vm.md

# VM (executing JavaScript)

<!--introduced_in=v0.10.0-->

> Stability: 2 - Stable

<!--name=vm-->

<!-- source_link=lib/vm.js -->

The `vm` module enables compiling and running code within V8 Virtual
Machine contexts.

<strong class="critical">The `vm` module is not a security
mechanism. Do not use it to run untrusted code.</strong>

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_][contextified] 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.

```js
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`

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

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

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added: v0.3.1
changes:
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import assertions to the
                 `importModuleDynamically` parameter.
  - version: v10.6.0
    pr-url: https://github.com/nodejs/node/pull/20300
    description: The `produceCachedData` is deprecated in favour of
                 `script.createCachedData()`.
  - version: v5.7.0
    pr-url: https://github.com/nodejs/node/pull/4777
    description: The `cachedData` and `produceCachedData` options are
                 supported now.
-->

* `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.createCachedData()`

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* 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.

```js
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])`

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-->

* `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`
    (<kbd>Ctrl</kbd>+<kbd>C</kbd>) 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.

```js
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]])`

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  - version: v10.0.0
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    description: The `contextCodeGeneration` option is supported now.
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    description: The `breakOnSigint` option is supported now.
-->

* `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`
    (<kbd>Ctrl</kbd>+<kbd>C</kbd>) 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][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 `Promise`s and `async function`s) 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`.

```js
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])`

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    description: The `breakOnSigint` option is supported now.
-->

* `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`
    (<kbd>Ctrl</kbd>+<kbd>C</kbd>) 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:

```js
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
```

## Class: `vm.Module`

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 - v13.0.0
 - v12.16.0
-->

> 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 Record][]s 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.

```mjs
import vm from '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();
```

```cjs
const vm = require('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();
})();
```

### `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 Record][]s 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 Record][]s
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`
    (<kbd>Ctrl</kbd>+<kbd>C</kbd>) 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
Record][]s 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:
    ```mjs
    import foo from 'foo';
    //              ^^^^^ the module specifier
    ```

  * `extra` {Object}
    * `assert` {Object} The data from the assertion:
      <!-- eslint-skip -->
      ```js
      import foo from 'foo' assert { name: 'value' };
      //                           ^^^^^^^^^^^^^^^^^ 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.

  * `referencingModule` {vm.Module} The `Module` object `link()` 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
Record][]s 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`

<!-- YAML
added: v9.6.0
-->

> 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.

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

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    description: Added support for import assertions to the
                 `importModuleDynamically` parameter.
-->

* `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.
  * `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.

```mjs
import vm from '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);
```

```cjs
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()`

<!-- YAML
added:
 - v13.7.0
 - v12.17.0
-->

* 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.

```js
// 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`

<!-- YAML
added:
 - v13.0.0
 - v12.16.0
-->

> 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.

```js
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...
```

### `new vm.SyntheticModule(exportNames, evaluateCallback[, options])`

<!-- YAML
added:
 - v13.0.0
 - v12.16.0
-->

* `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)`

<!-- YAML
added:
 - v13.0.0
 - v12.16.0
-->

* `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.

```mjs
import vm from 'vm';

const m = new vm.SyntheticModule(['x'], () => {
  m.setExport('x', 1);
});

await m.link(() => {});
await m.evaluate();

assert.strictEqual(m.namespace.x, 1);
```

```cjs
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]])`

<!-- YAML
added: v10.10.0
changes:
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import assertions to the
                 `importModuleDynamically` parameter.
  - version: v15.9.0
    pr-url: https://github.com/nodejs/node/pull/35431
    description: Added `importModuleDynamically` option again.
  - version: v14.3.0
    pr-url: https://github.com/nodejs/node/pull/33364
    description: Removal of `importModuleDynamically` due to compatibility
                 issues.
  - version:
    - v14.1.0
    - v13.14.0
    pr-url: https://github.com/nodejs/node/pull/32985
    description: The `importModuleDynamically` option is now supported.
-->

* `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]])`

<!-- YAML
added: v0.3.1
changes:
  - version: v14.6.0
    pr-url: https://github.com/nodejs/node/pull/34023
    description: The `microtaskMode` option is supported now.
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/19398
    description: The first argument can no longer be a function.
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/19016
    description: The `codeGeneration` option is supported now.
-->

* `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][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 `Promise`s and `async function`s) 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][contextified] 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.

```js
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)`

<!-- YAML
added: v0.11.7
-->

* `object` {Object}
* Returns: {boolean}

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

## `vm.measureMemory([options])`

<!-- YAML
added: v13.10.0
-->

> 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.

```js
const vm = require('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);
  });
```

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

<!-- YAML
added: v0.3.1
changes:
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import assertions to the
                 `importModuleDynamically` parameter.
  - version: v6.3.0
    pr-url: https://github.com/nodejs/node/pull/6635
    description: The `breakOnSigint` option is supported now.
-->

* `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`
    (<kbd>Ctrl</kbd>+<kbd>C</kbd>) 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. 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.
* 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:

```js
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]])`

<!-- YAML
added: v0.3.1
changes:
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import assertions to the
                 `importModuleDynamically` parameter.
  - version: v14.6.0
    pr-url: https://github.com/nodejs/node/pull/34023
    description: The `microtaskMode` option is supported now.
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/19016
    description: The `contextCodeGeneration` option is supported now.
  - version: v6.3.0
    pr-url: https://github.com/nodejs/node/pull/6635
    description: The `breakOnSigint` option is supported now.
-->

* `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`
    (<kbd>Ctrl</kbd>+<kbd>C</kbd>) 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][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. 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.
  * `microtaskMode` {string} If set to `afterEvaluate`, microtasks (tasks
    scheduled through `Promise`s and `async function`s) 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`.

```js
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])`

<!-- YAML
added: v0.3.1
changes:
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import assertions to the
                 `importModuleDynamically` parameter.
  - version: v6.3.0
    pr-url: https://github.com/nodejs/node/pull/6635
    description: The `breakOnSigint` option is supported now.
-->

* `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`
    (<kbd>Ctrl</kbd>+<kbd>C</kbd>) 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. 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.
* 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:

<!-- eslint-disable prefer-const -->

```js
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:

```js
'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 interactions with asynchronous tasks and Promises

`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:

```js
const vm = require('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`:

```js
const vm = require('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.

[Cyclic Module Record]: https://tc39.es/ecma262/#sec-cyclic-module-records
[ECMAScript Module Loader]: esm.md#modules-ecmascript-modules
[Evaluate() concrete method]: https://tc39.es/ecma262/#sec-moduleevaluation
[GetModuleNamespace]: https://tc39.es/ecma262/#sec-getmodulenamespace
[HostResolveImportedModule]: https://tc39.es/ecma262/#sec-hostresolveimportedmodule
[Link() concrete method]: https://tc39.es/ecma262/#sec-moduledeclarationlinking
[Module Record]: https://www.ecma-international.org/ecma-262/#sec-abstract-module-records
[Source Text Module Record]: https://tc39.es/ecma262/#sec-source-text-module-records
[Synthetic Module Record]: https://heycam.github.io/webidl/#synthetic-module-records
[V8 Embedder's Guide]: https://v8.dev/docs/embed#contexts
[`ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING`]: errors.md#err_vm_dynamic_import_callback_missing
[`ERR_VM_MODULE_STATUS`]: errors.md#err_vm_module_status
[`Error`]: errors.md#class-error
[`URL`]: url.md#class-url
[`eval()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval
[`optionsExpression`]: https://tc39.es/proposal-import-assertions/#sec-evaluate-import-call
[`script.runInContext()`]: #scriptrunincontextcontextifiedobject-options
[`script.runInThisContext()`]: #scriptruninthiscontextoptions
[`url.origin`]: url.md#urlorigin
[`vm.createContext()`]: #vmcreatecontextcontextobject-options
[`vm.runInContext()`]: #vmrunincontextcode-contextifiedobject-options
[`vm.runInThisContext()`]: #vmruninthiscontextcode-options
[contextified]: #what-does-it-mean-to-contextify-an-object
[global object]: https://es5.github.io/#x15.1
[indirect `eval()` call]: https://es5.github.io/#x10.4.2
[origin]: https://developer.mozilla.org/en-US/docs/Glossary/Origin