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 on the sandbox like a global variable. Any changes on 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.

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

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.
  • 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.
  • 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.

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

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' }

Note: 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.

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])

• sandbox <Object>

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.

vm.isContext(sandbox)

• sandbox <Object>

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

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

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.

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.runInDebugContext(code)

• code <string> The JavaScript code to compile and run.

The vm.runInDebugContext() method compiles and executes code inside the V8 debug context. The primary use case is to gain access to the V8 Debug object:

const vm = require('vm');
const Debug = vm.runInDebugContext('Debug');
console.log(Debug.findScript(process.emit).name);  // 'events.js'
console.log(Debug.findScript(process.exit).name);  // 'internal/process.js'

Note: The debug context and object are intrinsically tied to V8's debugger implementation and may change (or even be removed) without prior warning.

The Debug object can also be made available using the V8-specific --expose_debug_as= command line option.

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
  • 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.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.

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
  • 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.

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);

Note: 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 modules 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.