# VM (executing JavaScript)

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

> Stability: 2 - Stable

<!--name=vm-->

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

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

<strong class="critical">The `node: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('node:vm');

const x = 1;

const context = { x: 2 };
vm.createContext(context); // Contextify the object.

const code = 'x += 40; var y = 17;';
// `x` and `y` are global variables in the context.
// Initially, x has the value 2 because that is the value of context.x.
vm.runInContext(code, context);

console.log(context.x); // 42
console.log(context.y); // 17

console.log(x); // 1; y is not defined.
```

## 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:
    - v21.7.0
    - v20.12.0
    pr-url: https://github.com/nodejs/node/pull/51244
    description: Added support for
                `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import attributes 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|vm.constants.USE\_MAIN\_CONTEXT\_DEFAULT\_LOADER}
    Used to specify how the modules should be loaded during the evaluation
    of this script when `import()` is called. This option is part of the
    experimental modules API. We do not recommend using it in a production
    environment. For detailed information, see
    [Support of dynamic `import()` in compilation APIs][].

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`

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* {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()`

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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.

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.

```js
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()`.
```

### `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('node:vm');

const context = {
  animal: 'cat',
  count: 2,
};

const script = new vm.Script('count += 1; name = "kitty";');

vm.createContext(context);
for (let i = 0; i < 10; ++i) {
  script.runInContext(context);
}

console.log(context);
// Prints: { animal: 'cat', count: 12, name: 'kitty' }
```

Using the `timeout` or `breakOnSigint` options will result in new event loops
and corresponding threads being started, which have a non-zero performance
overhead.

### `script.runInNewContext([contextObject[, options]])`

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    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.
-->

* `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('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' }]
```

### `script.runInThisContext([options])`

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-->

* `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('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
```

### `script.sourceMapURL`

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* {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.

```mjs
import vm from 'node:vm';

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

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

```cjs
const vm = require('node:vm');

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

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

## Class: `vm.Module`

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> 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 '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();
```

```cjs
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();
})();
```

### `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)`

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* `linker` {Function}
  * `specifier` {string} The specifier of the requested module:
    ```mjs
    import foo from 'foo';
    //              ^^^^^ the module specifier
    ```

  * `referencingModule` {vm.Module} The `Module` object `link()` is called on.

  * `extra` {Object}
    * `attributes` {Object} The data from the attribute:
      ```mjs
      import foo from 'foo' with { name: 'value' };
      //                         ^^^^^^^^^^^^^^^^^ the attribute
      ```
      Per ECMA-262, hosts are expected to trigger an error if an
      unsupported attribute is present.
    * `assert` {Object} Alias for `extra.attributes`.

  * 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`

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> 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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* `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} Used to specify the
    how the modules should be loaded during the evaluation of this module
    when `import()` is called. This option is part of the experimental
    modules API. We do not recommend using it in a production environment.
    For detailed information, see
    [Support of dynamic `import()` in compilation APIs][].

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 '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);
```

```cjs
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);
})();
```

### `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.

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.

```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('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...
```

### `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 'node: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('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);
})();
```

## `vm.compileFunction(code[, params[, options]])`

<!-- YAML
added: v10.10.0
changes:
  - version:
    - v21.7.0
    - v20.12.0
    pr-url: https://github.com/nodejs/node/pull/51244
    description: Added support for
                `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
  - version:
    - v19.6.0
    - v18.15.0
    pr-url: https://github.com/nodejs/node/pull/46320
    description: The return value now includes `cachedDataRejected`
                 with the same semantics as the `vm.Script` version
                 if the `cachedData` option was passed.
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import attributes 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. This must be produced by a prior call to [`vm.compileFunction()`][]
    with the same `code` and `params`.
  * `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|vm.constants.USE\_MAIN\_CONTEXT\_DEFAULT\_LOADER}
  Used to specify the how the modules should be loaded during the evaluation of
  this function when `import()` is called. This option is part of the
  experimental modules API. We do not recommend using it in a production
  environment. For detailed information, see
  [Support of dynamic `import()` in compilation APIs][].
* 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.constants`

<!-- YAML
added:
  - v21.7.0
  - v20.12.0
-->

* {Object}

Returns an object containing commonly used constants for VM operations.

### `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`

<!-- YAML
added:
  - v21.7.0
  - v20.12.0
-->

> Stability: 1.1 - Active development

A constant that can be used as the `importModuleDynamically` option to
`vm.Script` and `vm.compileFunction()` so that Node.js uses the default
ESM loader from the main context to load the requested module.

For detailed information, see
[Support of dynamic `import()` in compilation APIs][].

## `vm.createContext([contextObject[, options]])`

<!-- YAML
added: v0.3.1
changes:
  - version:
    - v21.7.0
    - v20.12.0
    pr-url: https://github.com/nodejs/node/pull/51244
    description: Added support for
                 `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
  - version:
    - v21.2.0
    - v20.11.0
    pr-url: https://github.com/nodejs/node/pull/50360
    description: The `importModuleDynamically` option is supported now.
  - 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.
  * `importModuleDynamically`
    {Function|vm.constants.USE\_MAIN\_CONTEXT\_DEFAULT\_LOADER}
    Used to specify the how the modules should be loaded when `import()` is
    called in this context without a referrer script or module. This option is
    part of the experimental modules API. We do not recommend using it in a
    production environment. For detailed information, see
    [Support of dynamic `import()` in compilation APIs][].
* Returns: {Object} contextified object.

If given a `contextObject`, the `vm.createContext()` method will [prepare that
object][contextified] and return a reference to it 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('node:vm');

global.globalVar = 3;

const context = { globalVar: 1 };
vm.createContext(context);

vm.runInContext('globalVar *= 2;', context);

console.log(context);
// Prints: { globalVar: 2 }

console.log(global.globalVar);
// Prints: 3
```

If `contextObject` is omitted (or passed explicitly as `undefined`), a new,
empty [contextified][] object will be returned.

The `vm.createContext()` method is primarily useful for creating a single
context that can be used to run multiple scripts. For instance, if emulating a
web browser, the method can be used to create a single context representing a
window's global object, then run all `<script>` tags together within that
context.

The provided `name` and `origin` of the context are made visible through the
Inspector API.

## `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.
  Otherwise it will be rejected with an `ERR_CONTEXT_NOT_INITIALIZED` error.

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('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);
  });
```

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

<!-- YAML
added: v0.3.1
changes:
  - version:
    - v21.7.0
    - v20.12.0
    pr-url: https://github.com/nodejs/node/pull/51244
    description: Added support for
                `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import attributes 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.
  * `importModuleDynamically`
    {Function|vm.constants.USE\_MAIN\_CONTEXT\_DEFAULT\_LOADER}
    Used to specify the how the modules should be loaded during the evaluation
    of this script when `import()` is called. This option is part of the
    experimental modules API. We do not recommend using it in a production
    environment. For detailed information, see
    [Support of dynamic `import()` in compilation APIs][].

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('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 }
```

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

<!-- YAML
added: v0.3.1
changes:
  - version:
    - v21.7.0
    - v20.12.0
    pr-url: https://github.com/nodejs/node/pull/51244
    description: Added support for
                `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import attributes 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.
  * `importModuleDynamically`
    {Function|vm.constants.USE\_MAIN\_CONTEXT\_DEFAULT\_LOADER}
    Used to specify the how the modules should be loaded during the evaluation
    of this script when `import()` is called. This option is part of the
    experimental modules API. We do not recommend using it in a production
    environment. For detailed information, see
    [Support of dynamic `import()` in compilation APIs][].
  * `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('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' }
```

## `vm.runInThisContext(code[, options])`

<!-- YAML
added: v0.3.1
changes:
  - version:
    - v21.7.0
    - v20.12.0
    pr-url: https://github.com/nodejs/node/pull/51244
    description: Added support for
                `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
  - version:
    - v17.0.0
    - v16.12.0
    pr-url: https://github.com/nodejs/node/pull/40249
    description: Added support for import attributes 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.
  * `importModuleDynamically`
    {Function|vm.constants.USE\_MAIN\_CONTEXT\_DEFAULT\_LOADER}
    Used to specify the how the modules should be loaded during the evaluation
    of this script when `import()` is called. This option is part of the
    experimental modules API. We do not recommend using it in a production
    environment. For detailed information, see
    [Support of dynamic `import()` in compilation APIs][].
* 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('node:vm');
let localVar = 'initial value';

const vmResult = vm.runInThisContext('localVar = "vm";');
console.log(`vmResult: '${vmResult}', localVar: '${localVar}'`);
// Prints: vmResult: 'vm', localVar: 'initial value'

const evalResult = eval('localVar = "eval";');
console.log(`evalResult: '${evalResult}', localVar: '${localVar}'`);
// Prints: evalResult: 'eval', localVar: 'eval'
```

Because `vm.runInThisContext()` does not have access to the local scope,
`localVar` is unchanged. In contrast, [`eval()`][] _does_ have access to the
local scope, so the value `localVar` is changed. In this way
`vm.runInThisContext()` is much like an [indirect `eval()` call][], e.g.
`(0,eval)('code')`.

## 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:

```js
'use strict';
const vm = require('node:vm');

const code = `
((require) => {
  const http = require('node:http');

  http.createServer((request, response) => {
    response.writeHead(200, { 'Content-Type': 'text/plain' });
    response.end('Hello World\\n');
  }).listen(8124);

  console.log('Server running at http://127.0.0.1:8124/');
})`;

vm.runInThisContext(code)(require);
```

The `require()` in the above case shares the state with the context it is
passed from. This may introduce risks when untrusted code is executed, e.g.
altering objects in the context in unwanted ways.

## 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

`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('node:vm');

function loop() {
  console.log('entering loop');
  while (1) console.log(Date.now());
}

vm.runInNewContext(
  'Promise.resolve().then(() => loop());',
  { loop, console },
  { timeout: 5 },
);
// This is printed *before* 'entering loop' (!)
console.log('done executing');
```

This can be addressed by passing `microtaskMode: 'afterEvaluate'` to the code
that creates the `Context`:

```js
const vm = require('node:vm');

function loop() {
  while (1) console.log(Date.now());
}

vm.runInNewContext(
  'Promise.resolve().then(() => loop());',
  { loop, console },
  { timeout: 5, microtaskMode: 'afterEvaluate' },
);
```

In this case, the microtask scheduled through `promise.then()` will be run
before returning from `vm.runInNewContext()`, and will be interrupted
by the `timeout` functionality. This applies only to code running in a
`vm.Context`, so e.g. [`vm.runInThisContext()`][] does not take this option.

Promise callbacks are entered into the microtask queue of the context in which
they were created. For example, if `() => loop()` is replaced with just `loop`
in the above example, then `loop` will be pushed into the global microtask
queue, because it is a function from the outer (main) context, and thus will
also be able to escape the timeout.

If asynchronous scheduling functions such as `process.nextTick()`,
`queueMicrotask()`, `setTimeout()`, `setImmediate()`, etc. are made available
inside a `vm.Context`, functions passed to them will be added to global queues,
which are shared by all contexts. Therefore, callbacks passed to those functions
are not controllable through the timeout either.

## Support of dynamic `import()` in compilation APIs

The following APIs support an `importModuleDynamically` option to enable dynamic
`import()` in code compiled by the vm module.

* `new vm.Script`
* `vm.compileFunction()`
* `new vm.SourceTextModule`
* `vm.runInThisContext()`
* `vm.runInContext()`
* `vm.runInNewContext()`
* `vm.createContext()`

This option is still part of the experimental modules API. We do not recommend
using it in a production environment.

### When the `importModuleDynamically` option is not specified or undefined

If this option is not specified, or if it's `undefined`, code containing
`import()` can still be compiled by the vm APIs, but when the compiled code is
executed and it actually calls `import()`, the result will reject with
[`ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING`][].

### When `importModuleDynamically` is `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`

This option is currently not supported for `vm.SourceTextModule`.

With this option, when an `import()` is initiated in the compiled code, Node.js
would use the default ESM loader from the main context to load the requested
module and return it to the code being executed.

This gives access to Node.js built-in modules such as `fs` or `http`
to the code being compiled. If the code is executed in a different context,
be aware that the objects created by modules loaded from the main context
are still from the main context and not `instanceof` built-in classes in the
new context.

```cjs
const { Script, constants } = require('node:vm');
const script = new Script(
  'import("node:fs").then(({readFile}) => readFile instanceof Function)',
  { importModuleDynamically: constants.USE_MAIN_CONTEXT_DEFAULT_LOADER });

// false: URL loaded from the main context is not an instance of the Function
// class in the new context.
script.runInNewContext().then(console.log);
```

```mjs
import { Script, constants } from 'node:vm';

const script = new Script(
  'import("node:fs").then(({readFile}) => readFile instanceof Function)',
  { importModuleDynamically: constants.USE_MAIN_CONTEXT_DEFAULT_LOADER });

// false: URL loaded from the main context is not an instance of the Function
// class in the new context.
script.runInNewContext().then(console.log);
```

This option also allows the script or function to load user modules:

```mjs
import { Script, constants } from 'node:vm';
import { resolve } from 'node:path';
import { writeFileSync } from 'node:fs';

// Write test.js and test.txt to the directory where the current script
// being run is located.
writeFileSync(resolve(import.meta.dirname, 'test.mjs'),
              'export const filename = "./test.json";');
writeFileSync(resolve(import.meta.dirname, 'test.json'),
              '{"hello": "world"}');

// Compile a script that loads test.mjs and then test.json
// as if the script is placed in the same directory.
const script = new Script(
  `(async function() {
    const { filename } = await import('./test.mjs');
    return import(filename, { with: { type: 'json' } })
  })();`,
  {
    filename: resolve(import.meta.dirname, 'test-with-default.js'),
    importModuleDynamically: constants.USE_MAIN_CONTEXT_DEFAULT_LOADER,
  });

// { default: { hello: 'world' } }
script.runInThisContext().then(console.log);
```

```cjs
const { Script, constants } = require('node:vm');
const { resolve } = require('node:path');
const { writeFileSync } = require('node:fs');

// Write test.js and test.txt to the directory where the current script
// being run is located.
writeFileSync(resolve(__dirname, 'test.mjs'),
              'export const filename = "./test.json";');
writeFileSync(resolve(__dirname, 'test.json'),
              '{"hello": "world"}');

// Compile a script that loads test.mjs and then test.json
// as if the script is placed in the same directory.
const script = new Script(
  `(async function() {
    const { filename } = await import('./test.mjs');
    return import(filename, { with: { type: 'json' } })
  })();`,
  {
    filename: resolve(__dirname, 'test-with-default.js'),
    importModuleDynamically: constants.USE_MAIN_CONTEXT_DEFAULT_LOADER,
  });

// { default: { hello: 'world' } }
script.runInThisContext().then(console.log);
```

There are a few caveats with loading user modules using the default loader
from the main context:

1. The module being resolved would be relative to the `filename` option passed
   to `vm.Script` or `vm.compileFunction()`. The resolution can work with a
   `filename` that's either an absolute path or a URL string.  If `filename` is
   a string that's neither an absolute path or a URL, or if it's undefined,
   the resolution will be relative to the current working directory
   of the process. In the case of `vm.createContext()`, the resolution is always
   relative to the current working directory since this option is only used when
   there isn't a referrer script or module.
2. For any given `filename` that resolves to a specific path, once the process
   manages to load a particular module from that path, the result may be cached,
   and subsequent load of the same module from the same path would return the
   same thing. If the `filename` is a URL string, the cache would not be hit
   if it has different search parameters. For `filename`s that are not URL
   strings, there is currently no way to bypass the caching behavior.

### When `importModuleDynamically` is a function

When `importModuleDynamically` is a function, it will be invoked when `import()`
is called in the compiled code for users to customize how the requested module
should be compiled and evaluated. Currently, the Node.js instance must be
launched with the `--experimental-vm-modules` flag for this option to work. If
the flag isn't set, this callback will be ignored. If the code evaluated
actually calls to `import()`, the result will reject with
[`ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING_FLAG`][].

The callback `importModuleDynamically(specifier, referrer, importAttributes)`
has the following signature:

* `specifier` {string} specifier passed to `import()`
* `referrer` {vm.Script|Function|vm.SourceTextModule|Object}
  The referrer is the compiled `vm.Script` for `new vm.Script`,
  `vm.runInThisContext`, `vm.runInContext` and `vm.runInNewContext`. It's the
  compiled `Function` for `vm.compileFunction`, the compiled
  `vm.SourceTextModule` for `new vm.SourceTextModule`, and the context `Object`
  for `vm.createContext()`.
* `importAttributes` {Object} The `"with"` 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.

```mjs
// This script must be run with --experimental-vm-modules.
import { Script, SyntheticModule } from 'node:vm';

const script = new Script('import("foo.json", { with: { type: "json" } })', {
  async importModuleDynamically(specifier, referrer, importAttributes) {
    console.log(specifier);  // 'foo.json'
    console.log(referrer);   // The compiled script
    console.log(importAttributes);  // { type: 'json' }
    const m = new SyntheticModule(['bar'], () => { });
    await m.link(() => { });
    m.setExport('bar', { hello: 'world' });
    return m;
  },
});
const result = await script.runInThisContext();
console.log(result);  //  { bar: { hello: 'world' } }
```

```cjs
// This script must be run with --experimental-vm-modules.
const { Script, SyntheticModule } = require('node:vm');

(async function main() {
  const script = new Script('import("foo.json", { with: { type: "json" } })', {
    async importModuleDynamically(specifier, referrer, importAttributes) {
      console.log(specifier);  // 'foo.json'
      console.log(referrer);   // The compiled script
      console.log(importAttributes);  // { type: 'json' }
      const m = new SyntheticModule(['bar'], () => { });
      await m.link(() => { });
      m.setExport('bar', { hello: 'world' });
      return m;
    },
  });
  const result = await script.runInThisContext();
  console.log(result);  //  { bar: { hello: 'world' } }
})();
```

[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://262.ecma-international.org/14.0/#sec-abstract-module-records
[Source Text Module Record]: https://tc39.es/ecma262/#sec-source-text-module-records
[Support of dynamic `import()` in compilation APIs]: #support-of-dynamic-import-in-compilation-apis
[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_FLAG`]: errors.md#err_vm_dynamic_import_callback_missing_flag
[`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-attributes/#sec-evaluate-import-call
[`script.runInContext()`]: #scriptrunincontextcontextifiedobject-options
[`script.runInThisContext()`]: #scriptruninthiscontextoptions
[`url.origin`]: url.md#urlorigin
[`vm.compileFunction()`]: #vmcompilefunctioncode-params-options
[`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