- Assertion Testing
- Async Hooks
- Buffer
- C++ Addons
- C/C++ Addons with N-API
- C++ Embedder API
- Child Processes
- Cluster
- Command Line Options
- Console
- Crypto
- Debugger
- Deprecated APIs
- DNS
- Domain
- ECMAScript Modules
- Errors
- Events
- File System
- Globals
- HTTP
- HTTP/2
- HTTPS
- Inspector
- Internationalization
- Modules
- Net
- OS
- Path
- Performance Hooks
- Policies
- Process
- Punycode
- Query Strings
- Readline
- REPL
- Report
- Stream
- String Decoder
- Timers
- TLS/SSL
- Trace Events
- TTY
- UDP/Datagram
- URL
- Utilities
- V8
- VM
- WASI
- Worker Threads
- Zlib
Node.js v15.0.0-test20200430ccd18343cb Documentation
Table of Contents
ECMAScript Modules#
Introduction#
ECMAScript modules are the official standard format to package JavaScript
code for reuse. Modules are defined using a variety of import
and
export
statements.
Node.js fully supports ECMAScript modules as they are currently specified and provides limited interoperability between them and the existing module format, CommonJS.
Node.js contains support for ES Modules based upon the Node.js EP for ES Modules and the ECMAScript-modules implementation.
Expect major changes in the implementation including interoperability support, specifier resolution, and default behavior.
Enabling#
Experimental support for ECMAScript modules is enabled by default.
Node.js will treat the following as ES modules when passed to node
as the
initial input, or when referenced by import
statements within ES module code:
-
Files ending in
.mjs
. -
Files ending in
.js
when the nearest parentpackage.json
file contains a top-level field"type"
with a value of"module"
. -
Strings passed in as an argument to
--eval
or--print
, or piped tonode
viaSTDIN
, with the flag--input-type=module
.
Node.js will treat as CommonJS all other forms of input, such as .js
files
where the nearest parent package.json
file contains no top-level "type"
field, or string input without the flag --input-type
. This behavior is to
preserve backward compatibility. However, now that Node.js supports both
CommonJS and ES modules, it is best to be explicit whenever possible. Node.js
will treat the following as CommonJS when passed to node
as the initial input,
or when referenced by import
statements within ES module code:
-
Files ending in
.cjs
. -
Files ending in
.js
when the nearest parentpackage.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 tonode
viaSTDIN
, with the flag--input-type=commonjs
.
package.json
"type"
field#
Files ending with .js
will be 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 the root of the volume is reached.
// package.json
{
"type": "module"
}
# In same folder as above package.json
node my-app.js # Runs as ES module
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, Node.js defers to the default, a
package.json
with no "type"
field.
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
Package authors should include the "type"
field, 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.
Regardless of the value of the "type"
field, .mjs
files are always treated
as ES modules and .cjs
files are always treated as CommonJS.
Package Scope and File Extensions#
A folder containing a package.json
file, and all subfolders below that folder
down until the next folder containing another package.json
, is considered a
package scope. The "type"
field defines how .js
files should be treated
within a particular package.json
file’s package scope. Every package in a
project’s node_modules
folder contains its own package.json
file, so each
project’s dependencies have their own package scopes. A package.json
lacking a
"type"
field is treated as if it contained "type": "commonjs"
.
The package scope 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, in an ES module package scope 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 ES module package scope 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".
Files ending with .mjs
are always loaded as ES modules regardless of package
scope.
Files ending with .cjs
are always loaded as CommonJS regardless of package
scope.
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.
The .mjs
and .cjs
extensions may be used to mix types within the same
package scope:
-
Within a
"type": "module"
package scope, 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 scope). -
Within a
"type": "commonjs"
package scope, 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 scope).
--input-type
flag#
Strings passed in as an argument to --eval
or --print
(or -e
or -p
), or
piped to node
via STDIN
, will be treated as ES modules when the
--input-type=module
flag is set.
node --input-type=module --eval "import { sep } from 'path'; console.log(sep);"
echo "import { sep } from 'path'; console.log(sep);" | node --input-type=module
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.
Packages#
Package Entry Points#
In a package’s package.json
file, two fields can define entry points for a
package: "main"
and "exports"
. 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"
field provides an alternative to "main"
where the package
main entry point can be defined while also encapsulating the package, preventing
any other entry points besides those defined in "exports"
. If package entry
points are defined in both "main"
and "exports"
, the latter takes precedence
in versions of Node.js that support "exports"
. 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
.
If both "exports"
and "main"
are defined, the "exports"
field takes
precedence over "main"
.
Both "main"
and "exports"
entry points are not specific to ES modules or
CommonJS; "main"
will be overridden by "exports"
in a require
so it is
not a CommonJS fallback.
This is important with regard to require
, since require
of ES module files
throws an error in all versions of Node.js. To create a package that works both
in modern Node.js via import
and require
and also legacy Node.js versions,
see the dual CommonJS/ES module packages section.
Main Entry Point Export#
To set the main entry point for a package, it is advisable to define both
"exports"
and "main"
in the package’s package.json
file:
{
"main": "./main.js",
"exports": "./main.js"
}
The benefit of doing this is that when using the "exports"
field all
subpaths of the package will no longer be available to importers under
require('pkg/subpath.js')
, and instead they will get a new error,
ERR_PACKAGE_PATH_NOT_EXPORTED
.
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
.
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:
{
"main": "./main.js",
"exports": {
".": "./main.js",
"./submodule": "./src/submodule.js"
}
}
Now only the defined subpath in "exports"
can be imported by a
consumer:
import submodule from 'es-module-package/submodule';
// Loads ./node_modules/es-module-package/src/submodule.js
While other subpaths will error:
import submodule from 'es-module-package/private-module.js';
// Throws ERR_PACKAGE_PATH_NOT_EXPORTED
Entire folders can also be mapped with package exports:
// ./node_modules/es-module-package/package.json
{
"exports": {
"./features/": "./src/features/"
}
}
With the above, all modules within the ./src/features/
folder
are exposed deeply to import
and require
:
import feature from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js
When using folder mappings, ensure that you do want to expose every module inside the subfolder. Any modules which are not public should be moved to another folder to retain the encapsulation benefits of exports.
Package Exports Fallbacks#
For possible new specifier support in future, array fallbacks are supported for all invalid specifiers:
{
"exports": {
"./submodule": ["not:valid", "./submodule.js"]
}
}
Since "not:valid"
is not a valid specifier, "./submodule.js"
is used
instead as the fallback, as if it were the only target.
Exports Sugar#
If the "."
export is the only export, the "exports"
field provides sugar
for this case being the direct "exports"
field value.
If the "."
export has a fallback array or string value, then the "exports"
field can be set to this value directly.
{
"exports": {
".": "./main.js"
}
}
can be written:
{
"exports": "./main.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
{
"main": "./main-require.cjs",
"exports": {
"import": "./main-module.js",
"require": "./main-require.cjs"
},
"type": "module"
}
Node.js supports the following conditions:
"import"
- matched when the package is loaded viaimport
orimport()
. Can reference either an ES module or CommonJS file, as bothimport
andimport()
can load either ES module or CommonJS sources."require"
- matched when the package is loaded viarequire()
. Asrequire()
only supports CommonJS, the referenced file must be CommonJS."node"
- matched for any Node.js environment. Can be a CommonJS or ES module file. This condition should always come after"import"
or"require"
."default"
- the generic fallback that will always match. Can be a CommonJS or ES module file. This condition should always come last.
Condition matching is applied in object order from first to last within the
"exports"
object. The general rule is that conditions should be used
from most specific to least specific in object order.
Other conditions such as "browser"
, "electron"
, "deno"
, "react-native"
,
etc. are ignored by Node.js but may be used by other runtimes or tools.
Further restrictions, definitions or guidance on condition names may be
provided in the future.
Using the "import"
and "require"
conditions can lead to some hazards,
which are explained further in
the dual CommonJS/ES module packages section.
Conditional exports can also be extended to exports subpaths, for example:
{
"main": "./main.js",
"exports": {
".": "./main.js",
"./feature": {
"browser": "./feature-browser.js",
"default": "./feature.js"
}
}
}
Defines a package where require('pkg/feature')
and import 'pkg/feature'
could provide different implementations between the browser and Node.js,
given third-party tool support for a "browser"
condition.
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:
{
"main": "./main.js",
"exports": {
"browser": "./feature-browser.mjs",
"node": {
"import": "./feature-node.mjs",
"require": "./feature-node.cjs"
}
}
}
Conditions continue to be matched in order as with flat conditions. If
a nested conditional 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.
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": {
".": "./main.mjs",
"./foo": "./foo.js"
}
}
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 ./main.mjs.
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 package above, 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';
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'); // Loads from ./foo.js.
Dual CommonJS/ES Module Packages#
Prior to the introduction of support for ES modules in Node.js, it was a common
pattern for package authors to include both CommonJS and ES module JavaScript
sources in their package, with package.json
"main"
specifying the CommonJS
entry point and package.json
"module"
specifying the ES module entry point.
This enabled Node.js to run the CommonJS entry point while build tools such as
bundlers used the ES module entry point, since Node.js ignored (and still
ignores) the top-level "module"
field.
Node.js can now run ES module entry points, and a package can contain both
CommonJS and ES module entry points (either via separate specifiers such as
'pkg'
and 'pkg/es-module'
, or both at the same specifier via Conditional
Exports). Unlike in the scenario where "module"
is only used by bundlers,
or ES module files are transpiled into CommonJS on the fly before evaluation by
Node.js, the files referenced by the ES module entry point are evaluated as ES
modules.
Dual Package Hazard#
When an application is using a package that provides both CommonJS and ES module
sources, there is a risk of certain bugs if both versions of the package get
loaded. This potential comes from the fact that the pkgInstance
created by
const pkgInstance = require('pkg')
is not the same as the pkgInstance
created by import pkgInstance from 'pkg'
(or an alternative main path like
'pkg/module'
). This is the “dual package hazard,” where two versions of the
same package can be loaded within the same runtime environment. While it is
unlikely that an application or package would intentionally load both versions
directly, it is common for an application to load one version while a dependency
of the application loads the other version. This hazard can happen because
Node.js supports intermixing CommonJS and ES modules, and can lead to unexpected
behavior.
If the package main export is a constructor, an instanceof
comparison of
instances created by the two versions returns false
, and if the export is an
object, properties added to one (like pkgInstance.foo = 3
) are not present on
the other. This differs from how import
and require
statements work in
all-CommonJS or all-ES module environments, respectively, and therefore is
surprising to users. It also differs from the behavior users are familiar with
when using transpilation via tools like Babel or esm
.
Writing Dual Packages While Avoiding or Minimizing Hazards#
First, the hazard described in the previous section occurs when a package
contains both CommonJS and ES module sources and both sources are provided for
use in Node.js, either via separate main entry points or exported paths. A
package could instead be written where any version of Node.js receives only
CommonJS sources, and any separate ES module sources the package may contain
could be intended only for other environments such as browsers. Such a package
would be usable by any version of Node.js, since import
can refer to CommonJS
files; but it would not provide any of the advantages of using ES module syntax.
A package could also switch from CommonJS to ES module syntax in a breaking change version bump. This has the disadvantage that the newest version of the package would only be usable in ES module-supporting versions of Node.js.
Every pattern has tradeoffs, but there are two broad approaches that satisfy the following conditions:
- The package is usable via both
require
andimport
. - The package is usable in both current Node.js and older versions of Node.js that lack support for ES modules.
- The package main entry point, e.g.
'pkg'
can be used by bothrequire
to resolve to a CommonJS file and byimport
to resolve to an ES module file. (And likewise for exported paths, e.g.'pkg/feature'
.) - The package provides named exports, e.g.
import { name } from 'pkg'
rather thanimport pkg from 'pkg'; pkg.name
. - The package is potentially usable in other ES module environments such as browsers.
- The hazards described in the previous section are avoided or minimized.
Approach #1: Use an ES Module Wrapper#
Write the package in CommonJS or transpile ES module sources into CommonJS, and
create an ES module wrapper file that defines the named exports. Using
Conditional Exports, the ES module wrapper is used for import
and the
CommonJS entry point for require
.
// ./node_modules/pkg/package.json
{
"type": "module",
"main": "./index.cjs",
"exports": {
"import": "./wrapper.mjs",
"require": "./index.cjs"
}
}
// ./node_modules/pkg/index.cjs
exports.name = 'value';
// ./node_modules/pkg/wrapper.mjs
import cjsModule from './index.cjs';
export const name = cjsModule.name;
In this example, the name
from import { name } from 'pkg'
is the same
singleton as the name
from const { name } = require('pkg')
. Therefore ===
returns true
when comparing the two name
s and the divergent specifier hazard
is avoided.
If the module is not simply a list of named exports, but rather contains a
unique function or object export like module.exports = function () { ... }
,
or if support in the wrapper for the import pkg from 'pkg'
pattern is desired,
then the wrapper would instead be written to export the default optionally
along with any named exports as well:
import cjsModule from './index.cjs';
export const name = cjsModule.name;
export default cjsModule;
This approach is appropriate for any of the following use cases:
- The package is currently written in CommonJS and the author would prefer not to refactor it into ES module syntax, but wishes to provide named exports for ES module consumers.
- The package has other packages that depend on it, and the end user might
install both this package and those other packages. For example a
utilities
package is used directly in an application, and autilities-plus
package adds a few more functions toutilities
. Because the wrapper exports underlying CommonJS files, it doesn’t matter ifutilities-plus
is written in CommonJS or ES module syntax; it will work either way. - The package stores internal state, and the package author would prefer not to refactor the package to isolate its state management. See the next section.
A variant of this approach not requiring conditional exports for consumers could
be to add an export, e.g. "./module"
, to point to an all-ES module-syntax
version of the package. This could be used via import 'pkg/module'
by users
who are certain that the CommonJS version will not be loaded anywhere in the
application, such as by dependencies; or if the CommonJS version can be loaded
but doesn’t affect the ES module version (for example, because the package is
stateless):
// ./node_modules/pkg/package.json
{
"type": "module",
"main": "./index.cjs",
"exports": {
".": "./index.cjs",
"./module": "./wrapper.mjs"
}
}
Approach #2: Isolate State#
A package.json
file can define the separate CommonJS and ES module entry
points directly:
// ./node_modules/pkg/package.json
{
"type": "module",
"main": "./index.cjs",
"exports": {
"import": "./index.mjs",
"require": "./index.cjs"
}
}
This can be done if both the CommonJS and ES module versions of the package are equivalent, for example because one is the transpiled output of the other; and the package’s management of state is carefully isolated (or the package is stateless).
The reason that state is an issue is because both the CommonJS and ES module
versions of the package may get used within an application; for example, the
user’s application code could import
the ES module version while a dependency
require
s the CommonJS version. If that were to occur, two copies of the
package would be loaded in memory and therefore two separate states would be
present. This would likely cause hard-to-troubleshoot bugs.
Aside from writing a stateless package (if JavaScript’s Math
were a package,
for example, it would be stateless as all of its methods are static), there are
some ways to isolate state so that it’s shared between the potentially loaded
CommonJS and ES module instances of the package:
-
If possible, contain all state within an instantiated object. JavaScript’s
Date
, for example, needs to be instantiated to contain state; if it were a package, it would be used like this:import Date from 'date'; const someDate = new Date(); // someDate contains state; Date does not
The
new
keyword isn’t required; a package’s function can return a new object, or modify a passed-in object, to keep the state external to the package. -
Isolate the state in one or more CommonJS files that are shared between the CommonJS and ES module versions of the package. For example, if the CommonJS and ES module entry points are
index.cjs
andindex.mjs
, respectively:// ./node_modules/pkg/index.cjs const state = require('./state.cjs'); module.exports.state = state;
// ./node_modules/pkg/index.mjs import state from './state.cjs'; export { state };
Even if
pkg
is used via bothrequire
andimport
in an application (for example, viaimport
in application code and viarequire
by a dependency) each reference ofpkg
will contain the same state; and modifying that state from either module system will apply to both.
Any plugins that attach to the package’s singleton would need to separately attach to both the CommonJS and ES module singletons.
This approach is appropriate for any of the following use cases:
- The package is currently written in ES module syntax and the package author wants that version to be used wherever such syntax is supported.
- The package is stateless or its state can be isolated without too much difficulty.
- The package is unlikely to have other public packages that depend on it, or if it does, the package is stateless or has state that need not be shared between dependencies or with the overall application.
Even with isolated state, there is still the cost of possible extra code execution between the CommonJS and ES module versions of a package.
As with the previous approach, a variant of this approach not requiring
conditional exports for consumers could be to add an export, e.g.
"./module"
, to point to an all-ES module-syntax version of the package:
// ./node_modules/pkg/package.json
{
"type": "module",
"main": "./index.cjs",
"exports": {
".": "./index.cjs",
"./module": "./index.mjs"
}
}
import
Specifiers#
Terminology#
The specifier of an import
statement is the string after the from
keyword,
e.g. 'path'
in import { sep } from 'path'
. Specifiers are also used in
export from
statements, and as the argument to an import()
expression.
There are four types of specifiers:
-
Bare specifiers like
'some-package'
. They refer to an entry point of a package by the package name. -
Deep import specifiers like
'some-package/lib/shuffle.mjs'
. They refer to a path within a package prefixed by the package name. -
Relative specifiers like
'./startup.js'
or'../config.mjs'
. They refer to a path relative to the location of the importing file. -
Absolute specifiers like
'file:///opt/nodejs/config.js'
. They refer directly and explicitly to a full path.
Bare specifiers, and the bare specifier portion of deep import specifiers, are strings; but everything else in a specifier is a URL.
Only file:
and data:
URLs are supported. A specifier like
'https://example.com/app.js'
may be supported by browsers but it is not
supported in Node.js.
Specifiers may not begin with /
or //
. These are reserved for potential
future use. The root of the current volume may be referenced via file:///
.
data:
Imports#
data:
URLs are supported for importing with the following MIME types:
text/javascript
for ES Modulesapplication/json
for JSONapplication/wasm
for WASM.
data:
URLs only resolve Bare specifiers for builtin modules
and Absolute specifiers. Resolving
Relative specifiers will not work because data:
is not a
special scheme. For example, attempting to load ./foo
from data:text/javascript,import "./foo";
will fail to resolve since there
is no concept of relative resolution for data:
URLs. An example of a data:
URLs being used is:
import 'data:text/javascript,console.log("hello!");';
import _ from 'data:application/json,"world!"';
import.meta
#
The import.meta
metaproperty is an Object
that contains the following
property:
url
<string> The absolutefile:
URL of the module.
Differences Between ES Modules and CommonJS#
Mandatory file extensions#
A file extension must be provided when using the import
keyword. Directory
indexes (e.g. './startup/index.js'
) must also be fully specified.
This behavior matches how import
behaves in browser environments, assuming a
typically configured server.
No NODE_PATH
#
NODE_PATH
is not part of resolving import
specifiers. Please use symlinks
if this behavior is desired.
No require
, exports
, module.exports
, __filename
, __dirname
#
These CommonJS variables are not available in ES modules.
require
can be imported into an ES module using module.createRequire()
.
Equivalents of __filename
and __dirname
can be created inside of each file
via import.meta.url
.
import { fileURLToPath } from 'url';
import { dirname } from 'path';
const __filename = fileURLToPath(import.meta.url);
const __dirname = dirname(__filename);
No require.resolve
#
Former use cases relying on require.resolve
to determine the resolved path
of a module can be supported via import.meta.resolve
, which is experimental
and supported via the --experimental-import-meta-resolve
flag:
(async () => {
const dependencyAsset = await import.meta.resolve('component-lib/asset.css');
})();
import.meta.resolve
also accepts a second argument which is the parent module
from which to resolve from:
(async () => {
// Equivalent to import.meta.resolve('./dep')
await import.meta.resolve('./dep', import.meta.url);
})();
This function is asynchronous since the ES module resolver in Node.js is asynchronous. With the introduction of Top-Level Await, these use cases will be easier as they won't require an async function wrapper.
No require.extensions
#
require.extensions
is not used by import
. The expectation is that loader
hooks can provide this workflow in the future.
No require.cache
#
require.cache
is not used by import
. It has a separate cache.
URL-based paths#
ES modules are resolved and cached based upon
URL semantics. This means that files containing
special characters such as #
and ?
need to be escaped.
Modules will be loaded multiple times if the import
specifier used to resolve
them have a different query or fragment.
import './foo.mjs?query=1'; // loads ./foo.mjs with query of "?query=1"
import './foo.mjs?query=2'; // loads ./foo.mjs with query of "?query=2"
For now, only modules using the file:
protocol can be loaded.
Interoperability with CommonJS#
require
#
require
always treats the files it references as CommonJS. This applies
whether require
is used the traditional way within a CommonJS environment, or
in an ES module environment using module.createRequire()
.
To include an ES module into CommonJS, use import()
.
import
statements#
An import
statement can reference an ES module or a CommonJS module. Other
file types such as JSON or Native modules are not supported. For those, use
module.createRequire()
.
import
statements are permitted only in ES modules. For similar functionality
in CommonJS, see import()
.
The specifier of an import
statement (the string after the from
keyword)
can either be an URL-style relative path like './file.mjs'
or a package name
like 'fs'
.
Like in CommonJS, files within packages can be accessed by appending a path to
the package name; unless the package’s package.json
contains an "exports"
field, in which case files within packages need to be accessed via the path
defined in "exports"
.
import { sin, cos } from 'geometry/trigonometry-functions.mjs';
Only the “default export” is supported for CommonJS files or packages:
import packageMain from 'commonjs-package'; // Works
import { method } from 'commonjs-package'; // Errors
It is also possible to import an ES or CommonJS module for its side effects only.
import()
expressions#
Dynamic import()
is supported in both CommonJS and ES modules. It can be
used to include ES module files from CommonJS code.
CommonJS, JSON, and Native Modules#
CommonJS, JSON, and Native modules can be used with
module.createRequire()
.
// cjs.cjs
module.exports = 'cjs';
// esm.mjs
import { createRequire } from 'module';
const require = createRequire(import.meta.url);
const cjs = require('./cjs.cjs');
cjs === 'cjs'; // true
Builtin modules#
Builtin modules will provide named exports of their public API. A
default export is also provided which is the value of the CommonJS exports.
The default export can be used for, among other things, modifying the named
exports. Named exports of builtin modules are updated only by calling
module.syncBuiltinESMExports()
.
import EventEmitter from 'events';
const e = new EventEmitter();
import { readFile } from 'fs';
readFile('./foo.txt', (err, source) => {
if (err) {
console.error(err);
} else {
console.log(source);
}
});
import fs, { readFileSync } from 'fs';
import { syncBuiltinESMExports } from 'module';
fs.readFileSync = () => Buffer.from('Hello, ESM');
syncBuiltinESMExports();
fs.readFileSync === readFileSync;
Experimental JSON Modules#
Currently importing JSON modules are only supported in the commonjs
mode
and are loaded using the CJS loader. WHATWG JSON modules specification are
still being standardized, and are experimentally supported by including the
additional flag --experimental-json-modules
when running Node.js.
When the --experimental-json-modules
flag is included both the
commonjs
and module
mode will use the new experimental JSON
loader. The imported JSON only exposes a default
, there is no
support for named exports. A cache entry is created in the CommonJS
cache, to avoid duplication. The same object will be returned in
CommonJS if the JSON module has already been imported from the
same path.
Assuming an index.mjs
with
import packageConfig from './package.json';
The --experimental-json-modules
flag is needed for the module
to work.
node index.mjs # fails
node --experimental-json-modules index.mjs # works
Experimental Wasm Modules#
Importing Web Assembly modules is supported under the
--experimental-wasm-modules
flag, allowing any .wasm
files to be
imported as normal modules while also supporting their module imports.
This integration is in line with the ES Module Integration Proposal for Web Assembly.
For example, an index.mjs
containing:
import * as M from './module.wasm';
console.log(M);
executed under:
node --experimental-wasm-modules index.mjs
would provide the exports interface for the instantiation of module.wasm
.
Experimental Loaders#
Note: This API is currently being redesigned and will still change.
To customize the default module resolution, loader hooks can optionally be
provided via a --experimental-loader ./loader-name.mjs
argument to Node.js.
When hooks are used they only apply to ES module loading and not to any CommonJS modules loaded.
Hooks#
resolve
hook#
Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.
The resolve
hook returns the resolved file URL for a given module specifier
and parent URL. The module specifier is the string in an import
statement or
import()
expression, and the parent URL is the URL of the module that imported
this one, or undefined
if this is the main entry point for the application.
The conditions
property on the context
is an array of conditions for
Conditional Exports that apply to this resolution request. They can be used
for looking up conditional mappings elsewhere or to modify the list when calling
the default resolution logic.
The current set of Node.js default conditions will always
be in the context.conditions
list passed to the hook. If the hook wants to
ensure Node.js-compatible resolution logic, all items from this default
condition list must be passed through to the defaultResolve
function.
/**
* @param {string} specifier
* @param {object} context
* @param {string} context.parentURL
* @param {string[]} context.conditions
* @param {function} defaultResolve
* @returns {object} response
* @returns {string} response.url
*/
export async function resolve(specifier, context, defaultResolve) {
const { parentURL = null } = context;
if (someCondition) {
// For some or all specifiers, do some custom logic for resolving.
// Always return an object of the form {url: <string>}
return {
url: (parentURL) ?
new URL(specifier, parentURL).href : new URL(specifier).href
};
}
if (anotherCondition) {
// When calling the defaultResolve, the arguments can be modified. In this
// case it's adding another value for matching conditional exports.
return defaultResolve(specifier, {
...context,
conditions: [...context.conditions, 'another-condition'],
});
}
// Defer to Node.js for all other specifiers.
return defaultResolve(specifier, context, defaultResolve);
}
getFormat
hook#
Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.
The getFormat
hook provides a way to define a custom method of determining how
a URL should be interpreted. This can be one of the following:
format | Description |
---|---|
'builtin' | Load a Node.js builtin module |
'commonjs' | Load a Node.js CommonJS module |
'dynamic' | Use a dynamic instantiate hook |
'json' | Load a JSON file |
'module' | Load a standard JavaScript module (ES module) |
'wasm' | Load a WebAssembly module |
/**
* @param {string} url
* @param {object} context (currently empty)
* @param {function} defaultGetFormat
* @returns {object} response
* @returns {string} response.format
*/
export async function getFormat(url, context, defaultGetFormat) {
if (someCondition) {
// For some or all URLs, do some custom logic for determining format.
// Always return an object of the form {format: <string>}, where the
// format is one of the strings in the table above.
return {
format: 'module'
};
}
// Defer to Node.js for all other URLs.
return defaultGetFormat(url, context, defaultGetFormat);
}
getSource
hook#
Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.
The getSource
hook provides a way to define a custom method for retrieving
the source code of an ES module specifier. This would allow a loader to
potentially avoid reading files from disk.
/**
* @param {string} url
* @param {object} context
* @param {string} context.format
* @param {function} defaultGetSource
* @returns {object} response
* @returns {string|buffer} response.source
*/
export async function getSource(url, context, defaultGetSource) {
const { format } = context;
if (someCondition) {
// For some or all URLs, do some custom logic for retrieving the source.
// Always return an object of the form {source: <string|buffer>}.
return {
source: '...'
};
}
// Defer to Node.js for all other URLs.
return defaultGetSource(url, context, defaultGetSource);
}
transformSource
hook#
Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.
The transformSource
hook provides a way to modify the source code of a loaded
ES module file after the source string has been loaded but before Node.js has
done anything with it.
If this hook is used to convert unknown-to-Node.js file types into executable JavaScript, a resolve hook is also necessary in order to register any unknown-to-Node.js file extensions. See the transpiler loader example below.
/**
* @param {string|buffer} source
* @param {object} context
* @param {string} context.url
* @param {string} context.format
* @param {function} defaultTransformSource
* @returns {object} response
* @returns {string|buffer} response.source
*/
export async function transformSource(source,
context,
defaultTransformSource) {
const { url, format } = context;
if (someCondition) {
// For some or all URLs, do some custom logic for modifying the source.
// Always return an object of the form {source: <string|buffer>}.
return {
source: '...'
};
}
// Defer to Node.js for all other sources.
return defaultTransformSource(
source, context, defaultTransformSource);
}
getGlobalPreloadCode
hook#
Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.
Sometimes it can be necessary to run some code inside of the same global scope that the application will run in. This hook allows to return a string that will be ran as sloppy-mode script on startup.
Similar to how CommonJS wrappers work, the code runs in an implicit function
scope. The only argument is a require
-like function that can be used to load
builtins like "fs": getBuiltin(request: string)
.
If the code needs more advanced require
features, it will have to construct
its own require
using module.createRequire()
.
/**
* @returns {string} Code to run before application startup
*/
export function getGlobalPreloadCode() {
return `\
globalThis.someInjectedProperty = 42;
console.log('I just set some globals!');
const { createRequire } = getBuiltin('module');
const require = createRequire(process.cwd() + '/<preload>');
// [...]
`;
}
dynamicInstantiate
hook#
Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.
To create a custom dynamic module that doesn't correspond to one of the
existing format
interpretations, the dynamicInstantiate
hook can be used.
This hook is called only for modules that return format: 'dynamic'
from
the getFormat
hook.
/**
* @param {string} url
* @returns {object} response
* @returns {array} response.exports
* @returns {function} response.execute
*/
export async function dynamicInstantiate(url) {
return {
exports: ['customExportName'],
execute: (exports) => {
// Get and set functions provided for pre-allocated export names
exports.customExportName.set('value');
}
};
}
With the list of module exports provided upfront, the execute
function will
then be called at the exact point of module evaluation order for that module
in the import tree.
Examples#
The various loader hooks can be used together to accomplish wide-ranging customizations of Node.js’ code loading and evaluation behaviors.
HTTPS loader#
In current Node.js, specifiers starting with https://
are unsupported. The
loader below registers hooks to enable rudimentary support for such specifiers.
While this may seem like a significant improvement to Node.js core
functionality, there are substantial downsides to actually using this loader:
performance is much slower than loading files from disk, there is no caching,
and there is no security.
// https-loader.mjs
import { get } from 'https';
export function resolve(specifier, context, defaultResolve) {
const { parentURL = null } = context;
// Normally Node.js would error on specifiers starting with 'https://', so
// this hook intercepts them and converts them into absolute URLs to be
// passed along to the later hooks below.
if (specifier.startsWith('https://')) {
return {
url: specifier
};
} else if (parentURL && parentURL.startsWith('https://')) {
return {
url: new URL(specifier, parentURL).href
};
}
// Let Node.js handle all other specifiers.
return defaultResolve(specifier, context, defaultResolve);
}
export function getFormat(url, context, defaultGetFormat) {
// This loader assumes all network-provided JavaScript is ES module code.
if (url.startsWith('https://')) {
return {
format: 'module'
};
}
// Let Node.js handle all other URLs.
return defaultGetFormat(url, context, defaultGetFormat);
}
export function getSource(url, context, defaultGetSource) {
// For JavaScript to be loaded over the network, we need to fetch and
// return it.
if (url.startsWith('https://')) {
return new Promise((resolve, reject) => {
get(url, (res) => {
let data = '';
res.on('data', (chunk) => data += chunk);
res.on('end', () => resolve({ source: data }));
}).on('error', (err) => reject(err));
});
}
// Let Node.js handle all other URLs.
return defaultGetSource(url, context, defaultGetSource);
}
// main.mjs
import { VERSION } from 'https://coffeescript.org/browser-compiler-modern/coffeescript.js';
console.log(VERSION);
With this loader, running:
node --experimental-loader ./https-loader.mjs ./main.js
Will print the current version of CoffeeScript per the module at the URL in
main.mjs
.
Transpiler loader#
Sources that are in formats Node.js doesn’t understand can be converted into
JavaScript using the transformSource
hook. Before that hook gets called,
however, other hooks need to tell Node.js not to throw an error on unknown file
types; and to tell Node.js how to load this new file type.
This is less performant than transpiling source files before running Node.js; a transpiler loader should only be used for development and testing purposes.
// coffeescript-loader.mjs
import { URL, pathToFileURL } from 'url';
import CoffeeScript from 'coffeescript';
const baseURL = pathToFileURL(`${process.cwd()}/`).href;
// CoffeeScript files end in .coffee, .litcoffee or .coffee.md.
const extensionsRegex = /\.coffee$|\.litcoffee$|\.coffee\.md$/;
export function resolve(specifier, context, defaultResolve) {
const { parentURL = baseURL } = context;
// Node.js normally errors on unknown file extensions, so return a URL for
// specifiers ending in the CoffeeScript file extensions.
if (extensionsRegex.test(specifier)) {
return {
url: new URL(specifier, parentURL).href
};
}
// Let Node.js handle all other specifiers.
return defaultResolve(specifier, context, defaultResolve);
}
export function getFormat(url, context, defaultGetFormat) {
// Now that we patched resolve to let CoffeeScript URLs through, we need to
// tell Node.js what format such URLs should be interpreted as. For the
// purposes of this loader, all CoffeeScript URLs are ES modules.
if (extensionsRegex.test(url)) {
return {
format: 'module'
};
}
// Let Node.js handle all other URLs.
return defaultGetFormat(url, context, defaultGetFormat);
}
export function transformSource(source, context, defaultTransformSource) {
const { url, format } = context;
if (extensionsRegex.test(url)) {
return {
source: CoffeeScript.compile(source, { bare: true })
};
}
// Let Node.js handle all other sources.
return defaultTransformSource(source, context, defaultTransformSource);
}
# main.coffee
import { scream } from './scream.coffee'
console.log scream 'hello, world'
import { version } from 'process'
console.log "Brought to you by Node.js version #{version}"
# scream.coffee
export scream = (str) -> str.toUpperCase()
With this loader, running:
node --experimental-loader ./coffeescript-loader.mjs main.coffee
Will cause main.coffee
to be turned into JavaScript after its source code is
loaded from disk but before Node.js executes it; and so on for any .coffee
,
.litcoffee
or .coffee.md
files referenced via import
statements of any
loaded file.
Resolution Algorithm#
Features#
The resolver has the following properties:
- FileURL-based resolution as is used by ES modules
- Support for builtin module loading
- Relative and absolute URL resolution
- No default extensions
- No folder mains
- Bare specifier package resolution lookup through node_modules
Resolver Algorithm#
The algorithm to load an ES module specifier is given through the ESM_RESOLVE method below. It returns the resolved URL for a module specifier relative to a parentURL.
The algorithm to determine the module format of a resolved URL is provided by ESM_FORMAT, which returns the unique module format for any file. The "module" format is returned for an ECMAScript Module, while the "commonjs" format is used to indicate loading through the legacy CommonJS loader. Additional formats such as "addon" can be extended in future updates.
In the following algorithms, all subroutine errors are propagated as errors of these top-level routines unless stated otherwise.
defaultEnv is the conditional environment name priority array,
["node", "import"]
.
The resolver can throw the following errors:
- Invalid Module Specifier: Module specifier is an invalid URL, package name or package subpath specifier.
- Invalid Package Configuration: package.json configuration is invalid or contains an invalid configuration.
- Invalid Package Target: Package exports define a target module within the package that is an invalid type or string target.
- Package Path Not Exported: Package exports do not define or permit a target subpath in the package for the given module.
- Module Not Found: The package or module requested does not exist.
Resolver algorithm specification
ESM_RESOLVE(specifier, parentURL)
- Let resolvedURL be undefined.
If specifier is a valid URL, then
- Set resolvedURL to the result of parsing and reserializing specifier as a URL.
Otherwise, if specifier starts with "/", then
- Throw an Invalid Module Specifier error.
Otherwise, if specifier starts with "./" or "../", then
- Set resolvedURL to the URL resolution of specifier relative to parentURL.
Otherwise,
- Note: specifier is now a bare specifier.
- Set resolvedURL the result of PACKAGE_RESOLVE(specifier, parentURL).
If resolvedURL contains any percent encodings of "/" or "\" ("%2f" and "%5C" respectively), then
- Throw an Invalid Module Specifier error.
If resolvedURL does not end with a trailing "/" and the file at resolvedURL does not exist, then
- Throw a Module Not Found error.
- Set resolvedURL to the real path of resolvedURL.
- Let format be the result of ESM_FORMAT(resolvedURL).
- Load resolvedURL as module format, format.
- Return resolvedURL.
PACKAGE_RESOLVE(packageSpecifier, parentURL)
- Let packageName be undefined.
- Let packageSubpath be undefined.
If packageSpecifier is an empty string, then
- Throw an Invalid Module Specifier error.
Otherwise,
If packageSpecifier does not contain a "/" separator, then
- Throw an Invalid Module Specifier error.
- Set packageName to the substring of packageSpecifier until the second "/" separator or the end of the string.
If packageName starts with "." or contains "\" or "%", then
- Throw an Invalid Module Specifier error.
- Let packageSubpath be undefined.
If the length of packageSpecifier is greater than the length of packageName, then
- Set packageSubpath to "." concatenated with the substring of packageSpecifier from the position at the length of packageName.
If packageSubpath contains any "." or ".." segments or percent encoded strings for "/" or "\", then
- Throw an Invalid Module Specifier error.
- Set selfUrl to the result of SELF_REFERENCE_RESOLVE(packageName, packageSubpath, parentURL).
- If selfUrl isn't empty, return selfUrl.
If packageSubpath is undefined and packageName is a Node.js builtin module, then
- Return the string "nodejs:" concatenated with packageSpecifier.
While parentURL is not the file system root,
- Let packageURL be the URL resolution of "node_modules/" concatenated with packageSpecifier, relative to parentURL.
- Set parentURL to the parent folder URL of parentURL.
If the folder at packageURL does not exist, then
- Set parentURL to the parent URL path of parentURL.
- Continue the next loop iteration.
- Let pjson be the result of READ_PACKAGE_JSON(packageURL).
If packageSubpath is equal to "./", then
- Return packageURL + "/".
If packageSubpath is _undefined__, then
- Return the result of PACKAGE_MAIN_RESOLVE(packageURL, pjson).
Otherwise,
If pjson is not null and pjson has an "exports" key, then
- Let exports be pjson.exports.
If exports is not null or undefined, then
- Return PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports).
- Return the URL resolution of packageSubpath in packageURL.
- Throw a Module Not Found error.
SELF_REFERENCE_RESOLVE(packageName, packageSubpath, parentURL)
- Let packageURL be the result of READ_PACKAGE_SCOPE(parentURL).
If packageURL is null, then
- Return undefined.
- Let pjson be the result of READ_PACKAGE_JSON(packageURL).
If pjson does not include an "exports" property, then
- Return undefined.
If pjson.name is equal to packageName, then
If packageSubpath is equal to "./", then
- Return packageURL + "/".
If packageSubpath is undefined, then
- Return the result of PACKAGE_MAIN_RESOLVE(packageURL, pjson).
Otherwise,
If pjson is not null and pjson has an "exports" key, then
- Let exports be pjson.exports.
If exports is not null or undefined, then
- Return PACKAGE_EXPORTS_RESOLVE(packageURL, subpath, pjson.exports).
- Return the URL resolution of subpath in packageURL.
- Otherwise, return undefined.
PACKAGE_MAIN_RESOLVE(packageURL, pjson)
If pjson is null, then
- Throw a Module Not Found error.
If pjson.exports is not null or undefined, then
- If exports is an Object with both a key starting with "." and a key not starting with ".", throw an Invalid Package Configuration error.
If pjson.exports is a String or Array, or an Object containing no keys starting with ".", then
- Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, pjson.exports, "").
If pjson.exports is an Object containing a "." property, then
- Let mainExport be the "." property in pjson.exports.
- Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, mainExport, "").
- Throw a Package Path Not Exported error.
- Let legacyMainURL be the result applying the legacy LOAD_AS_DIRECTORY CommonJS resolver to packageURL, throwing a Module Not Found error for no resolution.
- Return legacyMainURL.
PACKAGE_EXPORTS_RESOLVE(packageURL, packagePath, exports)
- If exports is an Object with both a key starting with "." and a key not starting with ".", throw an Invalid Package Configuration error.
If exports is an Object and all keys of exports start with ".", then
- Set packagePath to "./" concatenated with packagePath.
If packagePath is a key of exports, then
- Let target be the value of exports[packagePath].
- Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, "", defaultEnv).
- Let directoryKeys be the list of keys of exports ending in "/", sorted by length descending.
For each key directory in directoryKeys, do
If packagePath starts with directory, then
- Let target be the value of exports[directory].
- Let subpath be the substring of target starting at the index of the length of directory.
- Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, subpath, defaultEnv).
- Throw a Package Path Not Exported error.
PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, subpath, env)
If target is a String, then
- If target does not start with "./" or contains any "node_modules" segments including "node_modules" percent-encoding, throw an Invalid Package Target error.
- Let resolvedTarget be the URL resolution of the concatenation of packageURL and target.
- If resolvedTarget is not contained in packageURL, throw an Invalid Package Target error.
- If subpath has non-zero length and target does not end with "/", throw an Invalid Module Specifier error.
- Let resolved be the URL resolution of the concatenation of subpath and resolvedTarget.
- If resolved is not contained in resolvedTarget, throw an Invalid Module Specifier error.
- Return resolved.
Otherwise, if target is a non-null Object, then
- If exports contains any index property keys, as defined in ECMA-262 6.1.7 Array Index, throw an Invalid Package Configuration error.
For each property p of target, in object insertion order as,
If p equals "default" or env contains an entry for p, then
- Let targetValue be the value of the p property in target.
- Return the result of PACKAGE_EXPORTS_TARGET_RESOLVE( packageURL, targetValue, subpath, env), continuing the loop on any Package Path Not Exported error.
- Throw a Package Path Not Exported error.
Otherwise, if target is an Array, then
- If target.length is zero, throw a _Package Path Not Exported error.
For each item targetValue in target, do
- If targetValue is an Array, continue the loop.
- Return the result of PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, targetValue, subpath, env), continuing the loop on any Package Path Not Exported or Invalid Package Target error.
- Throw the last fallback resolution error.
- Otherwise, if target is null, throw a Package Path Not Exported error.
- Otherwise throw an Invalid Package Target error.
ESM_FORMAT(url)
- Assert: url corresponds to an existing file.
- Let pjson be the result of READ_PACKAGE_SCOPE(url).
If url ends in ".mjs", then
- Return "module".
If url ends in ".cjs", then
- Return "commonjs".
If pjson?.type exists and is "module", then
If url ends in ".js", then
- Return "module".
- Throw an Unsupported File Extension error.
Otherwise,
- Throw an Unsupported File Extension error.
READ_PACKAGE_SCOPE(url)
- Let scopeURL be url.
While scopeURL is not the file system root,
- If scopeURL ends in a "node_modules" path segment, return null.
- Let pjson be the result of READ_PACKAGE_JSON(scopeURL).
If pjson is not null, then
- Return pjson.
- Set scopeURL to the parent URL of scopeURL.
- Return null.
READ_PACKAGE_JSON(packageURL)
- Let pjsonURL be the resolution of "package.json" within packageURL.
If the file at pjsonURL does not exist, then
- Return null.
If the file at packageURL does not parse as valid JSON, then
- Throw an Invalid Package Configuration error.
- Return the parsed JSON source of the file at pjsonURL.
Customizing ESM specifier resolution algorithm#
The current specifier resolution does not support all default behavior of the CommonJS loader. One of the behavior differences is automatic resolution of file extensions and the ability to import directories that have an index file.
The --experimental-specifier-resolution=[mode]
flag can be used to customize
the extension resolution algorithm. The default mode is explicit
, which
requires the full path to a module be provided to the loader. To enable the
automatic extension resolution and importing from directories that include an
index file use the node
mode.
$ node index.mjs
success!
$ node index # Failure!
Error: Cannot find module
$ node --experimental-specifier-resolution=node index
success!