Node.js v21.0.0-nightly20230423511d40cecd documentation

Class: vm.Script#

Instances of the vm.Script class contain precompiled scripts that can be executed in specific contexts.

new vm.Script(code[, options])#

• 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 optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source. When supplied, the cachedDataRejected value will be set to either true or false depending on acceptance of the data by V8.
  • produceCachedData <boolean> When true and no cachedData is present, V8 will attempt to produce code cache data for code. Upon success, a Buffer with V8's code cache data will be produced and stored in the cachedData property of the returned vm.Script instance. The cachedDataProduced value will be set to either true or false depending on whether code cache data is produced successfully. This option is deprecated in favor of script.createCachedData(). Default: false.
  • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_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 to import()
    • script <vm.Script>
    • importAssertions <Object> The "assert" value passed to the optionsExpression 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 contain then 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.cachedDataRejected#

• <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.

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.

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()`. copy

script.runInContext(contextifiedObject[, options])#

• contextifiedObject <Object> A contextified object as returned by the vm.createContext() method.
• options <Object>
  • displayErrors <boolean> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
  • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
  • breakOnSigint <boolean> If true, receiving SIGINT (Ctrl+C) will terminate execution and throw an Error. Existing handlers for the event that have been attached via process.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' } copy

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. If undefined, a new object will be created.
• options <Object>
  • displayErrors <boolean> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
  • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
  • breakOnSigint <boolean> If true, receiving SIGINT (Ctrl+C) will terminate execution and throw an Error. Existing handlers for the event that have been attached via process.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', where i 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 the url.origin property of a URL 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 to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
    • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.
  • microtaskMode <string> If set to afterEvaluate, microtasks (tasks scheduled through Promises and async functions) will be run immediately after the script has run. They are included in the timeout and breakOnSigint 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' }] copy

script.runInThisContext([options])#

• options <Object>
  • displayErrors <boolean> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
  • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
  • breakOnSigint <boolean> If true, receiving SIGINT (Ctrl+C) will terminate execution and throw an Error. Existing handlers for the event that have been attached via process.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 copy

script.sourceMapURL#

• <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.jsonconst vm = require('node:vm');

const script = new vm.Script(`
function myFunc() {}
//# sourceMappingURL=sourcemap.json
`);

console.log(script.sourceMapURL);
// Prints: sourcemap.jsoncopy

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 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();
})();copy

module.dependencySpecifiers#

• <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.

module.error#

• <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.

module.evaluate([options])#

• options <Object>
  • timeout <integer> Specifies the number of milliseconds to evaluate before terminating execution. If execution is interrupted, an Error will be thrown. This value must be a strictly positive integer.
  • breakOnSigint <boolean> If true, receiving SIGINT (Ctrl+C) will terminate execution and throw an Error. Existing handlers for the event that have been attached via process.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.

module.identifier#

• <string>

The identifier of the current module, as set in the constructor.

module.link(linker)#

• linker <Function>

  • specifier <string> The specifier of the requested module:

    import foo from 'foo';
    //              ^^^^^ the module specifier copy

  • referencingModule <vm.Module> The Module object link() is called on.

  • extra <Object>

    • assert <Object> The data from the assertion:

      import foo from 'foo' assert { name: 'value' };
      //                           ^^^^^^^^^^^^^^^^^ the assertion copy

      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.

  • 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#

• <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.

module.status#

• <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, but module.evaluate() has not yet been called.

• 'evaluating': The module is being evaluated through a module.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.

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.

new vm.SourceTextModule(code[, options])#

• code <string> JavaScript Module code to parse
• options
  • identifier <string> String used in stack traces. Default: 'vm:module(i)' where i is a context-specific ascending index.
  • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source. The code must be the same as the module from which this cachedData was created.
  • context <Object> The contextified object as returned by the vm.createContext() method, to compile and evaluate this Module 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 this Module. Default: 0.
  • columnOffset <integer> Specifies the first-line column number offset that is displayed in stack traces produced by this Module. Default: 0.
  • initializeImportMeta <Function> Called during evaluation of this Module to initialize the import.meta.
    • meta <import.meta>
    • module <vm.SourceTextModule>
  • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING.
    • specifier <string> specifier passed to import()
    • module <vm.Module>
    • importAssertions <Object> The "assert" value passed to the optionsExpression 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 contain then 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);
})();copy

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.

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 }); copy

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('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... copy

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)' where i is a context-specific ascending index.
  • context <Object> The contextified object as returned by the vm.createContext() method, to compile and evaluate this Module 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)#

• 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);
})();copy

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 first-line column number offset that is displayed in stack traces produced by this script. Default: 0.
  • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView 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 when import() is called. If this option is not specified, calls to import() will reject with ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING. This option is part of the experimental modules API, and should not be considered stable.
    • specifier <string> specifier passed to import()
    • function <Function>
    • importAssertions <Object> The "assert" value passed to the optionsExpression 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 contain then 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', where i 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 the url.origin property of a URL 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 to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
    • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.
  • microtaskMode <string> If set to afterEvaluate, microtasks (tasks scheduled through Promises and async functions) will be run immediately after a script has run through script.runInContext(). They are included in the timeout and breakOnSigint 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 copy

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)#

• object <Object>
• Returns: <boolean>

Returns true if the given object object has been contextified using vm.createContext().

vm.measureMemory([options])#

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);
  }); copy

vm.runInContext(code, contextifiedObject[, options])#

• code <string> The JavaScript code to compile and run.
• contextifiedObject <Object> The contextified object that will be used as the global when the code 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> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
  • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
  • breakOnSigint <boolean> If true, receiving SIGINT (Ctrl+C) will terminate execution and throw an Error. Existing handlers for the event that have been attached via process.on('SIGINT') are disabled during script execution, but continue to work after that. Default: false.
  • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source.
  • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_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 to import()
    • script <vm.Script>
    • importAssertions <Object> The "assert" value passed to the optionsExpression 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 contain then 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 } copy

vm.runInNewContext(code[, contextObject[, options]])#

• code <string> The JavaScript code to compile and run.
• contextObject <Object> An object that will be contextified. If undefined, 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> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
  • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
  • breakOnSigint <boolean> If true, receiving SIGINT (Ctrl+C) will terminate execution and throw an Error. Existing handlers for the event that have been attached via process.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', where i 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 the url.origin property of a URL 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 to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
    • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.
  • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source.
  • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_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 to import()
    • script <vm.Script>
    • importAssertions <Object> The "assert" value passed to the optionsExpression 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 contain then function exports.
  • microtaskMode <string> If set to afterEvaluate, microtasks (tasks scheduled through Promises and async functions) will be run immediately after the script has run. They are included in the timeout and breakOnSigint 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' } copy

vm.runInThisContext(code[, options])#

• 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> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
  • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
  • breakOnSigint <boolean> If true, receiving SIGINT (Ctrl+C) will terminate execution and throw an Error. Existing handlers for the event that have been attached via process.on('SIGINT') are disabled during script execution, but continue to work after that. Default: false.
  • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source.
  • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_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 to import()
    • script <vm.Script>
    • importAssertions <Object> The "assert" value passed to the optionsExpression 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 contain then 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' copy

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 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); copy

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 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.

Timeout interactions with asynchronous tasks and Promises#

Promises and async functions 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'); copy

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' },
); copy

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.