The vm module provides APIs for compiling and running code within V8 Virtual\nMachine contexts.
JavaScript code can be compiled and run immediately or\ncompiled, saved, and run later.
\nA common use case is to run the code in a sandboxed environment.\nThe sandboxed code uses a different V8 Context, meaning that\nit has a different global object than the rest of the code.
\nOne can provide the context by "contextifying" a sandbox\nobject. The sandboxed code treats any property on the sandbox like a\nglobal variable. Any changes on global variables caused by the sandboxed\ncode are reflected in the sandbox object.
\nconst vm = require('vm');\n\nconst x = 1;\n\nconst sandbox = { x: 2 };\nvm.createContext(sandbox); // Contextify the sandbox.\n\nconst code = 'x += 40; var y = 17;';\n// x and y are global variables in the sandboxed environment.\n// Initially, x has the value 2 because that is the value of sandbox.x.\nvm.runInContext(code, sandbox);\n\nconsole.log(sandbox.x); // 42\nconsole.log(sandbox.y); // 17\n\nconsole.log(x); // 1; y is not defined.\nThe vm module is not a security mechanism. Do not use it to run untrusted\ncode.
\n", "classes": [ { "textRaw": "Class: vm.Module", "type": "class", "name": "vm.Module", "meta": { "added": [ "v9.6.0" ], "changes": [] }, "stability": 1, "stabilityText": "Experimental", "desc": "This feature is only available with the --experimental-vm-modules command\nflag enabled.
The vm.Module class provides a low-level interface for using ECMAScript\nmodules in VM contexts. It is the counterpart of the vm.Script class that\nclosely mirrors Source Text Module Records as defined in the ECMAScript\nspecification.
Unlike vm.Script however, every vm.Module object is bound to a context from\nits creation. Operations on vm.Module objects are intrinsically asynchronous,\nin contrast with the synchronous nature of vm.Script objects. With the help\nof async functions, however, manipulating vm.Module objects is fairly\nstraightforward.
Using a vm.Module object requires four distinct steps: creation/parsing,\nlinking, instantiation, and evaluation. These four steps are illustrated in the\nfollowing example.
This implementation lies at a lower level than the ECMAScript Module\nloader. There is also currently no way to interact with the Loader, though\nsupport is planned.
\nconst vm = require('vm');\n\nconst contextifiedSandbox = vm.createContext({ secret: 42 });\n\n(async () => {\n // Step 1\n //\n // Create a Module by constructing a new `vm.Module` object. This parses the\n // provided source text, throwing a `SyntaxError` if anything goes wrong. By\n // default, a Module is created in the top context. But here, we specify\n // `contextifiedSandbox` as the context this Module belongs to.\n //\n // Here, we attempt to obtain the default export from the module "foo", and\n // put it into local binding "secret".\n\n const bar = new vm.Module(`\n import s from 'foo';\n s;\n `, { context: contextifiedSandbox });\n\n // Step 2\n //\n // "Link" the imported dependencies of this Module to it.\n //\n // The provided linking callback (the "linker") accepts two arguments: the\n // parent module (`bar` in this case) and the string that is the specifier of\n // the imported module. The callback is expected to return a Module that\n // corresponds to the provided specifier, with certain requirements documented\n // in `module.link()`.\n //\n // If linking has not started for the returned Module, the same linker\n // callback will be called on the returned Module.\n //\n // Even top-level Modules without dependencies must be explicitly linked. The\n // callback provided would never be called, however.\n //\n // The link() method returns a Promise that will be resolved when all the\n // Promises returned by the linker resolve.\n //\n // Note: This is a contrived example in that the linker function creates a new\n // "foo" module every time it is called. In a full-fledged module system, a\n // cache would probably be used to avoid duplicated modules.\n\n async function linker(specifier, referencingModule) {\n if (specifier === 'foo') {\n return new vm.Module(`\n // The "secret" variable refers to the global variable we added to\n // "contextifiedSandbox" when creating the context.\n export default secret;\n `, { context: referencingModule.context });\n\n // Using `contextifiedSandbox` instead of `referencingModule.context`\n // here would work as well.\n }\n throw new Error(`Unable to resolve dependency: ${specifier}`);\n }\n await bar.link(linker);\n\n // Step 3\n //\n // Instantiate the top-level Module.\n //\n // Only the top-level Module needs to be explicitly instantiated; its\n // dependencies will be recursively instantiated by instantiate().\n\n bar.instantiate();\n\n // Step 4\n //\n // Evaluate the Module. The evaluate() method returns a Promise with a single\n // property "result" that contains the result of the very last statement\n // executed in the Module. In the case of `bar`, it is `s;`, which refers to\n // the default export of the `foo` module, the `secret` we set in the\n // beginning to 42.\n\n const { result } = await bar.evaluate();\n\n console.log(result);\n // Prints 42.\n})();\nCreates a new ES Module object.
Evaluate the module.
\nThis must be called after the module has been instantiated; otherwise it will\nthrow an error. It could be called also when the module has already been\nevaluated, in which case it will do one of the following two things:
\nundefined if the initial evaluation ended in success (module.status\nis 'evaluated')module.status is 'errored')This method cannot be called while the module is being evaluated\n(module.status is 'evaluating') to prevent infinite recursion.
Corresponds to the Evaluate() concrete method field of Source Text Module\nRecords in the ECMAScript specification.
\n" }, { "textRaw": "module.instantiate()", "type": "method", "name": "instantiate", "desc": "Instantiate the module. This must be called after linking has completed\n(linkingStatus is 'linked'); otherwise it will throw an error. It may also\nthrow an exception if one of the dependencies does not provide an export the\nparent module requires.
However, if this function succeeded, further calls to this function after the\ninitial instantiation will be no-ops, to be consistent with the ECMAScript\nspecification.
\nUnlike other methods operating on Module, this function completes\nsynchronously and returns nothing.
Corresponds to the Instantiate() concrete method field of Source Text\nModule Records in the ECMAScript specification.
\n", "signatures": [ { "params": [] } ] }, { "textRaw": "module.link(linker)", "type": "method", "name": "link", "signatures": [ { "return": { "textRaw": "Returns: {Promise} ", "name": "return", "type": "Promise" }, "params": [ { "textRaw": "`linker` {Function} ", "name": "linker", "type": "Function" } ] }, { "params": [ { "name": "linker" } ] } ], "desc": "Link module dependencies. This method must be called before instantiation, and\ncan only be called once per module.
\nTwo parameters will be passed to the linker function:
specifier The specifier of the requested module:\nimport foo from 'foo';\n// ^^^^^ the module specifier\nreferencingModule The Module object link() is called on.The function is expected to return a Module object or a Promise that\neventually resolves to a Module object. The returned Module must satisfy the\nfollowing two invariants:
Module.linkingStatus must not be 'errored'.If the returned Module's linkingStatus is 'unlinked', this method will be\nrecursively called on the returned Module with the same provided linker\nfunction.
link() returns a Promise that will either get resolved when all linking\ninstances resolve to a valid Module, or rejected if the linker function either\nthrows an exception or returns an invalid Module.
The linker function roughly corresponds to the implementation-defined\nHostResolveImportedModule abstract operation in the ECMAScript\nspecification, with a few key differences:
\nThe actual HostResolveImportedModule implementation used during module\ninstantiation is one that returns the modules linked during linking. Since at\nthat point all modules would have been fully linked already, the\nHostResolveImportedModule implementation is fully synchronous per\nspecification.
\n" } ], "properties": [ { "textRaw": "`dependencySpecifiers` {string[]} ", "type": "string[]", "name": "dependencySpecifiers", "desc": "The specifiers of all dependencies of this module. The returned array is frozen\nto disallow any changes to it.
\nCorresponds to the [[RequestedModules]] field of Source Text Module Records\nin the ECMAScript specification.
\n" }, { "textRaw": "`error` {any} ", "type": "any", "name": "error", "desc": "If the module.status is 'errored', this property contains the exception \nthrown by the module during evaluation. If the status is anything else, \naccessing this property will result in a thrown exception.
The value undefined cannot be used for cases where there is not a thrown\nexception due to possible ambiguity with throw undefined;.
Corresponds to the [[EvaluationError]] field of Source Text Module Records\nin the ECMAScript specification.
\n" }, { "textRaw": "`linkingStatus` {string} ", "type": "string", "name": "linkingStatus", "desc": "The current linking status of module. It will be one of the following values:
'unlinked': module.link() has not yet been called.'linking': module.link() has been called, but not all Promises returned by\nthe linker function have been resolved yet.'linked': module.link() has been called, and all its dependencies have\nbeen successfully linked.'errored': module.link() has been called, but at least one of its\ndependencies failed to link, either because the callback returned a Promise\nthat is rejected, or because the Module the callback returned is invalid.The namespace object of the module. This is only available after instantiation\n(module.instantiate()) has completed.
Corresponds to the GetModuleNamespace abstract operation in the ECMAScript\nspecification.
\n" }, { "textRaw": "`status` {string} ", "type": "string", "name": "status", "desc": "The current status of the module. Will be one of:
\n'uninstantiated': The module is not instantiated. It may because of any of\nthe following reasons:
module.instantiate() has been called on this module, but it failed for\nsome reason.This status does not convey any information regarding if module.link() has\nbeen called. See module.linkingStatus for that.
'instantiating': The module is currently being instantiated through a\nmodule.instantiate() call on itself or a parent module.
'instantiated': The module has been instantiated successfully, but\nmodule.evaluate() has not yet been called.
'evaluating': The module is being evaluated through a module.evaluate() on\nitself or a parent module.
'evaluated': The module has been successfully evaluated.
'errored': The module has been evaluated, but an exception was thrown.
Other than 'errored', this status string corresponds to the specification's\nSource Text Module Record's [[Status]] field. 'errored' corresponds to\n'evaluated' in the specification, but with [[EvaluationError]] set to a value\nthat is not undefined.
The URL of the current module, as set in the constructor.
\n" } ] }, { "textRaw": "Class: vm.Script", "type": "class", "name": "vm.Script", "meta": { "added": [ "v0.3.1" ], "changes": [] }, "desc": "Instances of the vm.Script class contain precompiled scripts that can be\nexecuted in specific sandboxes (or "contexts").
Creating a new vm.Script object compiles code but does not run it. The\ncompiled vm.Script can be run later multiple times. The code is not bound to\nany global object; rather, it is bound before each run, just for that run.
Runs the compiled code contained by the vm.Script object within the given\ncontextifiedSandbox and returns the result. Running code does not have access\nto local scope.
The following example compiles code that increments a global variable, sets\nthe value of another global variable, then execute the code multiple times.\nThe globals are contained in the sandbox object.
const util = require('util');\nconst vm = require('vm');\n\nconst sandbox = {\n animal: 'cat',\n count: 2\n};\n\nconst script = new vm.Script('count += 1; name = "kitty";');\n\nconst context = vm.createContext(sandbox);\nfor (let i = 0; i < 10; ++i) {\n script.runInContext(context);\n}\n\nconsole.log(util.inspect(sandbox));\n\n// { animal: 'cat', count: 12, name: 'kitty' }\nUsing the timeout or breakOnSigint options will result in new event loops\nand corresponding threads being started, which have a non-zero performance\noverhead.
First contextifies the given sandbox, runs the compiled code contained by\nthe vm.Script object within the created sandbox, and returns the result.\nRunning code does not have access to local scope.
The following example compiles code that sets a global variable, then executes\nthe code multiple times in different contexts. The globals are set on and\ncontained within each individual sandbox.
const util = require('util');\nconst vm = require('vm');\n\nconst script = new vm.Script('globalVar = "set"');\n\nconst sandboxes = [{}, {}, {}];\nsandboxes.forEach((sandbox) => {\n script.runInNewContext(sandbox);\n});\n\nconsole.log(util.inspect(sandboxes));\n\n// [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]\nRuns the compiled code contained by the vm.Script within the context of the\ncurrent global object. Running code does not have access to local scope, but\ndoes have access to the current global object.
The following example compiles code that increments a global variable then\nexecutes that code multiple times:
const vm = require('vm');\n\nglobal.globalVar = 0;\n\nconst script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });\n\nfor (let i = 0; i < 1000; ++i) {\n script.runInThisContext();\n}\n\nconsole.log(globalVar);\n\n// 1000\nIf given a sandbox object, the vm.createContext() method will prepare\nthat sandbox so that it can be used in calls to\nvm.runInContext() or script.runInContext(). Inside such scripts,\nthe sandbox object will be the global object, retaining all of its existing\nproperties but also having the built-in objects and functions any standard\nglobal object has. Outside of scripts run by the vm module, global variables\nwill remain unchanged.
const util = require('util');\nconst vm = require('vm');\n\nglobal.globalVar = 3;\n\nconst sandbox = { globalVar: 1 };\nvm.createContext(sandbox);\n\nvm.runInContext('globalVar *= 2;', sandbox);\n\nconsole.log(util.inspect(sandbox)); // { globalVar: 2 }\n\nconsole.log(util.inspect(globalVar)); // 3\nIf sandbox is omitted (or passed explicitly as undefined), a new, empty\ncontextified sandbox object will be returned.
The vm.createContext() method is primarily useful for creating a single\nsandbox that can be used to run multiple scripts. For instance, if emulating a\nweb browser, the method can be used to create a single sandbox representing a\nwindow's global object, then run all <script> tags together within the context\nof that sandbox.
The provided name and origin of the context are made visible through the\nInspector API.
Returns true if the given sandbox object has been contextified using\nvm.createContext().
The vm.runInContext() method compiles code, runs it within the context of\nthe contextifiedSandbox, then returns the result. Running code does not have\naccess to the local scope. The contextifiedSandbox object must have been\npreviously contextified using the vm.createContext() method.
The following example compiles and executes different scripts using a single\ncontextified object:
\nconst util = require('util');\nconst vm = require('vm');\n\nconst sandbox = { globalVar: 1 };\nvm.createContext(sandbox);\n\nfor (let i = 0; i < 10; ++i) {\n vm.runInContext('globalVar *= 2;', sandbox);\n}\nconsole.log(util.inspect(sandbox));\n\n// { globalVar: 1024 }\nThe vm.runInDebugContext() method compiles and executes code inside the V8\ndebug context. The primary use case is to gain access to the V8 Debug object:
const vm = require('vm');\nconst Debug = vm.runInDebugContext('Debug');\nconsole.log(Debug.findScript(process.emit).name); // 'events.js'\nconsole.log(Debug.findScript(process.exit).name); // 'internal/process.js'\nNote: The debug context and object are intrinsically tied to V8's debugger\nimplementation and may change (or even be removed) without prior warning.
\nThe Debug object can also be made available using the V8-specific\n--expose_debug_as= command line option.
The vm.runInNewContext() first contextifies the given sandbox object (or\ncreates a new sandbox if passed as undefined), compiles the code, runs it\nwithin the context of the created context, then returns the result. Running code\ndoes not have access to the local scope.
The following example compiles and executes code that increments a global\nvariable and sets a new one. These globals are contained in the sandbox.
const util = require('util');\nconst vm = require('vm');\n\nconst sandbox = {\n animal: 'cat',\n count: 2\n};\n\nvm.runInNewContext('count += 1; name = "kitty"', sandbox);\nconsole.log(util.inspect(sandbox));\n\n// { animal: 'cat', count: 3, name: 'kitty' }\nvm.runInThisContext() compiles code, runs it within the context of the\ncurrent global and returns the result. Running code does not have access to\nlocal scope, but does have access to the current global object.
The following example illustrates using both vm.runInThisContext() and\nthe JavaScript eval() function to run the same code:
const vm = require('vm');\nlet localVar = 'initial value';\n\nconst vmResult = vm.runInThisContext('localVar = "vm";');\nconsole.log('vmResult:', vmResult);\nconsole.log('localVar:', localVar);\n\nconst evalResult = eval('localVar = "eval";');\nconsole.log('evalResult:', evalResult);\nconsole.log('localVar:', localVar);\n\n// vmResult: 'vm', localVar: 'initial value'\n// evalResult: 'eval', localVar: 'eval'\nBecause vm.runInThisContext() does not have access to the local scope,\nlocalVar is unchanged. In contrast, eval() does have access to the\nlocal scope, so the value localVar is changed. In this way\nvm.runInThisContext() is much like an indirect eval() call, e.g.\n(0,eval)('code').
When using either script.runInThisContext() or \nvm.runInThisContext(), the code is executed within the current V8 global \ncontext. 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\nthe context must either call require('http') on its own, or have a reference\nto the http module passed to it. For instance:
'use strict';\nconst vm = require('vm');\n\nconst code = `\n((require) => {\n const http = require('http');\n\n http.createServer((request, response) => {\n response.writeHead(200, { 'Content-Type': 'text/plain' });\n response.end('Hello World\\\\n');\n }).listen(8124);\n\n console.log('Server running at http://127.0.0.1:8124/');\n})`;\n\nvm.runInThisContext(code)(require);\nThe require() in the above case shares the state with the context it is\npassed from. This may introduce risks when untrusted code is executed, e.g.\naltering objects in the context in unwanted ways.
All JavaScript executed within Node.js runs within the scope of a "context".\nAccording to the V8 Embedder's Guide:
\n\n\nIn V8, a context is an execution environment that allows separate, unrelated,\nJavaScript applications to run in a single instance of V8. You must explicitly\nspecify the context in which you want any JavaScript code to be run.
\n
When the method vm.createContext() is called, the sandbox object that is\npassed in (or a newly created object if sandbox is undefined) is associated\ninternally with a new instance of a V8 Context. This V8 Context provides the\ncode run using the vm module's methods with an isolated global environment\nwithin which it can operate. The process of creating the V8 Context and\nassociating it with the sandbox object is what this document refers to as\n"contextifying" the sandbox.