depends on more than the specifier.

import { register } from 'import-in-the-middle/register-hooks.mjs'

register({
  shouldInclude (url, specifier) {
    return specifier === 'package-i-want-to-include' ||
      url.includes('/node_modules/some-scope/')
  }
})

The predicate receives only the URL and the specifier, never a resolved file path. Because module.register() transfers its data to the loader thread by structured clone — which cannot carry a function — shouldInclude is supported for synchronous registration (register-hooks.mjs, shown above) and for predicates constructed on the loader thread; it is not accepted through the data option of the asynchronous module.register('import-in-the-middle/hook.mjs', ...).

Opting out of CJS source stripping

On Node 22+, when a CJS module is loaded through the ESM translator and another loader hook provides its source (instead of leaving it null for Node to read natively), require() calls inside that CJS module for packages using the "module-sync" exports condition fail with ERR_VM_MODULE_LINK_FAILURE. To work around this Node bug, import-in-the-middle strips hook-provided source for CJS modules in the synchronous require chain, forcing Node to use its native CJS loader which handles this correctly.

If you rely on a downstream loader hook seeing (and acting on) the source of CJS modules in the require chain, you can opt out of this workaround by passing disableCjsSourceStripping: true in the hook config. The workaround remains enabled by default.

import { register } from 'import-in-the-middle/register-hooks.mjs'

register({
  include: ['package-i-want-to-include'],
  disableCjsSourceStripping: true
})

This option is accepted by both synchronous (register-hooks.mjs) and asynchronous (module.register('import-in-the-middle/hook.mjs', ...)) registration.

TypeScript modules

On Node.js versions that strip TypeScript types natively (those exposing module.stripTypeScriptTypes(),

= 22.13.0 / >= 23.9.0 / >= 24.0.0), import-in-the-middle intercepts .ts, .mts and .cts modules just like their JavaScript counterparts. The types are stripped before the module's exports are read, so type-only exports (export type, export interface) are not present on the intercepted namespace; value exports are.

// math.mts
export type Op = '+' | '-'
export function add (a: number, b: number): number {
  return a + b
}

Hook(['./math.mts'], (exported) => {
  // `exported.add` is interceptable; the `Op` type is not part of the namespace
})

On Node.js versions where type stripping is not enabled by default, run with --experimental-strip-types. Older versions that predate module.stripTypeScriptTypes() leave TypeScript modules untouched.

Limitations

• You cannot add new exports to a module. You can only modify existing ones.
• While bindings to module exports end up being "re-bound" when modified in a hook, dynamically imported modules cannot be altered after they're loaded.
• Modules loaded via require are not affected at all.
• A module's set of export names is assumed to be stable for the lifetime of the process. import-in-the-middle reads a module's source once to lex its exports and reuses that export set on later loads of the same URL. An upstream loader that returns a different set of exports for the same URL across calls — a stateful codegen loader, or one that varies its output by context — is not supported and will be instrumented with the export set from its first load. Idempotent transforms (type stripping, AST instrumentation, minifiers) are unaffected, and the actual module still executes the source its real load returns; only the interceptable export names are memoized.