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Executing JavaScript

Stability: 3 - Stable

You can access this module with:

var vm = require('vm');

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

vm.runInThisContext(code, [options])

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

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

var localVar = 'initial value';

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

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

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

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 this way vm.runInThisContext is much like an indirect eval call, e.g. (0,eval)('code'). However, it also has the following additional options:

  • filename: allows you to control the filename that shows up in any stack traces produced.
  • displayErrors: whether or not to print any errors to stderr, with the line of code that caused them highlighted, before throwing an exception. Will capture both syntax errors from compiling code and runtime errors thrown by executing the compiled code. Defaults to true.
  • timeout: a number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

vm.createContext([sandbox])

If given a sandbox object, will "contextify" that sandbox so that it can be used in calls to vm.runInContext or script.runInContext. Inside scripts run as such, sandbox will be the global object, retaining all 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, sandbox will be unchanged.

If not given a sandbox object, returns a new, empty contextified sandbox object you can use.

This function is useful for creating a sandbox that can be used to run multiple scripts, e.g. if you were emulating a web browser it could be used to create a single sandbox representing a window's global object, then run all <script> tags together inside that sandbox.

vm.isContext(sandbox)

Returns whether or not a sandbox object has been contextified by calling vm.createContext on it.

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

vm.runInContext compiles code, then runs it in contextifiedSandbox and returns the result. Running code does not have access to local scope. The contextifiedSandbox object must have been previously contextified via vm.createContext; it will be used as the global object for code.

vm.runInContext takes the same options as vm.runInThisContext.

Example: compile and execute differnt scripts in a single existing context.

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

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

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

// { globalVar: 1024 }

Note that running untrusted code is a tricky business requiring great care. vm.runInContext is quite useful, but safely running untrusted code requires a separate process.

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

vm.runInNewContext compiles code, contextifies sandbox if passed or creates a new contextified sandbox if it's omitted, and then runs the code with the sandbox as the global object and returns the result.

vm.runInNewContext takes the same options as vm.runInThisContext.

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');
var vm = require('vm'),

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

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

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

Note that running untrusted code is a tricky business requiring great care. vm.runInNewContext is quite useful, but safely running untrusted code requires a separate process.

Class: Script

A class for holding precompiled scripts, and running them in specific sandboxes.

new vm.Script(code, options)

Creating a new Script compiles code but does not run it. Instead, the created vm.Script object represents 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.

The options when creating a script are:

  • filename: allows you to control the filename that shows up in any stack traces produced from this script.
  • displayErrors: whether or not to print any errors to stderr, with the line of code that caused them highlighted, before throwing an exception. Applies only to syntax errors compiling the code; errors while running the code are controlled by the options to the script's methods.

script.runInThisContext([options])

Similar to vm.runInThisContext but a method of a precompiled Script object. script.runInThisContext runs script's compiled code and returns the result. Running code does not have access to local scope, but does have access to the current global object.

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

var vm = require('vm');

global.globalVar = 0;

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

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

console.log(globalVar);

// 1000

The options for running a script are:

  • displayErrors: whether or not to print any runtime errors to stderr, with the line of code that caused them highlighted, before throwing an exception. Applies only to runtime errors executing the code; it is impossible to create a Script instance with syntax errors, as the constructor will throw.
  • timeout: a number of milliseconds to execute the script before terminating execution. If execution is terminated, an Error will be thrown.

script.runInContext(contextifiedSandbox, [options])

Similar to vm.runInContext but a method of a precompiled Script object. script.runInContext runs script's compiled code in contextifiedSandbox and returns the result. Running code does not have access to local scope.

script.runInContext takes the same options as script.runInThisContext.

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

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

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

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

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

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

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

Note that running untrusted code is a tricky business requiring great care. script.runInContext is quite useful, but safely running untrusted code requires a separate process.

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

Similar to vm.runInNewContext but a method of a precompiled Script object. script.runInNewContext contextifies sandbox if passed or creates a new contextified sandbox if it's omitted, and then runs script's compiled code with the sandbox as the global object and returns the result. Running code does not have access to local scope.

script.runInNewContext takes the same options as script.runInThisContext.

Example: compile code that sets a global variable, then execute the code multiple times in different contexts. These globals are set on and contained in the sandboxes.

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

var sandboxes = [{}, {}, {}];

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

sandboxes.forEach(function (sandbox) {
  script.runInNewContext(sandbox);
});

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

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

Note that running untrusted code is a tricky business requiring great care. script.runInNewContext is quite useful, but safely running untrusted code requires a separate process.