Stability: 2 - Stable
The node:vm
module enables compiling and running code within V8 Virtual
Machine contexts.
The node:vm
module is not a security
mechanism. Do not use it to run untrusted code.
JavaScript code can be compiled and run immediately or compiled, saved, and run later.
A common use case is to run the code in a different V8 Context. This means invoked code has a different global object than the invoking code.
One can provide the context by contextifying an object. The invoked code treats any property in the context like a global variable. Any changes to global variables caused by the invoked code are reflected in the context object.
const vm = require('node:vm');
const x = 1;
const context = { x: 2 };
vm.createContext(context); // Contextify the object.
const code = 'x += 40; var y = 17;';
// `x` and `y` are global variables in the context.
// Initially, x has the value 2 because that is the value of context.x.
vm.runInContext(code, context);
console.log(context.x); // 42
console.log(context.y); // 17
console.log(x); // 1; y is not defined.
Instances of the vm.Script
class contain precompiled scripts that can be
executed in specific contexts.
code
{string} The JavaScript code to compile.options
{Object|string}filename
{string} Specifies the filename used in stack traces produced by this script. Default:'evalmachine.<anonymous>'
.lineOffset
{number} Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
{number} Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.cachedData
{Buffer|TypedArray|DataView} Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source. When supplied, thecachedDataRejected
value will be set to eithertrue
orfalse
depending on acceptance of the data by V8.produceCachedData
{boolean} Whentrue
and nocachedData
is present, V8 will attempt to produce code cache data forcode
. Upon success, aBuffer
with V8's code cache data will be produced and stored in thecachedData
property of the returnedvm.Script
instance. ThecachedDataProduced
value will be set to eithertrue
orfalse
depending on whether code cache data is produced successfully. This option is deprecated in favor ofscript.createCachedData()
. Default:false
.importModuleDynamically
{Function} Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API. We do not recommend using it in a production environment.specifier
{string} specifier passed toimport()
script
{vm.Script}importAssertions
{Object} The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: {Module Namespace Object|vm.Module} Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
If options
is a string, then it specifies the filename.
Creating a new vm.Script
object compiles code
but does not run it. The
compiled vm.Script
can be run later multiple times. The code
is not bound to
any global object; rather, it is bound before each run, just for that run.
- {boolean|undefined}
When cachedData
is supplied to create the vm.Script
, this value will be set
to either true
or false
depending on acceptance of the data by V8.
Otherwise the value is undefined
.
- Returns: {Buffer}
Creates a code cache that can be used with the Script
constructor's
cachedData
option. Returns a Buffer
. This method may be called at any
time and any number of times.
The code cache of the Script
doesn't contain any JavaScript observable
states. The code cache is safe to be saved along side the script source and
used to construct new Script
instances multiple times.
Functions in the Script
source can be marked as lazily compiled and they are
not compiled at construction of the Script
. These functions are going to be
compiled when they are invoked the first time. The code cache serializes the
metadata that V8 currently knows about the Script
that it can use to speed up
future compilations.
const script = new vm.Script(`
function add(a, b) {
return a + b;
}
const x = add(1, 2);
`);
const cacheWithoutAdd = script.createCachedData();
// In `cacheWithoutAdd` the function `add()` is marked for full compilation
// upon invocation.
script.runInThisContext();
const cacheWithAdd = script.createCachedData();
// `cacheWithAdd` contains fully compiled function `add()`.
contextifiedObject
{Object} A contextified object as returned by thevm.createContext()
method.options
{Object}displayErrors
{boolean} Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
{integer} Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
{boolean} Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.
- Returns: {any} the result of the very last statement executed in the script.
Runs the compiled code contained by the vm.Script
object within the given
contextifiedObject
and returns the result. Running code does not have access
to local scope.
The following example compiles code that increments a global variable, sets
the value of another global variable, then execute the code multiple times.
The globals are contained in the context
object.
const vm = require('node:vm');
const context = {
animal: 'cat',
count: 2,
};
const script = new vm.Script('count += 1; name = "kitty";');
vm.createContext(context);
for (let i = 0; i < 10; ++i) {
script.runInContext(context);
}
console.log(context);
// Prints: { animal: 'cat', count: 12, name: 'kitty' }
Using the timeout
or breakOnSigint
options will result in new event loops
and corresponding threads being started, which have a non-zero performance
overhead.
contextObject
{Object} An object that will be contextified. Ifundefined
, a new object will be created.options
{Object}displayErrors
{boolean} Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
{integer} Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
{boolean} Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.contextName
{string} Human-readable name of the newly created context. Default:'VM Context i'
, wherei
is an ascending numerical index of the created context.contextOrigin
{string} Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of theurl.origin
property of aURL
object. Most notably, this string should omit the trailing slash, as that denotes a path. Default:''
.contextCodeGeneration
{Object}strings
{boolean} If set to false any calls toeval
or function constructors (Function
,GeneratorFunction
, etc) will throw anEvalError
. Default:true
.wasm
{boolean} If set to false any attempt to compile a WebAssembly module will throw aWebAssembly.CompileError
. Default:true
.
microtaskMode
{string} If set toafterEvaluate
, microtasks (tasks scheduled throughPromise
s andasync function
s) will be run immediately after the script has run. They are included in thetimeout
andbreakOnSigint
scopes in that case.
- Returns: {any} the result of the very last statement executed in the script.
First contextifies the given contextObject
, runs the compiled code contained
by the vm.Script
object within the created context, and returns the result.
Running code does not have access to local scope.
The following example compiles code that sets a global variable, then executes
the code multiple times in different contexts. The globals are set on and
contained within each individual context
.
const vm = require('node:vm');
const script = new vm.Script('globalVar = "set"');
const contexts = [{}, {}, {}];
contexts.forEach((context) => {
script.runInNewContext(context);
});
console.log(contexts);
// Prints: [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]
options
{Object}displayErrors
{boolean} Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
{integer} Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
{boolean} Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.
- Returns: {any} the result of the very last statement executed in the script.
Runs the compiled code contained by the vm.Script
within the context of the
current global
object. Running code does not have access to local scope, but
does have access to the current global
object.
The following example compiles code that increments a global
variable then
executes that code multiple times:
const vm = require('node:vm');
global.globalVar = 0;
const script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });
for (let i = 0; i < 1000; ++i) {
script.runInThisContext();
}
console.log(globalVar);
// 1000
- {string|undefined}
When the script is compiled from a source that contains a source map magic comment, this property will be set to the URL of the source map.
import vm from 'node:vm';
const script = new vm.Script(`
function myFunc() {}
//# sourceMappingURL=sourcemap.json
`);
console.log(script.sourceMapURL);
// Prints: sourcemap.json
const vm = require('node:vm');
const script = new vm.Script(`
function myFunc() {}
//# sourceMappingURL=sourcemap.json
`);
console.log(script.sourceMapURL);
// Prints: sourcemap.json
Stability: 1 - Experimental
This feature is only available with the --experimental-vm-modules
command
flag enabled.
The vm.Module
class provides a low-level interface for using
ECMAScript modules in VM contexts. It is the counterpart of the vm.Script
class that closely mirrors Module Records as defined in the ECMAScript
specification.
Unlike vm.Script
however, every vm.Module
object is bound to a context from
its creation. Operations on vm.Module
objects are intrinsically asynchronous,
in contrast with the synchronous nature of vm.Script
objects. The use of
'async' functions can help with manipulating vm.Module
objects.
Using a vm.Module
object requires three distinct steps: creation/parsing,
linking, and evaluation. These three steps are illustrated in the following
example.
This implementation lies at a lower level than the ECMAScript Module loader. There is also no way to interact with the Loader yet, though support is planned.
import vm from 'node:vm';
const contextifiedObject = vm.createContext({
secret: 42,
print: console.log,
});
// Step 1
//
// Create a Module by constructing a new `vm.SourceTextModule` object. This
// parses the provided source text, throwing a `SyntaxError` if anything goes
// wrong. By default, a Module is created in the top context. But here, we
// specify `contextifiedObject` as the context this Module belongs to.
//
// Here, we attempt to obtain the default export from the module "foo", and
// put it into local binding "secret".
const bar = new vm.SourceTextModule(`
import s from 'foo';
s;
print(s);
`, { context: contextifiedObject });
// Step 2
//
// "Link" the imported dependencies of this Module to it.
//
// The provided linking callback (the "linker") accepts two arguments: the
// parent module (`bar` in this case) and the string that is the specifier of
// the imported module. The callback is expected to return a Module that
// corresponds to the provided specifier, with certain requirements documented
// in `module.link()`.
//
// If linking has not started for the returned Module, the same linker
// callback will be called on the returned Module.
//
// Even top-level Modules without dependencies must be explicitly linked. The
// callback provided would never be called, however.
//
// The link() method returns a Promise that will be resolved when all the
// Promises returned by the linker resolve.
//
// Note: This is a contrived example in that the linker function creates a new
// "foo" module every time it is called. In a full-fledged module system, a
// cache would probably be used to avoid duplicated modules.
async function linker(specifier, referencingModule) {
if (specifier === 'foo') {
return new vm.SourceTextModule(`
// The "secret" variable refers to the global variable we added to
// "contextifiedObject" when creating the context.
export default secret;
`, { context: referencingModule.context });
// Using `contextifiedObject` instead of `referencingModule.context`
// here would work as well.
}
throw new Error(`Unable to resolve dependency: ${specifier}`);
}
await bar.link(linker);
// Step 3
//
// Evaluate the Module. The evaluate() method returns a promise which will
// resolve after the module has finished evaluating.
// Prints 42.
await bar.evaluate();
const vm = require('node:vm');
const contextifiedObject = vm.createContext({
secret: 42,
print: console.log,
});
(async () => {
// Step 1
//
// Create a Module by constructing a new `vm.SourceTextModule` object. This
// parses the provided source text, throwing a `SyntaxError` if anything goes
// wrong. By default, a Module is created in the top context. But here, we
// specify `contextifiedObject` as the context this Module belongs to.
//
// Here, we attempt to obtain the default export from the module "foo", and
// put it into local binding "secret".
const bar = new vm.SourceTextModule(`
import s from 'foo';
s;
print(s);
`, { context: contextifiedObject });
// Step 2
//
// "Link" the imported dependencies of this Module to it.
//
// The provided linking callback (the "linker") accepts two arguments: the
// parent module (`bar` in this case) and the string that is the specifier of
// the imported module. The callback is expected to return a Module that
// corresponds to the provided specifier, with certain requirements documented
// in `module.link()`.
//
// If linking has not started for the returned Module, the same linker
// callback will be called on the returned Module.
//
// Even top-level Modules without dependencies must be explicitly linked. The
// callback provided would never be called, however.
//
// The link() method returns a Promise that will be resolved when all the
// Promises returned by the linker resolve.
//
// Note: This is a contrived example in that the linker function creates a new
// "foo" module every time it is called. In a full-fledged module system, a
// cache would probably be used to avoid duplicated modules.
async function linker(specifier, referencingModule) {
if (specifier === 'foo') {
return new vm.SourceTextModule(`
// The "secret" variable refers to the global variable we added to
// "contextifiedObject" when creating the context.
export default secret;
`, { context: referencingModule.context });
// Using `contextifiedObject` instead of `referencingModule.context`
// here would work as well.
}
throw new Error(`Unable to resolve dependency: ${specifier}`);
}
await bar.link(linker);
// Step 3
//
// Evaluate the Module. The evaluate() method returns a promise which will
// resolve after the module has finished evaluating.
// Prints 42.
await bar.evaluate();
})();
- {string[]}
The specifiers of all dependencies of this module. The returned array is frozen to disallow any changes to it.
Corresponds to the [[RequestedModules]]
field of Cyclic Module Records in
the ECMAScript specification.
- {any}
If the module.status
is 'errored'
, this property contains the exception
thrown by the module during evaluation. If the status is anything else,
accessing this property will result in a thrown exception.
The value undefined
cannot be used for cases where there is not a thrown
exception due to possible ambiguity with throw undefined;
.
Corresponds to the [[EvaluationError]]
field of Cyclic Module Records
in the ECMAScript specification.
options
{Object}timeout
{integer} Specifies the number of milliseconds to evaluate before terminating execution. If execution is interrupted, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
{boolean} Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.
- Returns: {Promise} Fulfills with
undefined
upon success.
Evaluate the module.
This must be called after the module has been linked; otherwise it will reject.
It could be called also when the module has already been evaluated, in which
case it will either do nothing if the initial evaluation ended in success
(module.status
is 'evaluated'
) or it will re-throw the exception that the
initial evaluation resulted in (module.status
is 'errored'
).
This method cannot be called while the module is being evaluated
(module.status
is 'evaluating'
).
Corresponds to the Evaluate() concrete method field of Cyclic Module Records in the ECMAScript specification.
- {string}
The identifier of the current module, as set in the constructor.
linker
{Function}-
specifier
{string} The specifier of the requested module:import foo from 'foo'; // ^^^^^ the module specifier
-
referencingModule
{vm.Module} TheModule
objectlink()
is called on. -
extra
{Object}assert
{Object} The data from the assertion:Per ECMA-262, hosts are expected to ignore assertions that they do not support, as opposed to, for example, triggering an error if an unsupported assertion is present.import foo from 'foo' assert { name: 'value' }; // ^^^^^^^^^^^^^^^^^ the assertion
-
Returns: {vm.Module|Promise}
-
- Returns: {Promise}
Link module dependencies. This method must be called before evaluation, and can only be called once per module.
The function is expected to return a Module
object or a Promise
that
eventually resolves to a Module
object. The returned Module
must satisfy the
following two invariants:
- It must belong to the same context as the parent
Module
. - Its
status
must not be'errored'
.
If the returned Module
's status
is 'unlinked'
, this method will be
recursively called on the returned Module
with the same provided linker
function.
link()
returns a Promise
that will either get resolved when all linking
instances resolve to a valid Module
, or rejected if the linker function either
throws an exception or returns an invalid Module
.
The linker function roughly corresponds to the implementation-defined HostResolveImportedModule abstract operation in the ECMAScript specification, with a few key differences:
- The linker function is allowed to be asynchronous while HostResolveImportedModule is synchronous.
The actual HostResolveImportedModule implementation used during module linking is one that returns the modules linked during linking. Since at that point all modules would have been fully linked already, the HostResolveImportedModule implementation is fully synchronous per specification.
Corresponds to the Link() concrete method field of Cyclic Module Records in the ECMAScript specification.
- {Object}
The namespace object of the module. This is only available after linking
(module.link()
) has completed.
Corresponds to the GetModuleNamespace abstract operation in the ECMAScript specification.
- {string}
The current status of the module. Will be one of:
-
'unlinked'
:module.link()
has not yet been called. -
'linking'
:module.link()
has been called, but not all Promises returned by the linker function have been resolved yet. -
'linked'
: The module has been linked successfully, and all of its dependencies are linked, butmodule.evaluate()
has not yet been called. -
'evaluating'
: The module is being evaluated through amodule.evaluate()
on itself or a parent module. -
'evaluated'
: The module has been successfully evaluated. -
'errored'
: The module has been evaluated, but an exception was thrown.
Other than 'errored'
, this status string corresponds to the specification's
Cyclic Module Record's [[Status]]
field. 'errored'
corresponds to
'evaluated'
in the specification, but with [[EvaluationError]]
set to a
value that is not undefined
.
Stability: 1 - Experimental
This feature is only available with the --experimental-vm-modules
command
flag enabled.
- Extends: {vm.Module}
The vm.SourceTextModule
class provides the Source Text Module Record as
defined in the ECMAScript specification.
code
{string} JavaScript Module code to parseoptions
identifier
{string} String used in stack traces. Default:'vm:module(i)'
wherei
is a context-specific ascending index.cachedData
{Buffer|TypedArray|DataView} Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source. Thecode
must be the same as the module from which thiscachedData
was created.context
{Object} The contextified object as returned by thevm.createContext()
method, to compile and evaluate thisModule
in. If no context is specified, the module is evaluated in the current execution context.lineOffset
{integer} Specifies the line number offset that is displayed in stack traces produced by thisModule
. Default:0
.columnOffset
{integer} Specifies the first-line column number offset that is displayed in stack traces produced by thisModule
. Default:0
.initializeImportMeta
{Function} Called during evaluation of thisModule
to initialize theimport.meta
.meta
{import.meta}module
{vm.SourceTextModule}
importModuleDynamically
{Function} Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
.specifier
{string} specifier passed toimport()
module
{vm.Module}importAssertions
{Object} The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: {Module Namespace Object|vm.Module} Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
Creates a new SourceTextModule
instance.
Properties assigned to the import.meta
object that are objects may
allow the module to access information outside the specified context
. Use
vm.runInContext()
to create objects in a specific context.
import vm from 'node:vm';
const contextifiedObject = vm.createContext({ secret: 42 });
const module = new vm.SourceTextModule(
'Object.getPrototypeOf(import.meta.prop).secret = secret;',
{
initializeImportMeta(meta) {
// Note: this object is created in the top context. As such,
// Object.getPrototypeOf(import.meta.prop) points to the
// Object.prototype in the top context rather than that in
// the contextified object.
meta.prop = {};
},
});
// Since module has no dependencies, the linker function will never be called.
await module.link(() => {});
await module.evaluate();
// Now, Object.prototype.secret will be equal to 42.
//
// To fix this problem, replace
// meta.prop = {};
// above with
// meta.prop = vm.runInContext('{}', contextifiedObject);
const vm = require('node:vm');
const contextifiedObject = vm.createContext({ secret: 42 });
(async () => {
const module = new vm.SourceTextModule(
'Object.getPrototypeOf(import.meta.prop).secret = secret;',
{
initializeImportMeta(meta) {
// Note: this object is created in the top context. As such,
// Object.getPrototypeOf(import.meta.prop) points to the
// Object.prototype in the top context rather than that in
// the contextified object.
meta.prop = {};
},
});
// Since module has no dependencies, the linker function will never be called.
await module.link(() => {});
await module.evaluate();
// Now, Object.prototype.secret will be equal to 42.
//
// To fix this problem, replace
// meta.prop = {};
// above with
// meta.prop = vm.runInContext('{}', contextifiedObject);
})();
- Returns: {Buffer}
Creates a code cache that can be used with the SourceTextModule
constructor's
cachedData
option. Returns a Buffer
. This method may be called any number
of times before the module has been evaluated.
The code cache of the SourceTextModule
doesn't contain any JavaScript
observable states. The code cache is safe to be saved along side the script
source and used to construct new SourceTextModule
instances multiple times.
Functions in the SourceTextModule
source can be marked as lazily compiled
and they are not compiled at construction of the SourceTextModule
. These
functions are going to be compiled when they are invoked the first time. The
code cache serializes the metadata that V8 currently knows about the
SourceTextModule
that it can use to speed up future compilations.
// Create an initial module
const module = new vm.SourceTextModule('const a = 1;');
// Create cached data from this module
const cachedData = module.createCachedData();
// Create a new module using the cached data. The code must be the same.
const module2 = new vm.SourceTextModule('const a = 1;', { cachedData });
Stability: 1 - Experimental
This feature is only available with the --experimental-vm-modules
command
flag enabled.
- Extends: {vm.Module}
The vm.SyntheticModule
class provides the Synthetic Module Record as
defined in the WebIDL specification. The purpose of synthetic modules is to
provide a generic interface for exposing non-JavaScript sources to ECMAScript
module graphs.
const vm = require('node:vm');
const source = '{ "a": 1 }';
const module = new vm.SyntheticModule(['default'], function() {
const obj = JSON.parse(source);
this.setExport('default', obj);
});
// Use `module` in linking...
exportNames
{string[]} Array of names that will be exported from the module.evaluateCallback
{Function} Called when the module is evaluated.options
identifier
{string} String used in stack traces. Default:'vm:module(i)'
wherei
is a context-specific ascending index.context
{Object} The contextified object as returned by thevm.createContext()
method, to compile and evaluate thisModule
in.
Creates a new SyntheticModule
instance.
Objects assigned to the exports of this instance may allow importers of
the module to access information outside the specified context
. Use
vm.runInContext()
to create objects in a specific context.
name
{string} Name of the export to set.value
{any} The value to set the export to.
This method is used after the module is linked to set the values of exports. If
it is called before the module is linked, an ERR_VM_MODULE_STATUS
error
will be thrown.
import vm from 'node:vm';
const m = new vm.SyntheticModule(['x'], () => {
m.setExport('x', 1);
});
await m.link(() => {});
await m.evaluate();
assert.strictEqual(m.namespace.x, 1);
const vm = require('node:vm');
(async () => {
const m = new vm.SyntheticModule(['x'], () => {
m.setExport('x', 1);
});
await m.link(() => {});
await m.evaluate();
assert.strictEqual(m.namespace.x, 1);
})();
code
{string} The body of the function to compile.params
{string[]} An array of strings containing all parameters for the function.options
{Object}filename
{string} Specifies the filename used in stack traces produced by this script. Default:''
.lineOffset
{number} Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
{number} Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.cachedData
{Buffer|TypedArray|DataView} Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source.produceCachedData
{boolean} Specifies whether to produce new cache data. Default:false
.parsingContext
{Object} The contextified object in which the said function should be compiled in.contextExtensions
{Object[]} An array containing a collection of context extensions (objects wrapping the current scope) to be applied while compiling. Default:[]
.importModuleDynamically
{Function} Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API, and should not be considered stable.specifier
{string} specifier passed toimport()
function
{Function}importAssertions
{Object} The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: {Module Namespace Object|vm.Module} Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
- Returns: {Function}
Compiles the given code into the provided context (if no context is
supplied, the current context is used), and returns it wrapped inside a
function with the given params
.
contextObject
{Object}options
{Object}name
{string} Human-readable name of the newly created context. Default:'VM Context i'
, wherei
is an ascending numerical index of the created context.origin
{string} Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of theurl.origin
property of aURL
object. Most notably, this string should omit the trailing slash, as that denotes a path. Default:''
.codeGeneration
{Object}strings
{boolean} If set to false any calls toeval
or function constructors (Function
,GeneratorFunction
, etc) will throw anEvalError
. Default:true
.wasm
{boolean} If set to false any attempt to compile a WebAssembly module will throw aWebAssembly.CompileError
. Default:true
.
microtaskMode
{string} If set toafterEvaluate
, microtasks (tasks scheduled throughPromise
s andasync function
s) will be run immediately after a script has run throughscript.runInContext()
. They are included in thetimeout
andbreakOnSigint
scopes in that case.
- Returns: {Object} contextified object.
If given a contextObject
, the vm.createContext()
method will prepare
that object so that it can be used in calls to
vm.runInContext()
or script.runInContext()
. Inside such scripts,
the contextObject
will be the global object, retaining all of its existing
properties but also having the built-in objects and functions any standard
global object has. Outside of scripts run by the vm module, global variables
will remain unchanged.
const vm = require('node:vm');
global.globalVar = 3;
const context = { globalVar: 1 };
vm.createContext(context);
vm.runInContext('globalVar *= 2;', context);
console.log(context);
// Prints: { globalVar: 2 }
console.log(global.globalVar);
// Prints: 3
If contextObject
is omitted (or passed explicitly as undefined
), a new,
empty contextified object will be returned.
The vm.createContext()
method is primarily useful for creating a single
context that can be used to run multiple scripts. For instance, if emulating a
web browser, the method can be used to create a single context representing a
window's global object, then run all <script>
tags together within that
context.
The provided name
and origin
of the context are made visible through the
Inspector API.
object
{Object}- Returns: {boolean}
Returns true
if the given object
object has been contextified using
vm.createContext()
.
Stability: 1 - Experimental
Measure the memory known to V8 and used by all contexts known to the current V8 isolate, or the main context.
options
{Object} Optional.mode
{string} Either'summary'
or'detailed'
. In summary mode, only the memory measured for the main context will be returned. In detailed mode, the memory measured for all contexts known to the current V8 isolate will be returned. Default:'summary'
execution
{string} Either'default'
or'eager'
. With default execution, the promise will not resolve until after the next scheduled garbage collection starts, which may take a while (or never if the program exits before the next GC). With eager execution, the GC will be started right away to measure the memory. Default:'default'
- Returns: {Promise} If the memory is successfully measured the promise will resolve with an object containing information about the memory usage.
The format of the object that the returned Promise may resolve with is specific to the V8 engine and may change from one version of V8 to the next.
The returned result is different from the statistics returned by
v8.getHeapSpaceStatistics()
in that vm.measureMemory()
measure the
memory reachable by each V8 specific contexts in the current instance of
the V8 engine, while the result of v8.getHeapSpaceStatistics()
measure
the memory occupied by each heap space in the current V8 instance.
const vm = require('node:vm');
// Measure the memory used by the main context.
vm.measureMemory({ mode: 'summary' })
// This is the same as vm.measureMemory()
.then((result) => {
// The current format is:
// {
// total: {
// jsMemoryEstimate: 2418479, jsMemoryRange: [ 2418479, 2745799 ]
// }
// }
console.log(result);
});
const context = vm.createContext({ a: 1 });
vm.measureMemory({ mode: 'detailed', execution: 'eager' })
.then((result) => {
// Reference the context here so that it won't be GC'ed
// until the measurement is complete.
console.log(context.a);
// {
// total: {
// jsMemoryEstimate: 2574732,
// jsMemoryRange: [ 2574732, 2904372 ]
// },
// current: {
// jsMemoryEstimate: 2438996,
// jsMemoryRange: [ 2438996, 2768636 ]
// },
// other: [
// {
// jsMemoryEstimate: 135736,
// jsMemoryRange: [ 135736, 465376 ]
// }
// ]
// }
console.log(result);
});
code
{string} The JavaScript code to compile and run.contextifiedObject
{Object} The contextified object that will be used as theglobal
when thecode
is compiled and run.options
{Object|string}filename
{string} Specifies the filename used in stack traces produced by this script. Default:'evalmachine.<anonymous>'
.lineOffset
{number} Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
{number} Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.displayErrors
{boolean} Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
{integer} Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
{boolean} Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.cachedData
{Buffer|TypedArray|DataView} Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source.importModuleDynamically
{Function} Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API. We do not recommend using it in a production environment.specifier
{string} specifier passed toimport()
script
{vm.Script}importAssertions
{Object} The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: {Module Namespace Object|vm.Module} Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
- Returns: {any} the result of the very last statement executed in the script.
The vm.runInContext()
method compiles code
, runs it within the context of
the contextifiedObject
, then returns the result. Running code does not have
access to the local scope. The contextifiedObject
object must have been
previously contextified using the vm.createContext()
method.
If options
is a string, then it specifies the filename.
The following example compiles and executes different scripts using a single contextified object:
const vm = require('node:vm');
const contextObject = { globalVar: 1 };
vm.createContext(contextObject);
for (let i = 0; i < 10; ++i) {
vm.runInContext('globalVar *= 2;', contextObject);
}
console.log(contextObject);
// Prints: { globalVar: 1024 }
code
{string} The JavaScript code to compile and run.contextObject
{Object} An object that will be contextified. Ifundefined
, a new object will be created.options
{Object|string}filename
{string} Specifies the filename used in stack traces produced by this script. Default:'evalmachine.<anonymous>'
.lineOffset
{number} Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
{number} Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.displayErrors
{boolean} Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
{integer} Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
{boolean} Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.contextName
{string} Human-readable name of the newly created context. Default:'VM Context i'
, wherei
is an ascending numerical index of the created context.contextOrigin
{string} Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of theurl.origin
property of aURL
object. Most notably, this string should omit the trailing slash, as that denotes a path. Default:''
.contextCodeGeneration
{Object}strings
{boolean} If set to false any calls toeval
or function constructors (Function
,GeneratorFunction
, etc) will throw anEvalError
. Default:true
.wasm
{boolean} If set to false any attempt to compile a WebAssembly module will throw aWebAssembly.CompileError
. Default:true
.
cachedData
{Buffer|TypedArray|DataView} Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source.importModuleDynamically
{Function} Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API. We do not recommend using it in a production environment.specifier
{string} specifier passed toimport()
script
{vm.Script}importAssertions
{Object} The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: {Module Namespace Object|vm.Module} Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
microtaskMode
{string} If set toafterEvaluate
, microtasks (tasks scheduled throughPromise
s andasync function
s) will be run immediately after the script has run. They are included in thetimeout
andbreakOnSigint
scopes in that case.
- Returns: {any} the result of the very last statement executed in the script.
The vm.runInNewContext()
first contextifies the given contextObject
(or
creates a new contextObject
if passed as undefined
), compiles the code
,
runs it within the created context, then returns the result. Running code
does not have access to the local scope.
If options
is a string, then it specifies the filename.
The following example compiles and executes code that increments a global
variable and sets a new one. These globals are contained in the contextObject
.
const vm = require('node:vm');
const contextObject = {
animal: 'cat',
count: 2,
};
vm.runInNewContext('count += 1; name = "kitty"', contextObject);
console.log(contextObject);
// Prints: { animal: 'cat', count: 3, name: 'kitty' }
code
{string} The JavaScript code to compile and run.options
{Object|string}filename
{string} Specifies the filename used in stack traces produced by this script. Default:'evalmachine.<anonymous>'
.lineOffset
{number} Specifies the line number offset that is displayed in stack traces produced by this script. Default:0
.columnOffset
{number} Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default:0
.displayErrors
{boolean} Whentrue
, if anError
occurs while compiling thecode
, the line of code causing the error is attached to the stack trace. Default:true
.timeout
{integer} Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown. This value must be a strictly positive integer.breakOnSigint
{boolean} Iftrue
, receivingSIGINT
(Ctrl+C) will terminate execution and throw anError
. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
are disabled during script execution, but continue to work after that. Default:false
.cachedData
{Buffer|TypedArray|DataView} Provides an optionalBuffer
orTypedArray
, orDataView
with V8's code cache data for the supplied source.importModuleDynamically
{Function} Called during evaluation of this module whenimport()
is called. If this option is not specified, calls toimport()
will reject withERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
. This option is part of the experimental modules API. We do not recommend using it in a production environment.specifier
{string} specifier passed toimport()
script
{vm.Script}importAssertions
{Object} The"assert"
value passed to theoptionsExpression
optional parameter, or an empty object if no value was provided.- Returns: {Module Namespace Object|vm.Module} Returning a
vm.Module
is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that containthen
function exports.
- Returns: {any} the result of the very last statement executed in the script.
vm.runInThisContext()
compiles code
, runs it within the context of the
current global
and returns the result. Running code does not have access to
local scope, but does have access to the current global
object.
If options
is a string, then it specifies the filename.
The following example illustrates using both vm.runInThisContext()
and
the JavaScript eval()
function to run the same code:
const vm = require('node:vm');
let localVar = 'initial value';
const vmResult = vm.runInThisContext('localVar = "vm";');
console.log(`vmResult: '${vmResult}', localVar: '${localVar}'`);
// Prints: vmResult: 'vm', localVar: 'initial value'
const evalResult = eval('localVar = "eval";');
console.log(`evalResult: '${evalResult}', localVar: '${localVar}'`);
// Prints: evalResult: 'eval', localVar: 'eval'
Because vm.runInThisContext()
does not have access to the local scope,
localVar
is unchanged. In contrast, eval()
does have access to the
local scope, so the value localVar
is changed. In this way
vm.runInThisContext()
is much like an indirect eval()
call, e.g.
(0,eval)('code')
.
When using either script.runInThisContext()
or
vm.runInThisContext()
, the code is executed within the current V8 global
context. The code passed to this VM context will have its own isolated scope.
In order to run a simple web server using the node:http
module the code passed
to the context must either call require('node:http')
on its own, or have a
reference to the node:http
module passed to it. For instance:
'use strict';
const vm = require('node:vm');
const code = `
((require) => {
const http = require('node:http');
http.createServer((request, response) => {
response.writeHead(200, { 'Content-Type': 'text/plain' });
response.end('Hello World\\n');
}).listen(8124);
console.log('Server running at http://127.0.0.1:8124/');
})`;
vm.runInThisContext(code)(require);
The require()
in the above case shares the state with the context it is
passed from. This may introduce risks when untrusted code is executed, e.g.
altering objects in the context in unwanted ways.
All JavaScript executed within Node.js runs within the scope of a "context". According to the V8 Embedder's Guide:
In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.
When the method vm.createContext()
is called, the contextObject
argument
(or a newly-created object if contextObject
is undefined
) is associated
internally with a new instance of a V8 Context. This V8 Context provides the
code
run using the node:vm
module's methods with an isolated global
environment within which it can operate. The process of creating the V8 Context
and associating it with the contextObject
is what this document refers to as
"contextifying" the object.
Promise
s and async function
s can schedule tasks run by the JavaScript
engine asynchronously. By default, these tasks are run after all JavaScript
functions on the current stack are done executing.
This allows escaping the functionality of the timeout
and
breakOnSigint
options.
For example, the following code executed by vm.runInNewContext()
with a
timeout of 5 milliseconds schedules an infinite loop to run after a promise
resolves. The scheduled loop is never interrupted by the timeout:
const vm = require('node:vm');
function loop() {
console.log('entering loop');
while (1) console.log(Date.now());
}
vm.runInNewContext(
'Promise.resolve().then(() => loop());',
{ loop, console },
{ timeout: 5 },
);
// This is printed *before* 'entering loop' (!)
console.log('done executing');
This can be addressed by passing microtaskMode: 'afterEvaluate'
to the code
that creates the Context
:
const vm = require('node:vm');
function loop() {
while (1) console.log(Date.now());
}
vm.runInNewContext(
'Promise.resolve().then(() => loop());',
{ loop, console },
{ timeout: 5, microtaskMode: 'afterEvaluate' },
);
In this case, the microtask scheduled through promise.then()
will be run
before returning from vm.runInNewContext()
, and will be interrupted
by the timeout
functionality. This applies only to code running in a
vm.Context
, so e.g. vm.runInThisContext()
does not take this option.
Promise callbacks are entered into the microtask queue of the context in which
they were created. For example, if () => loop()
is replaced with just loop
in the above example, then loop
will be pushed into the global microtask
queue, because it is a function from the outer (main) context, and thus will
also be able to escape the timeout.
If asynchronous scheduling functions such as process.nextTick()
,
queueMicrotask()
, setTimeout()
, setImmediate()
, etc. are made available
inside a vm.Context
, functions passed to them will be added to global queues,
which are shared by all contexts. Therefore, callbacks passed to those functions
are not controllable through the timeout either.