GeezerVison

Modules: Packages#

Introduction#

A package is a folder tree described by a package.json file. The package consists of the folder containing the package.json file and all subfolders until the next folder containing another package.json file, or a folder named node_modules.

This page provides guidance for package authors writing package.json files along with a reference for the package.json fields defined by Node.js.

Determining module system#

Introduction#

Node.js will treat the following as ES modules when passed to node as the initial input, or when referenced by import statements or import() expressions:

• Files with an .mjs extension.

• Files with a .js extension when the nearest parent package.json file contains a top-level "type" field with a value of "module".

• Strings passed in as an argument to --eval, or piped to node via STDIN, with the flag --input-type=module.

• Code containing syntax only successfully parsed as ES modules, such as import or export statements or import.meta, with no explicit marker of how it should be interpreted. Explicit markers are .mjs or .cjs extensions, package.json "type" fields with either "module" or "commonjs" values, or the --input-type flag. Dynamic import() expressions are supported in either CommonJS or ES modules and would not force a file to be treated as an ES module. See Syntax detection.

Node.js will treat the following as CommonJS when passed to node as the initial input, or when referenced by import statements or import() expressions:

• Files with a .cjs extension.

• Files with a .js extension when the nearest parent package.json file contains a top-level field "type" with a value of "commonjs".

• Strings passed in as an argument to --eval or --print, or piped to node via STDIN, with the flag --input-type=commonjs.

• Files with a .js extension with no parent package.json file or where the nearest parent package.json file lacks a type field, and where the code can evaluate successfully as CommonJS. In other words, Node.js tries to run such "ambiguous" files as CommonJS first, and will retry evaluating them as ES modules if the evaluation as CommonJS fails because the parser found ES module syntax.

Writing ES module syntax in "ambiguous" files incurs a performance cost, and therefore it is encouraged that authors be explicit wherever possible. In particular, package authors should always include the "type" field in their package.json files, even in packages where all sources are CommonJS. Being explicit about the type of the package will future-proof the package in case the default type of Node.js ever changes, and it will also make things easier for build tools and loaders to determine how the files in the package should be interpreted.

Syntax detection#

Node.js will inspect the source code of ambiguous input to determine whether it contains ES module syntax; if such syntax is detected, the input will be treated as an ES module.

Ambiguous input is defined as:

• Files with a .js extension or no extension; and either no controlling package.json file or one that lacks a type field.
• String input (--eval or STDIN) when --input-typeis not specified.

ES module syntax is defined as syntax that would throw when evaluated as CommonJS. This includes the following:

• import statements (but not import() expressions, which are valid in CommonJS).
• export statements.
• import.meta references.
• await at the top level of a module.
• Lexical redeclarations of the CommonJS wrapper variables (require, module, exports, __dirname, __filename).

Module resolution and loading#

Node.js has two types of module resolution and loading, chosen based on how the module is requested.

When a module is requested via require() (available by default in CommonJS modules, and can be dynamically generated using createRequire() in both CommonJS and ES Modules):

• Resolution:
  • The resolution initiated by require() supports folders as modules.
  • When resolving a specifier, if no exact match is found, require() will try to add extensions (.js, .json, and finally .node) and then attempt to resolve folders as modules.
  • It does not support URLs as specifiers by default.
• Loading:
  • .json files are treated as JSON text files.
  • .node files are interpreted as compiled addon modules loaded with process.dlopen().
  • .ts, .mts and .cts files are treated as TypeScript text files.
  • Files with any other extension, or without extensions, are treated as JavaScript text files.
  • require() can only be used to load ECMAScript modules from CommonJS modules if the ECMAScript module and its dependencies are synchronous (i.e. they do not contain top-level await).

When a module is requested via static import statements (only available in ES Modules) or import() expressions (available in both CommonJS and ES Modules):

• Resolution:
  • The resolution of import/import() does not support folders as modules, directory indexes (e.g. './startup/index.js') must be fully specified.
  • It does not perform extension searching. A file extension must be provided when the specifier is a relative or absolute file URL.
  • It supports file:// and data: URLs as specifiers by default.
• Loading:
  • .json files are treated as JSON text files. When importing JSON modules, an import type attribute is required (e.g. import json from './data.json' with { type: 'json' }).
  • .node files are interpreted as compiled addon modules loaded with process.dlopen(), if --experimental-addon-modules is enabled.
  • .ts, .mts and .cts files are treated as TypeScript text files.
  • It accepts only .js, .mjs, and .cjs extensions for JavaScript text files.
  • .wasm files are treated as WebAssembly modules.
  • Any other file extensions will result in a ERR_UNKNOWN_FILE_EXTENSION error. Additional file extensions can be facilitated via customization hooks.
  • import/import() can be used to load JavaScript CommonJS modules. Such modules are passed through merve to try to identify named exports, which are available if they can be determined through static analysis.

Regardless of how a module is requested, the resolution and loading process can be customized using customization hooks.

package.json and file extensions#

Within a package, the package.json "type" field defines how Node.js should interpret .js files. If a package.json file does not have a "type" field, .js files are treated as CommonJS.

A package.json "type" value of "module" tells Node.js to interpret .js files within that package as using ES module syntax.

The "type" field applies not only to initial entry points (node my-app.js) but also to files referenced by import statements and import() expressions.

// my-app.js, treated as an ES module because there is a package.json
// file in the same folder with "type": "module".

import './startup/init.js';
// Loaded as ES module since ./startup contains no package.json file,
// and therefore inherits the "type" value from one level up.

import 'commonjs-package';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".

import './node_modules/commonjs-package/index.js';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".
js

Files ending with .mjs are always loaded as ES modules regardless of the nearest parent package.json.

Files ending with .cjs are always loaded as CommonJS regardless of the nearest parent package.json.

import './legacy-file.cjs';
// Loaded as CommonJS since .cjs is always loaded as CommonJS.

import 'commonjs-package/src/index.mjs';
// Loaded as ES module since .mjs is always loaded as ES module.
js

The .mjs and .cjs extensions can be used to mix types within the same package:

• Within a "type": "module" package, Node.js can be instructed to interpret a particular file as CommonJS by naming it with a .cjs extension (since both .js and .mjs files are treated as ES modules within a "module" package).

• Within a "type": "commonjs" package, Node.js can be instructed to interpret a particular file as an ES module by naming it with an .mjs extension (since both .js and .cjs files are treated as CommonJS within a "commonjs" package).

--input-type flag#

Strings passed in as an argument to --eval (or -e), or piped to node via STDIN, are treated as ES modules when the --input-type=module flag is set.

node --input-type=module --eval "import { sep } from 'node:path'; console.log(sep);"

echo "import { sep } from 'node:path'; console.log(sep);" | node --input-type=module
bash

For completeness there is also --input-type=commonjs, for explicitly running string input as CommonJS. This is the default behavior if --input-type is unspecified.

Package entry points#

In a package's package.json file, two fields can define entry points for a package: "main" and "exports". Both fields apply to both ES module and CommonJS module entry points.

The "main" field is supported in all versions of Node.js, but its capabilities are limited: it only defines the main entry point of the package.

The "exports" provides a modern alternative to "main" allowing multiple entry points to be defined, conditional entry resolution support between environments, and preventing any other entry points besides those defined in "exports". This encapsulation allows module authors to clearly define the public interface for their package.

For new packages targeting the currently supported versions of Node.js, the "exports" field is recommended. For packages supporting Node.js 10 and below, the "main" field is required. If both "exports" and "main" are defined, the "exports" field takes precedence over "main" in supported versions of Node.js.

Conditional exports can be used within "exports" to define different package entry points per environment, including whether the package is referenced via require or via import. For more information about supporting both CommonJS and ES modules in a single package please consult the dual CommonJS/ES module packages section.

Existing packages introducing the "exports" field will prevent consumers of the package from using any entry points that are not defined, including the package.json (e.g. require('your-package/package.json')). This will likely be a breaking change.

To make the introduction of "exports" non-breaking, ensure that every previously supported entry point is exported. It is best to explicitly specify entry points so that the package's public API is well-defined. For example, a project that previously exported main, lib, feature, and the package.json could use the following package.exports:

{
  "name": "my-package",
  "exports": {
    ".": "./lib/index.js",
    "./lib": "./lib/index.js",
    "./lib/index": "./lib/index.js",
    "./lib/index.js": "./lib/index.js",
    "./feature": "./feature/index.js",
    "./feature/index": "./feature/index.js",
    "./feature/index.js": "./feature/index.js",
    "./package.json": "./package.json"
  }
}
json

Alternatively a project could choose to export entire folders both with and without extensioned subpaths using export patterns:

{
  "name": "my-package",
  "exports": {
    ".": "./lib/index.js",
    "./lib": "./lib/index.js",
    "./lib/*": "./lib/*.js",
    "./lib/*.js": "./lib/*.js",
    "./feature": "./feature/index.js",
    "./feature/*": "./feature/*.js",
    "./feature/*.js": "./feature/*.js",
    "./package.json": "./package.json"
  }
}
json

With the above providing backwards-compatibility for any minor package versions, a future major change for the package can then properly restrict the exports to only the specific feature exports exposed:

{
  "name": "my-package",
  "exports": {
    ".": "./lib/index.js",
    "./feature/*.js": "./feature/*.js",
    "./feature/internal/*": null
  }
}
json

Main entry point export#

When writing a new package, it is recommended to use the "exports" field:

{
  "exports": "./index.js"
}
json

When the "exports" field is defined, all subpaths of the package are encapsulated and no longer available to importers. For example, require('pkg/subpath.js') throws an ERR_PACKAGE_PATH_NOT_EXPORTED error.

This encapsulation of exports provides more reliable guarantees about package interfaces for tools and when handling semver upgrades for a package. It is not a strong encapsulation since a direct require of any absolute subpath of the package such as require('/path/to/node_modules/pkg/subpath.js') will still load subpath.js.

All currently supported versions of Node.js and modern build tools support the "exports" field. For projects using an older version of Node.js or a related build tool, compatibility can be achieved by including the "main" field alongside "exports" pointing to the same module:

{
  "main": "./index.js",
  "exports": "./index.js"
}
json

Subpath exports#

When using the "exports" field, custom subpaths can be defined along with the main entry point by treating the main entry point as the "." subpath:

{
  "exports": {
    ".": "./index.js",
    "./submodule.js": "./src/submodule.js"
  }
}
json

Now only the defined subpath in "exports" can be imported by a consumer:

import submodule from 'es-module-package/submodule.js';
// Loads ./node_modules/es-module-package/src/submodule.js
js

While other subpaths will error:

import submodule from 'es-module-package/private-module.js';
// Throws ERR_PACKAGE_PATH_NOT_EXPORTED
js

Extensions in subpaths#

Package authors should provide either extensioned (import 'pkg/subpath.js') or extensionless (import 'pkg/subpath') subpaths in their exports. This ensures that there is only one subpath for each exported module so that all dependents import the same consistent specifier, keeping the package contract clear for consumers and simplifying package subpath completions.

Traditionally, packages tended to use the extensionless style, which has the benefits of readability and of masking the true path of the file within the package.

With import maps now providing a standard for package resolution in browsers and other JavaScript runtimes, using the extensionless style can result in bloated import map definitions. Explicit file extensions can avoid this issue by enabling the import map to utilize a packages folder mapping to map multiple subpaths where possible instead of a separate map entry per package subpath export. This also mirrors the requirement of using the full specifier path in relative and absolute import specifiers.

Path Rules and Validation for Export Targets#

When defining paths as targets in the "exports" field, Node.js enforces several rules to ensure security, predictability, and proper encapsulation. Understanding these rules is crucial for authors publishing packages.

Targets must be relative URLs#

All target paths in the "exports" map (the values associated with export keys) must be relative URL strings starting with ./.

// package.json
{
  "name": "my-package",
  "exports": {
    ".": "./dist/main.js",          // Correct
    "./feature": "./lib/feature.js", // Correct
    // "./origin-relative": "/dist/main.js", // Incorrect: Must start with ./
    // "./absolute": "file:///dev/null", // Incorrect: Must start with ./
    // "./outside": "../common/util.js" // Incorrect: Must start with ./
  }
}
json

Reasons for this behavior include:

• Security: Prevents exporting arbitrary files from outside the package's own directory.
• Encapsulation: Ensures all exported paths are resolved relative to the package root, making the package self-contained.

No path traversal or invalid segments#

Export targets must not resolve to a location outside the package's root directory. Additionally, path segments like . (single dot), .. (double dot), or node_modules (and their URL-encoded equivalents) are generally disallowed within the target string after the initial ./ and in any subpath part substituted into a target pattern.

// package.json
{
  "name": "my-package",
  "exports": {
    // ".": "./dist/../../elsewhere/file.js", // Invalid: path traversal
    // ".": "././dist/main.js",             // Invalid: contains "." segment
    // ".": "./dist/../dist/main.js",       // Invalid: contains ".." segment
    // "./utils/./helper.js": "./utils/helper.js" // Key has invalid segment
  }
}
json

Exports sugar#

If the "." export is the only export, the "exports" field provides sugar for this case being the direct "exports" field value.

{
  "exports": {
    ".": "./index.js"
  }
}
json

can be written:

{
  "exports": "./index.js"
}
json

Subpath imports#

In addition to the "exports" field, there is a package "imports" field to create private mappings that only apply to import specifiers from within the package itself.

Entries in the "imports" field must always start with # to ensure they are disambiguated from external package specifiers.

For example, the imports field can be used to gain the benefits of conditional exports for internal modules:

// package.json
{
  "imports": {
    "#dep": {
      "node": "dep-node-native",
      "default": "./dep-polyfill.js"
    }
  },
  "dependencies": {
    "dep-node-native": "^1.0.0"
  }
}
json

where import '#dep' does not get the resolution of the external package dep-node-native (including its exports in turn), and instead gets the local file ./dep-polyfill.js relative to the package in other environments.

Unlike the "exports" field, the "imports" field permits mapping to external packages.

The resolution rules for the imports field are otherwise analogous to the exports field.

Subpath patterns#

For packages with a small number of exports or imports, we recommend explicitly listing each exports subpath entry. But for packages that have large numbers of subpaths, this might cause package.json bloat and maintenance issues.

For these use cases, subpath export patterns can be used instead:

// ./node_modules/es-module-package/package.json
{
  "exports": {
    "./features/*.js": "./src/features/*.js"
  },
  "imports": {
    "#internal/*.js": "./src/internal/*.js"
  }
}
json

* maps expose nested subpaths as it is a string replacement syntax only.

All instances of * on the right hand side will then be replaced with this value, including if it contains any / separators.

import featureX from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js

import featureY from 'es-module-package/features/y/y.js';
// Loads ./node_modules/es-module-package/src/features/y/y.js

import internalZ from '#internal/z.js';
// Loads ./src/internal/z.js
js

This is a direct static matching and replacement without any special handling for file extensions. Including the "*.js" on both sides of the mapping restricts the exposed package exports to only JS files.

The property of exports being statically enumerable is maintained with exports patterns since the individual exports for a package can be determined by treating the right hand side target pattern as a ** glob against the list of files within the package. Because node_modules paths are forbidden in exports targets, this expansion is dependent on only the files of the package itself.

To exclude private subfolders from patterns, null targets can be used:

// ./node_modules/es-module-package/package.json
{
  "exports": {
    "./features/*.js": "./src/features/*.js",
    "./features/private-internal/*": null
  }
}
json

import featureInternal from 'es-module-package/features/private-internal/m.js';
// Throws: ERR_PACKAGE_PATH_NOT_EXPORTED

import featureX from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js
js

Conditional exports#

Conditional exports provide a way to map to different paths depending on certain conditions. They are supported for both CommonJS and ES module imports.

For example, a package that wants to provide different ES module exports for require() and import can be written:

// package.json
{
  "exports": {
    "import": "./index-module.js",
    "require": "./index-require.cjs"
  },
  "type": "module"
}
json

Node.js implements the following conditions, listed in order from most specific to least specific as conditions should be defined:

• "node-addons" - similar to "node" and matches for any Node.js environment. This condition can be used to provide an entry point which uses native C++ addons as opposed to an entry point which is more universal and doesn't rely on native addons. This condition can be disabled via the --no-addons flag.
• "node" - matches for any Node.js environment. Can be a CommonJS or ES module file. In most cases explicitly calling out the Node.js platform is not necessary.
• "import" - matches when the package is loaded via import or import(), or via any top-level import or resolve operation by the ECMAScript module loader. Applies regardless of the module format of the target file. Always mutually exclusive with "require".
• "require" - matches when the package is loaded via require(). The referenced file should be loadable with require() although the condition matches regardless of the module format of the target file. Expected formats include CommonJS, JSON, native addons, and ES modules. Always mutually exclusive with "import".
• "module-sync" - matches no matter the package is loaded via import, import() or require(). The format is expected to be ES modules that does not contain top-level await in its module graph - if it does, ERR_REQUIRE_ASYNC_MODULE will be thrown when the module is require()-ed.
• "default" - the generic fallback that always matches. Can be a CommonJS or ES module file. This condition should always come last.

Within the "exports" object, key order is significant. During condition matching, earlier entries have higher priority and take precedence over later entries. The general rule is that conditions should be from most specific to least specific in object order.

Using the "import" and "require" conditions can lead to some hazards, which are further explained in the dual CommonJS/ES module packages section.

The "node-addons" condition can be used to provide an entry point which uses native C++ addons. However, this condition can be disabled via the --no-addons flag. When using "node-addons", it's recommended to treat "default" as an enhancement that provides a more universal entry point, e.g. using WebAssembly instead of a native addon.

Conditional exports can also be extended to exports subpaths, for example:

{
  "exports": {
    ".": "./index.js",
    "./feature.js": {
      "node": "./feature-node.js",
      "default": "./feature.js"
    }
  }
}
json

Defines a package where require('pkg/feature.js') and import 'pkg/feature.js' could provide different implementations between Node.js and other JS environments.

When using environment branches, always include a "default" condition where possible. Providing a "default" condition ensures that any unknown JS environments are able to use this universal implementation, which helps avoid these JS environments from having to pretend to be existing environments in order to support packages with conditional exports. For this reason, using "node" and "default" condition branches is usually preferable to using "node" and "browser" condition branches.

Nested conditions#

In addition to direct mappings, Node.js also supports nested condition objects.

For example, to define a package that only has dual mode entry points for use in Node.js but not the browser:

{
  "exports": {
    "node": {
      "import": "./feature-node.mjs",
      "require": "./feature-node.cjs"
    },
    "default": "./feature.mjs"
  }
}
json

Conditions continue to be matched in order as with flat conditions. If a nested condition does not have any mapping it will continue checking the remaining conditions of the parent condition. In this way nested conditions behave analogously to nested JavaScript if statements.

Resolving user conditions#

When running Node.js, custom user conditions can be added with the --conditions flag:

node --conditions=development index.js
bash

which would then resolve the "development" condition in package imports and exports, while resolving the existing "node", "node-addons", "default", "import", and "require" conditions as appropriate.

Any number of custom conditions can be set with repeat flags.

Typical conditions should only contain alphanumerical characters, using ":", "-", or "=" as separators if necessary. Anything else may run into compatibility issues outside of node.

In node, conditions have very few restrictions, but specifically these include:

1. They must contain at least one character.
2. They cannot start with "." since they may appear in places that also allow relative paths.
3. They cannot contain "," since they may be parsed as a comma-separated list by some CLI tools.
4. They cannot be integer property keys like "10" since that can have unexpected effects on property key ordering for JS objects.

Community Conditions Definitions#

Condition strings other than the "import", "require", "node", "module-sync", "node-addons" and "default" conditions implemented in Node.js core are ignored by default.

Other platforms may implement other conditions and user conditions can be enabled in Node.js via the --conditions / -C flag.

Since custom package conditions require clear definitions to ensure correct usage, a list of common known package conditions and their strict definitions is provided below to assist with ecosystem coordination.

• "types" - can be used by typing systems to resolve the typing file for the given export. This condition should always be included first.
• "browser" - any web browser environment.
• "development" - can be used to define a development-only environment entry point, for example to provide additional debugging context such as better error messages when running in a development mode. Must always be mutually exclusive with "production".
• "production" - can be used to define a production environment entry point. Must always be mutually exclusive with "development".

For other runtimes, platform-specific condition key definitions are maintained by the WinterCG in the Runtime Keys proposal specification.

New conditions definitions may be added to this list by creating a pull request to the Node.js documentation for this section. The requirements for listing a new condition definition here are that:

• The definition should be clear and unambiguous for all implementers.
• The use case for why the condition is needed should be clearly justified.
• There should exist sufficient existing implementation usage.
• The condition name should not conflict with another condition definition or condition in wide usage.
• The listing of the condition definition should provide a coordination benefit to the ecosystem that wouldn't otherwise be possible. For example, this would not necessarily be the case for company-specific or application-specific conditions.
• The condition should be such that a Node.js user would expect it to be in Node.js core documentation. The "types" condition is a good example: It doesn't really belong in the Runtime Keys proposal but is a good fit here in the Node.js docs.

The above definitions may be moved to a dedicated conditions registry in due course.

Self-referencing a package using its name#

Within a package, the values defined in the package's package.json "exports" field can be referenced via the package's name. For example, assuming the package.json is:

// package.json
{
  "name": "a-package",
  "exports": {
    ".": "./index.mjs",
    "./foo.js": "./foo.js"
  }
}
json

Then any module in that package can reference an export in the package itself:

// ./a-module.mjs
import { something } from 'a-package'; // Imports "something" from ./index.mjs.
js

Self-referencing is available only if package.json has "exports", and will allow importing only what that "exports" (in the package.json) allows. So the code below, given the previous package, will generate a runtime error:

// ./another-module.mjs

// Imports "another" from ./m.mjs. Fails because
// the "package.json" "exports" field
// does not provide an export named "./m.mjs".
import { another } from 'a-package/m.mjs';
js

Self-referencing is also available when using require, both in an ES module, and in a CommonJS one. For example, this code will also work:

// ./a-module.js
const { something } = require('a-package/foo.js'); // Loads from ./foo.js.
cjs

Finally, self-referencing also works with scoped packages. For example, this code will also work:

// package.json
{
  "name": "@my/package",
  "exports": "./index.js"
}
json

// ./index.js
module.exports = 42;
cjs

// ./other.js
console.log(require('@my/package'));
cjs

$ node other.js
42
console

Dual CommonJS/ES module packages#

See the package examples repository for details.

Package maps#

Package maps provide a mechanism to control package resolution without relying on the node_modules folder structure. When enabled via the --experimental-package-map flag, Node.js uses a JSON configuration file to determine how bare specifiers are resolved.

This feature is useful for:

• Monorepos: Define explicit dependency relationships between workspace packages without symlinks or hoisting complexities.
• Dependency isolation: Prevent packages from accessing undeclared dependencies (phantom dependencies).
• Low file system coupling: The package resolution algorithm runs without inspecting the file system, relying instead on static data tables.

Configuration file format#

The package map configuration file is a JSON file with a packages object. Each key in packages is called a package ID and is a unique identifier for a package entry:

{
  "packages": {
    "app": {
      "url": "./packages/app",
      "dependencies": {
        "@myorg/utils": "utils",
        "@myorg/ui-lib": "ui-lib"
      }
    },
    "utils": {
      "url": "./packages/utils"
    },
    "ui-lib": {
      "url": "./packages/ui-lib",
      "dependencies": {
        "@myorg/utils": "utils"
      }
    }
  }
}
json

Each package entry has the following fields:

• url <string> Required. An absolute or relative URL. This is parsed using the WHATWG URL API, using the configuration file URL as base. Only file: protocol is supported. Multiple packages are allowed to share the same URL; consumers must key module instances by both module url and package IDs to differentiate them.
• dependencies <Object> An object mapping bare specifiers to package keys. Each key is the import name used in source code, and each value is the corresponding package key in the packages object. Defaults to an empty object.

Resolution algorithm#

When a bare specifier is encountered:

1. Node.js determines which package performs the resolution request.
   • If possible the package ID for the importer file should be provided to the resolution algorithm.
   • Failing that, the resolution will check if the file path is within any package location decoded from its url.
2. If no package ID is provided and the importing file is not within any mapped package, an ERR_PACKAGE_MAP_EXTERNAL_FILE error is thrown.
3. Node.js looks up the specifier's package name in the importing package's dependencies object to find the corresponding package key.
4. If found, the resolution algorithm locates the target package location from the package's url field in the package map.
5. If the specifier is not in dependencies, a MODULE_NOT_FOUND error is thrown.
6. The package location is forwarded to the regular Node.js resolution algorithm to finish the resolution (index.js, exports field, etc).

More details can be found in the resolution algorithm pseudo-code.

Multiple package versions#

Different packages can depend on different versions of the same package. Because dependencies maps bare specifiers to package keys, two packages can map the same specifier to different targets:

{
  "packages": {
    "app": {
      "url": "./app",
      "dependencies": {
        "component": "component-v2"
      }
    },
    "legacy": {
      "url": "./legacy",
      "dependencies": {
        "component": "component-v1"
      }
    },
    "component-v1": {
      "url": "./vendor/component-1.0.0"
    },
    "component-v2": {
      "url": "./vendor/component-2.0.0"
    }
  }
}
json

Both app and legacy can import 'component', but they resolve to different paths based on their declared dependencies.

Multiple packages for the same URL#

To address complex hoisting situations, multiple packages may share the same URL, which introduces ambiguity when determining which package an import originates from:

{
  "packages": {
    "app-old": {
      "url": "./app-old",
      "dependencies": {
        "lib": "lib-old"
      }
    },
    "app-new": {
      "url": "./app-new",
      "dependencies": {
        "lib": "lib-new"
      }
    },
    "lib-old": {
      "url": "./lib",
      "dependencies": {
        "react": "react-15"
      }
    },
    "lib-new": {
      "url": "./lib",
      "dependencies": {
        "react": "react-18"
      }
    }
  }
}
json

In the example above both lib-old and lib-new use the same ./lib folder to store their sources, the only difference being in which version of react they'll access when performing require calls or using import.

Because multiple package entries share the same URL, resolving a bare specifier from a file within that URL is ambiguous unless the originating package ID is known. If the package ID cannot be determined (for example, because the caller did not propagate it from a previous resolution), Node.js will throw an error rather than guess.

To support this pattern, implementers must key module instances by package ID and propagate it from each resolution result to subsequent resolution requests. This ensures that when lib requires react, the runtime knows whether the request comes from lib-old or lib-new and can select the correct dependency.

Interaction with other resolution#

Package maps only apply to bare specifiers that are not Node.js builtin modules. The following cases are not affected by package maps and continue to use standard resolution:

• Relative paths or URLs (./ or ../).
• Absolute paths or URLs.
• Node.js builtin modules (node:fs, etc.).

Limitations#

• Package maps must be a single static file; dynamic configuration is not supported.
• Circular dependency detection is not performed by the package map resolver.
• The package map file is loaded synchronously at startup.

Node.js package.json field definitions#

This section describes the fields used by the Node.js runtime. Other tools (such as npm) use additional fields which are ignored by Node.js and not documented here.

The following fields in package.json files are used in Node.js:

• "name" - Relevant when using named imports within a package. Also used by package managers as the name of the package.
• "main" - The default module when loading the package, if exports is not specified, and in versions of Node.js prior to the introduction of exports.
• "type" - The package type determining whether to load .js files as CommonJS or ES modules.
• "exports" - Package exports and conditional exports. When present, limits which submodules can be loaded from within the package.
• "imports" - Package imports, for use by modules within the package itself.

"name"#

• Type: <string>

{
  "name": "package-name"
}
json

The "name" field defines your package's name. Publishing to the npm registry requires a name that satisfies certain requirements.

The "name" field can be used in addition to the "exports" field to self-reference a package using its name.

"main"#

• Type: <string>

{
  "main": "./index.js"
}
json

The "main" field defines the entry point of a package when imported by name via a node_modules lookup. Its value is a path.

The "exports" field, if it exists, takes precedence over the "main" field when importing the package by name.

It also defines the script that is used when the package directory is loaded via require().

// This resolves to ./path/to/directory/index.js.
require('./path/to/directory');
cjs

"type"#

• Type: <string>

The "type" field defines the module format that Node.js uses for all .js files that have that package.json file as their nearest parent.

Files ending with .js are loaded as ES modules when the nearest parent package.json file contains a top-level field "type" with a value of "module".

The nearest parent package.json is defined as the first package.json found when searching in the current folder, that folder's parent, and so on up until a node_modules folder or the volume root is reached.

// package.json
{
  "type": "module"
}
json

# In same folder as preceding package.json
node my-app.js # Runs as ES module
bash

If the nearest parent package.json lacks a "type" field, or contains "type": "commonjs", .js files are treated as CommonJS. If the volume root is reached and no package.json is found, .js files are treated as CommonJS.

import statements of .js files are treated as ES modules if the nearest parent package.json contains "type": "module".

// my-app.js, part of the same example as above
import './startup.js'; // Loaded as ES module because of package.json
js

Regardless of the value of the "type" field, .mjs files are always treated as ES modules and .cjs files are always treated as CommonJS.

"exports"#

• Type: <Object> | <string> | <string>[]

{
  "exports": "./index.js"
}
json

The "exports" field allows defining the entry points of a package when imported by name loaded either via a node_modules lookup or a self-reference to its own name. It is supported in Node.js 12+ as an alternative to the "main" that can support defining subpath exports and conditional exports while encapsulating internal unexported modules.

Conditional Exports can also be used within "exports" to define different package entry points per environment, including whether the package is referenced via require or via import.

All paths defined in the "exports" must be relative file URLs starting with ./.

"imports"#

• Type: <Object>

// package.json
{
  "imports": {
    "#dep": {
      "node": "dep-node-native",
      "default": "./dep-polyfill.js"
    }
  },
  "dependencies": {
    "dep-node-native": "^1.0.0"
  }
}
json

Entries in the imports field must be strings starting with #.

Package imports permit mapping to external packages.

This field defines subpath imports for the current package.