eshost

eshost

eshost is a library for executing ECMAScript code uniformly across any ECMAScript host environment. eshost consists of a wrapper around the various ways of executing a host and processing its output (called a Runner) and a runtime library for host-agnostic scripts to use.

For a CLI tool that uses this library to make comparing ECMAScript hosts super easy, see eshost-cli.

Installation

npm install eshost

Supported Hosts

Host	Supported Platforms	Download	Notes
browser	Any		Errors reported from Microsoft Edge are all of type Error.
node	Any	https://nodejs.org
ch	Windows	Built from source	Chakra console host.
d8	Any	Built 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	Mac	Built from source¹
nashorn	Any	Built from source

1: Also available on your Mac system at /System/Library/Frameworks/JavaScriptCore.framework/Versions/A/Resources/jsc.

Examples

const runner = js.getRunner('path/to/d8.exe', 'd8');
runner.exec(`
  print(1+1);
`).then(result => console.log(result.stdout));

Documentation

eshost API

supportedHosts

An array of supported host types.

getRunner(path, type, arguments)

Gets an instance of a runner for a particular host type. Supported host types:

• browser
• node
• d8
• jsshell
• ch

Creating a runner may start the host (eg. for the browser, creating the host executes the browser process).

You can pass command line arguments to the host process using the arguments option. It is an array of strings as you might pass to Node's spawn API.

Runner API

exec(code)

Executes code in the host. Returns a result object.

Result Object

An object with the following keys:

• 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 the error object you get in the host itself. Namely, it has the following keys:

• name: Error name (eg. SyntaxError, TypeError, etc.)
• message: Error message
• stack: An array of stack frames.

Runtime Library

print(str)

Prints str to stdout.

$.global

A reference to the global object.

$.createRealm(globals)

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

For example, creating two nested realms:

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

$.evalInNewRealm(code, onError)

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

$.evalInNewScript(code, onError)

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.

$.getGlobal(name)

Gets a global property name.

$.setGlobal(name, value)

Sets a global property name to value.