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Node.js v15.0.0-test20200429329279fc84 Documentation
Table of Contents
-
vm.measureMemory([options])
vm.compileFunction(code[, params[, options]])
vm.createContext([contextObject[, options]])
vm.isContext(object)
vm.runInContext(code, contextifiedObject[, options])
vm.runInNewContext(code[, contextObject[, options]])
vm.runInThisContext(code[, options])
- Example: Running an HTTP Server within a VM
- What does it mean to "contextify" an object?
- Timeout limitations when using
process.nextTick()
, Promises, andqueueMicrotask()
VM (Executing JavaScript)#
The vm
module enables compiling and running code within V8 Virtual
Machine contexts. The 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('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.
Class: vm.Script
#
Instances of the vm.Script
class contain precompiled scripts that can be
executed in specific contexts.
Constructor: new vm.Script(code[, options])
#
code
<string> The JavaScript code to compile.-
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 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, and should not be considered stable.specifier
<string> specifier passed toimport()
script
<vm.Script>- 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.
script.createCachedData()
#
- 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.
const script = new vm.Script(`
function add(a, b) {
return a + b;
}
const x = add(1, 2);
`);
const cacheWithoutX = script.createCachedData();
script.runInThisContext();
const cacheWithX = script.createCachedData();
script.runInContext(contextifiedObject[, options])
#
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
, the execution will be terminated whenSIGINT
(Ctrl+C) is received. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
will be disabled during script execution, but will continue to work after that. If execution is terminated, anError
will be thrown. 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 util = require('util');
const vm = require('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.
script.runInNewContext([contextObject[, options]])
#
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
, the execution will be terminated whenSIGINT
(Ctrl+C) is received. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
will be disabled during script execution, but will continue to work after that. If execution is terminated, anError
will be thrown. 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>
- 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 util = require('util');
const vm = require('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' }]
script.runInThisContext([options])
#
-
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
, the execution will be terminated whenSIGINT
(Ctrl+C) is received. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
will be disabled during script execution, but will continue to work after that. If execution is terminated, anError
will be thrown. 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('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
vm.measureMemory([options])
#
Measure the memory known to V8 and used by the current execution context or a specified context.
-
options
<Object> Optional.mode
<string> Either'summary'
or'detailed'
. Default:'summary'
context
<Object> Optional. A contextified object returned byvm.createContext()
. If not specified, measure the memory usage of the current context wherevm.measureMemory()
is invoked.
- 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()
measures
the memory reachable by V8 from a specific context, while
v8.getHeapSpaceStatistics()
measures the memory used by an instance
of V8 engine, which can switch among multiple contexts that reference
objects in the heap of one engine.
const vm = require('vm');
// Measure the memory used by the current context and return the result
// in summary.
vm.measureMemory({ mode: 'summary' })
// Is the same as vm.measureMemory()
.then((result) => {
// The current format is:
// { total: { jsMemoryEstimate: 2211728, jsMemoryRange: [ 0, 2211728 ] } }
console.log(result);
});
const context = vm.createContext({});
vm.measureMemory({ mode: 'detailed' }, context)
.then((result) => {
// At the moment the detailed format is the same as the summary one.
console.log(result);
});
Class: vm.Module
#
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 currently no way to interact with the Loader, though support is planned.
const vm = require('vm');
const contextifiedObject = vm.createContext({ secret: 42 });
(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;
`, { 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 with a single
// property "result" that contains the result of the very last statement
// executed in the Module. In the case of `bar`, it is `s;`, which refers to
// the default export of the `foo` module, the `secret` we set in the
// beginning to 42.
const { result } = await bar.evaluate();
console.log(result);
// Prints 42.
})();
module.dependencySpecifiers
#
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.
module.error
#
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.
module.evaluate([options])
#
-
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
, the execution will be terminated whenSIGINT
(Ctrl+C) is received. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
will be disabled during script execution, but will continue to work after that. If execution is interrupted, anError
will be thrown. Default:false
.
- Returns: <Promise>
Evaluate the module.
This must be called after the module has been linked; otherwise it will throw an error. It could be called also when the module has already been evaluated, in which case it will do one of the following two things:
- return
undefined
if the initial evaluation ended in success (module.status
is'evaluated'
) - rethrow the same exception the initial evaluation threw if the initial
evaluation ended in an error (
module.status
is'errored'
)
This method cannot be called while the module is being evaluated
(module.status
is 'evaluating'
) to prevent infinite recursion.
Corresponds to the Evaluate() concrete method field of Cyclic Module Records in the ECMAScript specification.
module.link(linker)
#
-
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. -
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.
module.namespace
#
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.
module.status
#
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
.
module.identifier
#
The identifier of the current module, as set in the constructor.
Class: vm.SourceTextModule
#
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.
Constructor: new vm.SourceTextModule(code[, options])
#
code
<string> JavaScript Module code to parse-
options
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.lineOffset
<integer> Specifies the line number offset that is displayed in stack traces produced by thisModule
. Default:0
.columnOffset
<integer> Specifies the 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>- 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.
const vm = require('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);
})();
sourceTextModule.createCachedData()
#
- 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.
// 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 });
Class: vm.SyntheticModule
#
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('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...
Constructor: new vm.SyntheticModule(exportNames, evaluateCallback[, options])
#
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.
syntheticModule.setExport(name, value)
#
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.
const vm = require('vm');
(async () => {
const m = new vm.SyntheticModule(['x'], () => {
m.setExport('x', 1);
});
await m.link(() => {});
await m.evaluate();
assert.strictEqual(m.namespace.x, 1);
})();
vm.compileFunction(code[, params[, options]])
#
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 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>- 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
.
vm.createContext([contextObject[, options]])
#
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>
- 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 util = require('util');
const vm = require('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.
vm.isContext(object)
#
Returns true
if the given oject
object has been contextified using
vm.createContext()
.
vm.runInContext(code, contextifiedObject[, options])
#
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.-
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 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
, the execution will be terminated whenSIGINT
(Ctrl+C) is received. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
will be disabled during script execution, but will continue to work after that. If execution is terminated, anError
will be thrown. Default:false
.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, and should not be considered stable.specifier
<string> specifier passed toimport()
module
<vm.Module>- 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 util = require('util');
const vm = require('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 }
vm.runInNewContext(code[, contextObject[, options]])
#
code
<string> The JavaScript code to compile and run.contextObject
<Object> An object that will be contextified. Ifundefined
, a new object will be created.-
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 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
, the execution will be terminated whenSIGINT
(Ctrl+C) is received. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
will be disabled during script execution, but will continue to work after that. If execution is terminated, anError
will be thrown. 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> 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, and should not be considered stable.specifier
<string> specifier passed toimport()
module
<vm.Module>- 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.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 util = require('util');
const vm = require('vm');
const contextObject = {
animal: 'cat',
count: 2
};
vm.runInNewContext('count += 1; name = "kitty"', contextObject);
console.log(contextObject);
// Prints: { animal: 'cat', count: 3, name: 'kitty' }
vm.runInThisContext(code[, options])
#
code
<string> The JavaScript code to compile and run.-
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 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
, the execution will be terminated whenSIGINT
(Ctrl+C) is received. Existing handlers for the event that have been attached viaprocess.on('SIGINT')
will be disabled during script execution, but will continue to work after that. If execution is terminated, anError
will be thrown. Default:false
.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, and should not be considered stable.specifier
<string> specifier passed toimport()
module
<vm.Module>- 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('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')
.
Example: Running an HTTP Server within a VM#
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 http
module the code passed to
the context must either call require('http')
on its own, or have a reference
to the http
module passed to it. For instance:
'use strict';
const vm = require('vm');
const code = `
((require) => {
const http = require('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.
What does it mean to "contextify" an object?#
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 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.
Timeout limitations when using process.nextTick()
, Promises, and queueMicrotask()
#
Because of the internal mechanics of how the process.nextTick()
queue and
the microtask queue that underlies Promises are implemented within V8 and
Node.js, it is possible for code running within a context to "escape" the
timeout
set using vm.runInContext()
, vm.runInNewContext()
, and
vm.runInThisContext()
.
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('vm');
function loop() {
while (1) console.log(Date.now());
}
vm.runInNewContext(
'Promise.resolve().then(loop);',
{ loop, console },
{ timeout: 5 }
);
This issue also occurs when the loop()
call is scheduled using
the process.nextTick()
and queueMicrotask()
functions.
This issue occurs because all contexts share the same microtask and nextTick queues.