eshost

eshost

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.

Installation

npm install eshost

Supported Hosts

Host	Supported Platforms	Download	Notes
node	Any	https://nodejs.org
ch	Any	Download or build	Chakra console host.
d8	Any	Build from source	V8 console host. Errors are reported on stdout. Use $.getGlobal and $.setGlobal to get and set properties of global objects in other realms.
jsshell	Any	Download	SpiderMonkey console host.
jsc	Mac¹	Build from source²
xs	Any	Build from source
nashorn	Any	Build from source
edge	Windows		Errors reported from Microsoft Edge are all of type Error. Requires Microsoft WebDriver in your path.
chrome	Any		Requires ChromeDriver in your path.
firefox	Any		Requires GeckoDriver in your path (possibly renamed to wires).
safari	Mac		Requires (SafariDriver browser extension)[https://github.com/SeleniumHQ/selenium/wiki/SafariDriver].

• 1: It is possible to build jsc on other platforms, but not supported.
• 2: Also available on your Mac system at /System/Library/Frameworks/JavaScriptCore.framework/Versions/A/Resources/jsc.

Example Usage

const eshost = require('eshost');
const agent = await eshost.createAgent('d8', { hostPath: 'path/to/d8.exe' });
const result = await agent.evalScript(`
  print(1+1);
`);
console.log(result.stdout);

Documentation

eshost

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

eshost.supportedHosts

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.

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

Agent

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 Test262Stream 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 $.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 $.destroy() on the root realm when it's finished. When false, $.destroy() is added for you.	false

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.).

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
stack	An array of stack frames.

destroy(): Promise<void>

Tears down the agent. For browsers, this will close the browser window.

Runtime Library

print(str)

Prints str to stdout.

$.global

A reference to the global object.

$.createRealm(options)

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

For example, creating two nested realms:

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

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

$sub = $.createRealm({
  destroy() {
    print('destroyed!')
  }
});

$sub.evalScript('$.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 $.destroy()).

$.evalScript(code)

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.

$.destroy()

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

$.getGlobal(name)

Gets a global property name.

$.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:

ESHOST_SKIP_JSC=1 ESHOST_SKIP_D8=1 npm test

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:

ESHOST_REMOTE_BROWSERNAME=firefox npm test