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Executing JavaScript

Stability: 2 - Unstable. See Caveats, below.

You can access this module with:

var vm = require('vm');

JavaScript code can be compiled and run immediately or compiled, saved, and run later.

Caveats

The vm module has many known issues and edge cases. If you run into issues or unexpected behavior, please consult the open issues on GitHub. Some of the biggest problems are described below.

Sandboxes

The sandbox argument to vm.runInNewContext and vm.createContext, along with the initSandbox argument to vm.createContext, do not behave as one might normally expect and their behavior varies between different versions of Node.

The key issue to be aware of is that V8 provides no way to directly control the global object used within a context. As a result, while properties of your sandbox object will be available in the context, any properties from the prototypes of the sandbox may not be available. Furthermore, the this expression within the global scope of the context evaluates to the empty object ({}) instead of to your sandbox.

Your sandbox's properties are also not shared directly with the script. Instead, the properties of the sandbox are copied into the context at the beginning of execution, and then after execution, the properties are copied back out in an attempt to propagate any changes.

Globals

Properties of the global object, like Array and String, have different values inside of a context. This means that common expressions like [] instanceof Array or Object.getPrototypeOf([]) === Array.prototype may not produce expected results when used inside of scripts evaluated via the vm module.

Some of these problems have known workarounds listed in the issues for vm on GitHub. for example, Array.isArray works around the example problem with Array.

vm.runInThisContext(code, [filename])

vm.runInThisContext() compiles code, runs it and returns the result. Running code does not have access to local scope. filename is optional, it's used only in stack traces.

Example of using vm.runInThisContext and eval to run the same code:

var localVar = 123,
    usingscript, evaled,
    vm = require('vm');

usingscript = vm.runInThisContext('localVar = 1;',
  'myfile.vm');
console.log('localVar: ' + localVar + ', usingscript: ' +
  usingscript);
evaled = eval('localVar = 1;');
console.log('localVar: ' + localVar + ', evaled: ' +
  evaled);

// localVar: 123, usingscript: 1
// localVar: 1, evaled: 1

vm.runInThisContext does not have access to the local scope, so localVar is unchanged. eval does have access to the local scope, so localVar is changed.

In case of syntax error in code, vm.runInThisContext emits the syntax error to stderr and throws an exception.

vm.runInNewContext(code, [sandbox], [filename], [timeout])

vm.runInNewContext compiles code, then runs it in sandbox and returns the result. Running code does not have access to local scope. The object sandbox will be used as the global object for code. sandbox and filename are optional, filename is only used in stack traces. timeout specifies an optional number of milliseconds to execute code before terminating execution. If execution is terminated, null will be thrown.

Example: compile and execute code that increments a global variable and sets a new one. These globals are contained in the sandbox.

var util = require('util'),
    vm = require('vm'),
    sandbox = {
      animal: 'cat',
      count: 2
    };

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

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

Note that running untrusted code is a tricky business requiring great care. To prevent accidental global variable leakage, vm.runInNewContext is quite useful, but safely running untrusted code requires a separate process.

In case of syntax error in code, vm.runInNewContext emits the syntax error to stderr and throws an exception.

vm.runInContext(code, context, [filename], [timeout])

vm.runInContext compiles code, then runs it in context and returns the result. A (V8) context comprises a global object, together with a set of built-in objects and functions. Running code does not have access to local scope and the global object held within context will be used as the global object for code. filename is optional, it's used only in stack traces. timeout specifies an optional number of milliseconds to execute code before terminating execution. If execution is terminated, null will be thrown.

Example: compile and execute code in a existing context.

var util = require('util'),
    vm = require('vm'),
    initSandbox = {
      animal: 'cat',
      count: 2
    },
    context = vm.createContext(initSandbox);

vm.runInContext('count += 1; name = "CATT"', context, 'myfile.vm');
console.log(util.inspect(context));

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

Note that createContext will perform a shallow clone of the supplied sandbox object in order to initialize the global object of the freshly constructed context.

Note that running untrusted code is a tricky business requiring great care. To prevent accidental global variable leakage, vm.runInContext is quite useful, but safely running untrusted code requires a separate process.

In case of syntax error in code, vm.runInContext emits the syntax error to stderr and throws an exception.

vm.createContext([initSandbox])

vm.createContext creates a new context which is suitable for use as the 2nd argument of a subsequent call to vm.runInContext. A (V8) context comprises a global object together with a set of build-in objects and functions. The optional argument initSandbox will be shallow-copied to seed the initial contents of the global object used by the context.

vm.createScript(code, [filename])

createScript compiles code but does not run it. Instead, it returns a vm.Script object representing this compiled code. This script can be run later many times using methods below. The returned script is not bound to any global object. It is bound before each run, just for that run. filename is optional, it's only used in stack traces.

In case of syntax error in code, createScript prints the syntax error to stderr and throws an exception.

Class: Script

A class for running scripts. Returned by vm.createScript.

script.runInThisContext([timeout])

Similar to vm.runInThisContext but a method of a precompiled Script object. script.runInThisContext runs the code of script and returns the result. Running code does not have access to local scope, but does have access to the global object (v8: in actual context). timeout specifies an optional number of milliseconds to execute code before terminating execution. If execution is terminated, null will be thrown.

Example of using script.runInThisContext to compile code once and run it multiple times:

var vm = require('vm');

globalVar = 0;

var script = vm.createScript('globalVar += 1', 'myfile.vm');

for (var i = 0; i < 1000 ; i += 1) {
  script.runInThisContext();
}

console.log(globalVar);

// 1000

script.runInNewContext([sandbox], [timeout])

Similar to vm.runInNewContext a method of a precompiled Script object. script.runInNewContext runs the code of script with sandbox as the global object and returns the result. Running code does not have access to local scope. sandbox is optional. timeout specifies an optional number of milliseconds to execute code before terminating execution. If execution is terminated, null will be thrown.

Example: compile code that increments a global variable and sets one, then execute this code multiple times. These globals are contained in the sandbox.

var util = require('util'),
    vm = require('vm'),
    sandbox = {
      animal: 'cat',
      count: 2
    };

var script = vm.createScript('count += 1; name = "kitty"', 'myfile.vm');

for (var i = 0; i < 10 ; i += 1) {
  script.runInNewContext(sandbox);
}

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

// { animal: 'cat', count: 12, name: 'kitty' }

Note that running untrusted code is a tricky business requiring great care. To prevent accidental global variable leakage, script.runInNewContext is quite useful, but safely running untrusted code requires a separate process.

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