Table of Contents

• VM (Executing JavaScript)
  • Class: vm.Module
    • Constructor: new vm.Module(code[, options])
    • module.dependencySpecifiers
    • module.error
    • module.linkingStatus
    • module.namespace
    • module.status
    • module.url
    • module.evaluate([options])
    • module.instantiate()
    • module.link(linker)
  • Class: vm.Script
    • new vm.Script(code, options)
    • script.createCachedData()
    • script.runInContext(contextifiedSandbox[, options])
    • script.runInNewContext([sandbox[, options]])
    • script.runInThisContext([options])
  • vm.createContext([sandbox[, options]])
  • vm.isContext(sandbox)
  • vm.runInContext(code, contextifiedSandbox[, options])
  • vm.runInNewContext(code[, sandbox[, options]])
  • vm.runInThisContext(code[, options])
  • Example: Running an HTTP Server within a VM
  • What does it mean to "contextify" an object?

VM (Executing JavaScript)#

The vm module provides APIs for compiling and running code within V8 Virtual Machine contexts.

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 sandboxed environment. The sandboxed code uses a different V8 Context, meaning that it has a different global object than the rest of the code.

One can provide the context by "contextifying" a sandbox object. The sandboxed code treats any property in the sandbox like a global variable. Any changes to global variables caused by the sandboxed code are reflected in the sandbox object.

const vm = require('vm');

const x = 1;

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

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

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

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

The vm module is not a security mechanism. Do not use it to run untrusted code.

Class: vm.Module#

This feature is only available with the --experimental-vm-modules command flag enabled.

The vm.Module class provides a low-level interface for using ECMAScript modules in VM contexts. It is the counterpart of the vm.Script class that closely mirrors Source Text Module Records as defined in the ECMAScript specification.

Unlike vm.Script however, every vm.Module object is bound to a context from its creation. Operations on vm.Module objects are intrinsically asynchronous, in contrast with the synchronous nature of vm.Script objects. With the help of async functions, however, manipulating vm.Module objects is fairly straightforward.

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

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

const vm = require('vm');

const 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".

  const bar = new vm.Module(`
    import s from 'foo';
    s;
  `, { 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 `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.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}`);
  }
  await bar.link(linker);

  // 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().

  bar.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();

  console.log(result);
  // 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> Specifies the column number offset that is displayed in stack traces produced by this Module.
  • initalizeImportMeta <Function> Called during evaluation of this Module to initialize the import.meta. This function has the signature (meta, module), where meta is the import.meta object in the Module, and module is this vm.Module object.

Creates a new ES Module object.

Properties assigned to the import.meta object that are objects may allow the Module to access information outside the specified context, if the object is created in the top level context. Use vm.runInContext() to create objects in a specific context.

const vm = require('vm');

const contextifiedSandbox = vm.createContext({ secret: 42 });

(async () => {
  const module = new vm.Module(
    '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 sandbox.
        meta.prop = {};
      }
    });
  // Since module has no dependencies, the linker function will never be called.
  await module.link(() => {});
  module.initialize();
  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('{}', contextifiedSandbox);
})();

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 Records in the ECMAScript specification.

module.error#

• <any>

If the module.status is 'errored', this property contains the exception thrown by the module during evaluation. If the status is anything else, accessing this property will result in a thrown exception.

The value undefined cannot be used for cases where there is not a thrown exception due to possible ambiguity with throw undefined;.

Corresponds to the [[EvaluationError]] field of Source Text Module Records in the ECMAScript specification.

module.linkingStatus#

• <string>

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

• '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': module.link() has been called, and all its dependencies have been successfully linked.
• 'errored': module.link() 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 specification.

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 module.link() 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 Records 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 specification.

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

Corresponds to the Instantiate() concrete method field of Source Text Module Records in the ECMAScript specification.

module.link(linker)#

• 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:

• specifier The specifier of the requested module:

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

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

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

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 specification.

Class: vm.Script#

Instances of the vm.Script class contain precompiled scripts that can be executed in specific sandboxes (or "contexts").

new vm.Script(code, options)#

• code <string> The JavaScript code to compile.
• options
  • filename <string> Specifies the filename used in stack traces produced by this script.
  • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
  • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
  • cachedData <Buffer> Provides an optional Buffer 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().

Creating a new vm.Script object compiles code but does not run it. The compiled vm.Script can be run later multiple times. The code is not bound to any global object; rather, it is bound before each run, just for that run.

script.createCachedData()#

• Returns: <Buffer>

Creates a code cache that can be used with the Script constructor's cachedData option. Returns a Buffer. This method may be called at any time and any number of times.

const script = new vm.Script(`
function add(a, b) {
  return a + b;
}

const x = add(1, 2);
`);

const cacheWithoutX = script.createCachedData();

script.runInThisContext();

const cacheWithX = script.createCachedData();

script.runInContext(contextifiedSandbox[, options])#

• contextifiedSandbox <Object> A contextified object as returned by the vm.createContext() method.
• options <Object>
  • filename <string> Specifies the filename used in stack traces produced by this script.
  • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
  • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
  • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
  • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.
  • breakOnSigint: 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 terminated, an Error will be thrown.

Runs the compiled code contained by the vm.Script object within the given contextifiedSandbox 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 sandbox object.

const util = require('util');
const vm = require('vm');

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

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

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

console.log(util.inspect(sandbox));

// { 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([sandbox[, options]])#

• sandbox <Object> An object that will be contextified. If undefined, a new object will be created.
• options <Object>
  • filename <string> Specifies the filename used in stack traces produced by this script.
  • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
  • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
  • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
  • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.
  • contextName <string> Human-readable name of the newly created context. Default: 'VM Context i', where i is an ascending numerical index of the created context.
  • contextOrigin <string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the url.origin property of a URL object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: ''.
  • contextCodeGeneration <Object>
    • strings <boolean> If set to false any calls to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
    • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.

First contextifies the given sandbox, runs the compiled code contained by the vm.Script object within the created sandbox, 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 sandbox.

const util = require('util');
const vm = require('vm');

const script = new vm.Script('globalVar = "set"');

const sandboxes = [{}, {}, {}];
sandboxes.forEach((sandbox) => {
  script.runInNewContext(sandbox);
});

console.log(util.inspect(sandboxes));

// [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]

script.runInThisContext([options])#

• options <Object>
  • filename <string> Specifies the filename used in stack traces produced by this script.
  • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
  • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
  • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
  • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

Runs the compiled code contained by the vm.Script within the context of the current global object. Running code does not have access to local scope, but does have access to the current global object.

The following example compiles code that increments a global variable then executes that code multiple times:

const vm = require('vm');

global.globalVar = 0;

const script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });

for (let i = 0; i < 1000; ++i) {
  script.runInThisContext();
}

console.log(globalVar);

// 1000

vm.createContext([sandbox[, options]])#

• sandbox <Object>
• options <Object>
  • name <string> Human-readable name of the newly created context. Default: 'VM Context i', where i is an ascending numerical index of the created context.
  • origin <string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the url.origin property of a URL object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: ''.
  • codeGeneration <Object>
    • strings <boolean> If set to false any calls to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
    • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.

If given a sandbox object, the vm.createContext() method will prepare that sandbox so that it can be used in calls to vm.runInContext() or script.runInContext(). Inside such scripts, the sandbox object will be the global object, retaining all of its existing properties but also having the built-in objects and functions any standard global object has. Outside of scripts run by the vm module, global variables will remain unchanged.

const util = require('util');
const vm = require('vm');

global.globalVar = 3;

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

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

console.log(util.inspect(sandbox)); // { globalVar: 2 }

console.log(util.inspect(globalVar)); // 3

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

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

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

vm.isContext(sandbox)#

• sandbox <Object>
• Returns: <boolean>

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

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

• code <string> The JavaScript code to compile and run.
• contextifiedSandbox <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.
  • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
  • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
  • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
  • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

The vm.runInContext() method compiles code, runs it within the context of the contextifiedSandbox, then returns the result. Running code does not have access to the local scope. The contextifiedSandbox object must have been previously contextified using the vm.createContext() method.

If options is a string, then it specifies the filename.

The following example compiles and executes different scripts using a single contextified object:

const util = require('util');
const vm = require('vm');

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

for (let i = 0; i < 10; ++i) {
  vm.runInContext('globalVar *= 2;', sandbox);
}
console.log(util.inspect(sandbox));

// { globalVar: 1024 }

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

• code <string> The JavaScript code to compile and run.
• sandbox <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.
  • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
  • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
  • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
  • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.
  • contextName <string> Human-readable name of the newly created context. Default: 'VM Context i', where i is an ascending numerical index of the created context.
  • contextOrigin <string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the url.origin property of a URL object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: ''.

The vm.runInNewContext() first contextifies the given sandbox object (or creates a new sandbox if passed as undefined), compiles the code, runs it within the context of 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 sandbox.

const util = require('util');
const vm = require('vm');

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

vm.runInNewContext('count += 1; name = "kitty"', sandbox);
console.log(util.inspect(sandbox));

// { animal: 'cat', count: 3, name: 'kitty' }

vm.runInThisContext(code[, options])#

• code <string> The JavaScript code to compile and run.
• options <Object> | <string>
  • filename <string> Specifies the filename used in stack traces produced by this script.
  • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
  • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
  • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
  • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

vm.runInThisContext() compiles code, runs it within the context of the current global and returns the result. Running code does not have access to local scope, but does have access to the current global object.

If options is a string, then it specifies the filename.

The following example illustrates using both vm.runInThisContext() and the JavaScript eval() function to run the same code:

const vm = require('vm');
let localVar = 'initial value';

const vmResult = vm.runInThisContext('localVar = "vm";');
console.log('vmResult:', vmResult);
console.log('localVar:', localVar);

const evalResult = eval('localVar = "eval";');
console.log('evalResult:', evalResult);
console.log('localVar:', localVar);

// vmResult: 'vm', localVar: 'initial value'
// evalResult: 'eval', localVar: 'eval'

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

Example: Running an HTTP Server within a VM#

When using either script.runInThisContext() or vm.runInThisContext(), the code is executed within the current V8 global context. The code passed to this VM context will have its own isolated scope.

In order to run a simple web server using the http module the code passed to the context must either call require('http') on its own, or have a reference to the http module passed to it. For instance:

'use strict';
const vm = require('vm');

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

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

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

vm.runInThisContext(code)(require);

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

What does it mean to "contextify" an object?#

All JavaScript executed within Node.js runs within the scope of a "context". According to the V8 Embedder's Guide:

In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.

When the method vm.createContext() is called, the sandbox object that is passed in (or a newly created object if sandbox 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 sandbox object is what this document refers to as "contextifying" the sandbox.