Executing JavaScript

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

    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:

    1. var localVar = 'initial value';
    3. var vmResult = vm.runInThisContext('localVar = "vm";');
    4. console.log('vmResult: ', vmResult);
    5. console.log('localVar: ', localVar);
    7. var evalResult = eval('localVar = "eval";');
    8. console.log('evalResult: ', evalResult);
    9. console.log('localVar: ', localVar);
    11. // vmResult: 'vm', localVar: 'initial value'
    12. // 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:

    • 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.

    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 .

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

    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 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.

    1. var util = require('util');
    2. var vm = require('vm'),
    4. var sandbox = {
    5.   animal: 'cat',
    6.   count: 2
    7. };
    9. vm.runInNewContext('count += 1; name = "kitty"', sandbox);
    10. console.log(util.inspect(sandbox));
    12. // { 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.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:

    1. var Debug = vm.runInDebugContext('Debug');
    2. 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.

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

    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.

    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:

    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.

    Similar to vm.runInContext but a method of a precompiled Script object.
    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.

    1. var util = require('util');
    2. var vm = require('vm');
    4. var sandbox = {
    5.   count: 2
    6. };
    8. var script = new vm.Script('count += 1; name = "kitty"');
    10. for (var i = 0; i < 10; ++i) {
    11.   script.runInContext(sandbox);
    12. }
    14. console.log(util.inspect(sandbox));
    16. // { 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.

    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.

    1. var util = require('util');
    2. var vm = require('vm');
    4. var sandboxes = [{}, {}, {}];
    6. var script = new vm.Script('globalVar = "set"');
    8. sandboxes.forEach(function (sandbox) {
    9.   script.runInNewContext(sandbox);
    10. });
    12. console.log(util.inspect(sandboxes));
    13. // [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]

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