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Execute ECMAScript code uniformly across any ECMAScript host environment. See also eshost-cli for an easy way to use this library from the command line.

Using eshost, you can create an agent (eg. a web browser or a command-line ECMAScript host) and evaluate scripts within that agent. Code running within the agent has access to the eshost runtime API which enables code to evaluate scripts, create new realms, handle errors, and so forth all without worrying about the host-specific mechanisms for these capabilities are.

eshost consists of a wrapper around the various ways of executing a host and processing its output (called an Agent) and a runtime library for host-agnostic scripts to use.


npm install eshost

Supported Hosts

Host Name Type Supported Platforms Download Notes
ch¹ ChakraCore CLI Any Download or build Chakra console host.
d8¹ V8 CLI Any Build from source V8 console host. Errors are reported on stdout. Use $262.getGlobal and $262.setGlobal to get and set properties of global objects in other realms.
engine262 Engine262 CLI Any Build from source An implementation of ECMA-262 in JavaScript.
graaljs GraalJS CLI Any Download
jsshell¹ SpiderMonkey CLI Any Download SpiderMonkey console host.
jsc¹ JavaScriptCore CLI Mac² Build from source³
serenity-js Serenity LibJS CLI Any Build from source
nashorn Nashorn CLI Any Build from source
node Node.js CLI Any Active LTS versions only
qjs4 QuickJS CLI Any Build from source
xs Moddable XS CLI Any Build from source
chrome Google Chrome Browser Any Requires ChromeDriver in your path.
edge Microsoft Edge Browser Windows Errors reported from Microsoft Edge are all of type Error. Requires Microsoft WebDriver in your path.
firefox Mozilla Firefox Browser Any Requires GeckoDriver in your path (possibly renamed to wires).
safari Apple Safari Browser Mac Requires SafariDriver browser extension.
  • 1: eshost accepts esvu or jsvu style binary name values as the first argument to eshost.createAgent(type: string, options = {}): Agent. See Installing Engines.
  • 2: It is possible to build jsc on other platforms, but not supported.
  • 3: Also available on your Mac system at /System/Library/Frameworks/JavaScriptCore.framework/Versions/A/Resources/jsc.
  • 4: For QuickJS installation help, see Install and Configure Hosts.

Installing Engines

esvu or jsvu are the recommended tools for maintaining JavaScript engines for testing purposes. Take a look at the esvu supported engines or jsvu supported engines for more information.

Example Usage

const eshost = require('eshost');
const agent = await eshost.createAgent('d8', { hostPath: 'path/to/d8.exe' });
const result = await agent.evalScript(`



The eshost object is the main export of the "eshost" module.


An array of supported host types.

eshost.createAgent(type: string, options = {}): Agent

Creates an instance of a host agent for a particular host type. See the table above for supported host types.

  • type


    Host Type All Acceptable type Values
    ChakraCore chakra, ch
    Engine262 engine262
    GraalJS graaljs
    Hermes hermes
    JavaScriptCore javascriptcore, jsc
    Serenity LibJS serenity-js, libjs
    Nashorn nashorn
    Node node
    QuickJS qjs 1
    SpiderMonkey jsshell, spidermonkey, sm
    V8 d8, v8
    XS xs


    Host Type All Acceptable type Values
    chrome chrome
    edge edge
    firefox firefox
    safari safari
  • options

    Property Description
    hostPath Path to host to execute. For console hosts, this argument is required. For the specific browser runners, hostPath is optional and if omitted, the location for that browser will be detected automatically.
    hostArguments Command line arguments used when invoking your host. Not supported for browser hosts. hostArguments is an array of strings as you might pass to Node's spawn API.
    transform A function to map the source to some other source before running the result on the underlying host.
    webHost for web browser hosts only; URL host name from which to serve browser assets; optional; defaults to "localhost"
    webPort for web browser hosts only; URL port number from which to serve browser assets; optional; defaults to 1337
    capabilities for remote host only; the Selenium/WebDriver capabilities to request for the remote session; all specified attributes will be forwarded to the server; a listing of available attributes is available in the Selenium project's wiki; the following attributes are required: { browserName, platform, version }
    webdriverServer for remote host only; URL of the WebDriver server to which commands should be issued


initialize(): Promise<void>

Initializes the host and returns a promise that is resolved once the host is initialized. Command line hosts have no initialization as a new process is started for each execution.

This is called for you if you use the createAgent factory.

evalScript(code, options = {}): Promise<Result>

Executes code in the host using the Script goal symbol. Returns a promise for a result object.

evalScript(record, options = {}): Promise<Result>

When evalScript receives a Test262File test record, it executes record.contents in the host using the Script goal symbol, unless record.attrs.flags.module === true, in which case it will execute record.contents in the host using the Module goal symbol. Returns a promise for a result object.

By default, a script will run in eshost until the realm is destroyed. For most command-line hosts, this is done automatically when the script execution queues are empty. However, browsers will remain open waiting for more code to become available. Therefore, eshost will automatically append $262.destroy() to the end of your scripts. This behavior is not correct if you are attempting to execute asynchronous code. In such cases, add async: true to the options.

  • options

    Property Description Default Value
    async Set to true if the test is expected to call $262.destroy() on the root realm when it's finished. When false, $262.destroy() is added for you. false

Result Object

An object with the following keys:

Property Description
stdout Anything printed to stdout (mostly what you print using print).
stderr Anything printed to stderr
error If the script threw an error, it will be an error object. Else, it will be null.

The error object is similar to an error object you get in the host itself. Namely, it has the following keys:

Property Description
name Error name (eg. SyntaxError, TypeError, etc.)
message Error message, if available.
stack An array of stack frames, if available.

stop(): Promise<void>

Stops the currently executing script. For a console host, this simply kills the child process. For browser hosts, it will kill the current window and create a new one.

destroy(): Promise<void>

Destroys the agent, closing any of its associated resources (eg. browser windows, child processes, etc.).

destroy(): Promise<void>

Tears down the agent. For browsers, this will close the browser window. For most CLI/Shell hosts, this is a no-op.

Runtime Library


Prints str to stdout.


A reference to the global object.


Creates a new realm, returning that realm's runtime library ($).

For example, creating two nested realms:

$sub = $262.createRealm();
$subsub = $sub.createRealm();

You can also use a destroy callback that gets called when the code inside the realm calls $262.destroy(). For example:

$sub = $262.createRealm({
  destroy() {

$sub.evalScript('$262.destroy()'); // prints "destroyed!"
  • options

    Property Description
    globals An object containing properties to add to the global object in the new realm.
    destroy A callback that is called when the code executing in the realm destroys its realm (ie. by calling $262.destroy()).


Creates a new script and evals code in that realm. If an error is thrown, it will be passed to the onError callback.

Scripts are different from eval in that lexical bindings go into the global lexical contour rather than being scoped to the eval.


Destroys the realm. Note that in some hosts, $262.destroy may not actually stop executing code in the realm or even destroy the realm.


Gets a global property name.

$262.setGlobal(name, value)

Sets a global property name to value.

Running the tests

This project's tests can be executed with the following command:

npm test

The above command will cause tests to be run against all supported hosts. Executables for each host must be available on the system's PATH environment variable.

One or more hosts may be skipped from the test run by setting corresponding environment variables whose name match the pattern ESHOST_SKIP_*, where * is the capitalized name of the host. For example, in a Unix-like system, the following command executes the project's tests but skips JavaScriptCore and D8 tests:


Tests for the "remote" agent can be configured to run against any arbitrary Selenium/WebDriver configuration through the specification of the following environment variables: ESHOST_REMOTE_BROWSERNAME, ESHOST_REMOTE_VERSION, ESHOST_REMOTE_PLATFORM. These values are used to define the host's capabilities; see the above documentation of eshost.createAgent for more details. For example, in a Unix-like system, the following command executes the project's tests in a remote instance of the Firefox web browser:





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