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vm: add modules

Adds vm.Module, which wraps around ModuleWrap to provide an interface
for developers to work with modules in a more reflective manner.

Co-authored-by: Timothy Gu <>
PR-URL: #17560
Reviewed-By: Michaël Zasso <>
Reviewed-By: Tiancheng "Timothy" Gu <>
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devsnek and TimothyGu committed Jan 31, 2018
1 parent 2033a9f commit 0993fbe5b213d0fe746c3162bcda85f6c66bb552
@@ -1639,6 +1639,43 @@ entry types were found.

Superseded by `ERR_OUT_OF_RANGE`


The module attempted to be linked is not eligible for linking, because of one of
the following reasons:

- It has already been linked (`linkingStatus` is `'linked'`)
- It is being linked (`linkingStatus` is `'linking'`)
- Linking has failed for this module (`linkingStatus` is `'errored'`)


The module being returned from the linker function is from a different context
than the parent module. Linked modules must share the same context.


The linker function returned a module for which linking has failed.


The module must be successfully linked before instantiation.


The fulfilled value of a linking promise is not a `vm.Module` object.


The current module's status does not allow for this operation. The specific
meaning of the error depends on the specific function.


@@ -43,6 +43,322 @@ console.log(x); // 1; y is not defined.
*Note*: The vm module is not a security mechanism.
**Do not use it to run untrusted code**.

## Class: vm.Module
<!-- YAML

> 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 [Source Text Module Record][]s as defined in the ECMAScript

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. With the help
of async functions, however, manipulating `vm.Module` objects is fairly

Using a `vm.Module` object requires four distinct steps: creation/parsing,
linking, instantiation, and evaluation. These four steps are illustrated in the
following example.

*Note*: This implementation lies at a lower level than the [ECMAScript Module
loader][]. There is also currently no way to interact with the Loader, though
support is planned.

const vm = require('vm');
const contextifiedSandbox = vm.createContext({ secret: 42 });
(async () => {
// Step 1
// Create a Module by constructing a new `vm.Module` 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
// `contextifiedSandbox` 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".

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jdalton Feb 7, 2018


Typo 👆 local binding "s".

const bar = new vm.Module(`
import s from 'foo';
`, { context: contextifiedSandbox });
// 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 ``.
// 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(referencingModule, specifier) {
if (specifier === 'foo') {
return new vm.Module(`
// The "secret" variable refers to the global variable we added to
// "contextifiedSandbox" when creating the context.
export default secret;
`, { context: referencingModule.context });
// Using `contextifiedSandbox` instead of `referencingModule.context`
// here would work as well.
throw new Error(`Unable to resolve dependency: ${specifier}`);
// Step 3
// Instantiate the top-level Module.
// Only the top-level Module needs to be explicitly instantiated; its
// dependencies will be recursively instantiated by instantiate().
// Step 4
// Evaluate the Module. The evaluate() method returns a Promise with a single
// property "result" that contains the result of the very last statement
// executed in the Module. In the case of `bar`, it is `s;`, which refers to
// the default export of the `foo` module, the `secret` we set in the
// beginning to 42.
const { result } = await bar.evaluate();
// Prints 42.

### Constructor: new vm.Module(code[, options])

* `code` {string} JavaScript Module code to parse
* `options`
* `url` {string} URL used in module resolution and 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.
* `lineOffset` {integer} Specifies the line number offset that is displayed
in stack traces produced by this Module.
* `columnOffset` {integer} Spcifies the column number offset that is displayed
in stack traces produced by this Module.

Creates a new ES `Module` object.

### 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 [Source Text 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.

*Note*: `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 [Source Text Module Record][]s
in the ECMAScript specification.

### module.linkingStatus

* {string}

The current linking status of `module`. It will be one of the following values:

- `'unlinked'`: `` has not yet been called.
- `'linking'`: `` has been called, but not all Promises returned by
the linker function have been resolved yet.
- `'linked'`: `` has been called, and all its dependencies have
been successfully linked.
- `'errored'`: `` has been called, but at least one of its
dependencies failed to link, either because the callback returned a Promise
that is rejected, or because the Module the callback returned is invalid.

### module.namespace

* {Object}

The namespace object of the module. This is only available after instantiation
(`module.instantiate()`) has completed.

Corresponds to the [GetModuleNamespace][] abstract operation in the ECMAScript

### module.status

* {string}

The current status of the module. Will be one of:

- `'uninstantiated'`: The module is not instantiated. It may because of any of
the following reasons:

- The module was just created.
- `module.instantiate()` has been called on this module, but it failed for
some reason.

This status does not convey any information regarding if `` has
been called. See `module.linkingStatus` for that.

- `'instantiating'`: The module is currently being instantiated through a
`module.instantiate()` call on itself or a parent module.

- `'instantiated'`: The module has been instantiated successfully, 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
[Source Text Module Record][]'s [[Status]] field. `'errored'` corresponds to
`'evaluated'` in the specification, but with [[EvaluationError]] set to a value
that is not `undefined`.

### module.url

* {string}

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

### module.evaluate([options])

* `options` {Object}
* `timeout` {number} Specifies the number of milliseconds to evaluate
before terminating execution. If execution is interrupted, an [`Error`][]
will be thrown.
* `breakOnSigint` {boolean} If `true`, the execution will be terminated when
`SIGINT` (Ctrl+C) is received. Existing handlers for the event that have
been attached via `process.on("SIGINT")` will be disabled during script
execution, but will continue to work after that. If execution is
interrupted, an [`Error`][] will be thrown.
* Returns: {Promise}

Evaluate the module.

This must be called after the module has been instantiated; otherwise it will
throw an error. It could be called also when the module has already been
evaluated, in which case it will do one of the following two things:

- return `undefined` if the initial evaluation ended in success (`module.status`
is `'evaluated'`)
- rethrow the same exception the initial evaluation threw if the initial
evaluation ended in an error (`module.status` is `'errored'`)

This method cannot be called while the module is being evaluated
(`module.status` is `'evaluating'`) to prevent infinite recursion.

Corresponds to the [Evaluate() concrete method][] field of [Source Text Module
Record][]s in the ECMAScript specification.

### module.instantiate()

Instantiate the module. This must be called after linking has completed
(`linkingStatus` is `'linked'`); otherwise it will throw an error. It may also
throw an exception if one of the dependencies does not provide an export the
parent module requires.

However, if this function succeeded, further calls to this function after the
initial instantiation will be no-ops, to be consistent with the ECMAScript

Unlike other methods operating on `Module`, this function completes
synchronously and returns nothing.

Corresponds to the [Instantiate() concrete method][] field of [Source Text
Module Record][]s in the ECMAScript specification.


* `linker` {Function}
* Returns: {Promise}

Link module dependencies. This method must be called before instantiation, and
can only be called once per module.

Two parameters will be passed to the `linker` function:

- `referencingModule` The `Module` object `link()` is called on.
- `specifier` The specifier of the requested module:

<!-- eslint-skip -->
import foo from 'foo';
// ^^^^^ the module specifier

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 `linkingStatus` must not be `'errored'`.

If the returned `Module`'s `linkingStatus` is `'unlinked'`, this method will be
recursively called on the returned `Module` with the same provided `linker`

`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 linker function is executed during linking, a Node.js-specific stage
before instantiation, while [HostResolveImportedModule][] is called during

The actual [HostResolveImportedModule][] implementation used during module
instantiation 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

## Class: vm.Script
<!-- YAML
added: v0.3.1
@@ -518,8 +834,14 @@ associating it with the `sandbox` object is what this document refers to as
[`vm.createContext()`]: #vm_vm_createcontext_sandbox_options
[`vm.runInContext()`]: #vm_vm_runincontext_code_contextifiedsandbox_options
[`vm.runInThisContext()`]: #vm_vm_runinthiscontext_code_options
[ECMAScript Module Loader]: esm.html#esm_ecmascript_modules
[Evaluate() concrete method]:
[Instantiate() concrete method]:
[V8 Embedder's Guide]:'s%20Guide#contexts
[contextified]: #vm_what_does_it_mean_to_contextify_an_object
[global object]:
[indirect `eval()` call]:
[Source Text Module Record]:
@@ -676,6 +676,15 @@ E('ERR_V8BREAKITERATOR', 'Full ICU data not installed. ' +
'At least one valid performance entry type is required');
E('ERR_VM_MODULE_ALREADY_LINKED', 'Module has already been linked');
'Linked modules must use the same context');
'Linking has already failed for the provided module');
'Module must be linked before it can be instantiated');
E('ERR_VM_MODULE_NOT_MODULE', 'Provided module is not an instance of Module');
E('ERR_VM_MODULE_STATUS', 'Module status %s');
E('ERR_ZLIB_BINDING_CLOSED', 'zlib binding closed');
E('ERR_ZLIB_INITIALIZATION_FAILED', 'Initialization failed');

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