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# Executing JavaScript#

    Stability: 2 - Stable

You can access this module with:

    const vm = require('vm');

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

## 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.
- `lineOffset`: allows you to add an offset to the line number that is displayed in stack traces
- `columnOffset`: allows you to add an offset to the column number that is displayed in stack traces
- `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.
- `timeout`: a number of milliseconds to execute `code` 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.

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

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

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

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

    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.

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

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

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

    sandboxes.forEach((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.

### 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:

    const vm = require('vm');

    global.globalVar = 0;

    const 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:

- `filename`: allows you to control the filename that shows up in any stack traces produced.
- `lineOffset`: allows you to add an offset to the line number that is displayed in stack traces
- `columnOffset`: allows you to add an offset to the column number that is displayed in stack traces
- `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 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.

## 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 different scripts in a single existing context.

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

    const 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.runInDebugContext(code)#

`vm.runInDebugContext` compiles and executes `code` inside the V8 debug context. The primary use case is to get access to the V8 debug object:

    const Debug = vm.runInDebugContext('Debug');
    Debug.scripts().forEach(function(script) { console.log(script.name); });

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

The debug object can also be exposed with the `--expose_debug_as=` switch.

## 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.

    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' }

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.

## 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:

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

`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.
- `lineOffset`: allows you to add an offset to the line number that is displayed in stack traces
- `columnOffset`: allows you to add an offset to the column number that is displayed in stack traces
- `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.