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JavaScript Compatibility

GraalVM provides an ECMAScript-compliant JavaScript language runtime. This document explains the public API it presents for user applications written in JavaScript.

• ECMAScript Language Compliance
• Compatibility Extensions
• GraalVM JavaScript Extensions

ECMAScript Language Compliance #

GraalVM JavaScript implements JavaScript as prescribed in the ECMAScript (ECMA-262) specification. It is fully compatible with the ECMAScript 2022 specification (sometimes referred to as the 13th edition or “ES13”). New features are frequently added to GraalVM when they are confirmed to be part of ECMAScript 2023, see the CHANGELOG.md for details. Older versions starting from ECMAScript 5 can be enabled with a config flag (by number: --js.ecmascript-version=5 or by year: --js.ecmascript-version=2019). In a production setup you might consider specifying a fixed ECMAScript version to be used, as future versions of GraalVM JavaScript will use newer versions of the specification once available.

GraalVM JavaScript provides the following function objects in the global scope as specified by ECMAScript, representing the JavaScript core library: Array, ArrayBuffer, Boolean, DataView, Date, Error, Function, JSON, Map, Math, Number, Object, Promise, Proxy, Reflect, RegExp, Set, SharedArrayBuffer, String, Symbol, TypedArray, WeakMap, and WeakSet.

Additional objects are available under flags, for instance Intl (flag: --js.intl-402). Run js --help or js --help:languages for the list of available flags.

Several of these function objects and some of their members are only available when a certain version of the specification is selected for execution. For a list of methods provided, inspect the ECMAScript specification. Extensions to the specification are specified below.

Internationalization API (ECMA-402) #

Internationalization API implementation (see https://tc39.github.io/ecma402) can be activated using the following flag: --js.intl-402=true. If you run in native mode (default option), you also need to specify the path to your ICU data directory using the following option: --vm.Dcom.ibm.icu.impl.ICUBinary.dataPath=$GRAAL_VM_DIR/jre/languages/js/icu4j/icudt, where $GRAAL_VM_DIR refers to your GraalVM installation directory. If you run in the JVM mode (the --jvm flag is used), you do not need to specify where your ICU data are located, although you can do it with the above option.

Once you activate the Internationalization API, you can use the following built-ins:

• Intl.NumberFormat
• Intl.DateTimeFormat
• Intl.Collator
• Intl.PluralRules

The functionality of a few other built-ins is then also updated according to the specification linked above.

JavaScript Modules #

GraalVM JavaScript supports modules as defined by ECMAScript 6 and later. Be aware that the support for this feature grew and still grows over time. Be sure to use the latest ECMAScript version for the all the latest features.

When loading modules via a polyglot Source, you can use the inofficial application/javascript+module mime type to specify you are loading a module. When loading with JavaScript code from a file, make sure the module is loaded from a file with the .mjs extension. Loading with the import keyword is not limited by that, and can import from a file of any extension.

Compatibility Extensions #

The following objects and methods are available in GraalVM JavaScript for compatibility with other JavaScript execution engines. Note that the behavior of such methods might not strictly match the semantics of those methods in all existing engines.

Language Features #

Conditional Catch Clauses

GraalVM JavaScript supports conditional catch clauses if the js.syntax-extensions option is enabled:

try {
    myMethod(); // can throw
} catch (e if e instanceof TypeError) {
    print("TypeError caught");
} catch (e) {
    print("another Error caught");
}

Global Properties #

load(source)

• loads (parses and executes) the specified JavaScript source code

Source can be of type:

• a String: the path of the source file or a URL to execute.
• java.lang.URL: the URL is queried for the source code to execute if the js.load-from-url option is set to true.
• java.io.File: the file is read for the source code to execute.
• a JavaScript object: the object is queried for a name and a script property, which represent the source name and code, respectively.
• all other types: the source is converted to a String.

load is available by default and can be deactivated by setting the js.load option to false.

print(...arg) and printErr(...arg)

• prints the arguments on the console (stdout and stderr, respectively)
• provides a best-effort human readable output

print and printErr are available by default and can be deactivated by setting the js.print option to false.

Methods of the console Global Object

A global console object is provided that offers several methods for debugging purposes. These methods strive to provide similar functionality as provided in other engines, but do not guarantee identical results.

Note that those methods behave differently when GraalVM JavaScript is executed in Node.js mode (i.e., the node executable is started instead of js). Node.js provides its own implementation that is used instead.

• console.log, console.info, and console.debug: an alias for print(...arg)
• console.error, and console.warn: similar to print, but using the error IO stream
• console.assert(check, message): prints message when check is falsy
• console.clear: clears the console window if possible
• console.count(), and console.countReset(): counts and print how many times it has been called, or resets this counter
• console.group, and console.groupEnd: increases or decreases the indentation for succeeding outputs to the console
• console.time(), console.timeLog(), and console.timeEnd(): starts a timer, prints the duration the timer has been active, or prints the duration and stops the timer, respectively

The console object is available by default and can be deactivated by setting the option js.console to false.

Additional Global Functions in the js Shell #

quit(status)

• exits the engine and returns the specified status code

read(file)

• reads the content of file

The result is returned as a String.

The argument file can be of type:

• java.io.File: the file is used directly.
• all other types: file is converted to a String and interpreted as a file name.

readbuffer(file)

• reads the content of file similar to the read function

The result is returned as a JavaScript ArrayBuffer object.

readline()

• reads one line of input from the input stream

The result is returned as a String.

Object #

Object.prototype.__defineGetter__(prop, func)

• defines the prop property of this to be the getter function func

This functionality is deprecated in most JavaScript engines. In recent ECMAScript versions, getters and setters are natively supported by the language.

Object.prototype.__defineSetter__(prop, func)

• defines the prop property of this to be the setter function func

This functionality is deprecated in most JavaScript engines. In recent ECMAScript versions, getters and setters are natively supported by the language.

Object.prototype.__lookupGetter__(prop)

• returns the getter function for property prop of the object as set by __defineGetter__

This functionality is deprecated in most JavaScript engines. In recent ECMAScript versions, getters and setters are natively supported by the language.

Object.prototype.__lookupSetter__(prop)

• returns the setter function for property prop of the object as set by __defineSetter__

This functionality is deprecated in most JavaScript engines. In recent ECMAScript versions, getters and setters are natively supported by the language.

Nashorn Scripting Mode #

GraalVM JavaScript provides a scripting mode compatible with the one provided by the Nashorn engine. It is enabled with the js.scripting option. Make sure to have --experimental-options set:

js --experimental-options --js.scripting=true

In scripting mode, several properties and functions are added to the global object, including readFully, readLine, $ARG, $ENV, and $EXEC.

There are migration guides available for code previously targeted to the Nashorn or Rhino engines.

GraalVM JavaScript Extensions #

Graal Object #

The Graal object is provided as a property of the global object. It provides Graal-specific information. The existence of the property can be used to identify whether the GraalVM JavaScript engine is the current language engine:

if (typeof Graal != 'undefined') {
    print(Graal.versionECMAScript);
    print(Graal.versionGraalVM);
    print(Graal.isGraalRuntime());
}

The Graal object is available in GraalVM JavaScript by default, unless deactivated by an option (js.graal-builtin=false).

Graal.versionECMAScript

• provides the version number (year value) of GraalVM JavaScript’s ECMAScript compatibility mode.

Graal.versionGraalVM

• provides the version of GraalVM, if the current engine is executed on GraalVM

Graal.isGraalRuntime()

• provides whether GraalVM JavaScript is executed on a GraalVM-enabled runtime
• If true, hot code is compiled by the GraalVM compiler, resulting in high peak performance.
• If false, GraalVM JavaScript will not be optimized by the GraalVM Compiler, typically resulting in lower performance.

Graal.setUnhandledPromiseRejectionHandler(handler) #

• provides the unhandled promise rejection handler when using option (js.unhandled-rejections=handler).
• the handler is called with two arguments: (rejection, promise).
• Graal.setUnhandledPromiseRejectionHandler can be called with null, undefined, or empty args to clear the handler.

Java #

The Java object is only available when the engine is started in JVM mode (--jvm flag).

Note that some functions require a Nashorn compatibility mode flag to be set. On GraalVM, this flag can be set with:

js --jvm --experimental-options --js.nashorn-compat=true

Java.type(className)

• loads the specified Java class and provides it as an object
• fields of this object can be read directly from it, and new instances can be created with the JavaScript new keyword:

  var BigDec = Java.type('java.math.BigDecimal');
  var bd = new BigDec("0.1");
  console.log(bd.add(bd).toString());
  

Java.from(javaData)

• creates a shallow copy of the Java datastructure (Array, List) as a JavaScript array

In many cases, this is not necessary; you can typically use the Java datastructure directly from JavaScript.

Java.to(jsData, toType)

• converts the argument to a Java dataype

The source object jsData is expected to be a JavaScript array, or an object with a length property. The target toType can either be a String (e.g. "int[]") or a type object (e.g., Java.type("int[]")). Valid target types are Java arrays. When no target type is provided, Object[] is assumed:

var jsArr = ["a", "b", "c"];
var strArrType = Java.type("java.lang.String[]");
var javaArr = Java.to(jsArr, strArrType);
assertEquals('class java.lang.String[]', String(javaArr.getClass()));

The conversion methods as defined by ECMAScript (e.g., ToString and ToDouble) are executed when a JavaScript value has to be converted to a Java type. Lossy conversion is disallowed and results in a TypeError.

Java.isJavaObject(obj)

• returns whether obj is an object of the Java language
• returns false for native JavaScript objects, as well as for objects of other polyglot languages

Java.isType(obj)

• returns true if obj is an object representing the constructor and static members of a Java class, as obtained (for example) by Java.type()
• returns false for all other arguments

Java.typeName(obj)

• returns the Java Class name of obj when obj represents a Java type (isType(obj) === true) or Java Class instance
• returns undefined otherwise

Java.isJavaFunction(fn)

• returns whether fn is an object of the Java language that represents a Java function
• returns false for all other types, including native JavaScript function, and functions of other polyglot languages

This function requires the Nashorn compatibility mode flag.

Java.isScriptObject(obj)

• returns whether obj is an object of the JavaScript language
• returns false for all other types, including objects of Java and other polyglot languages

This function requires the Nashorn compatibility mode flag.

Java.isScriptFunction(fn)

• returns whether fn is a JavaScript function
• returns false for all other types, including Java function, and functions of other polyglot languages

This function requires the Nashorn compatibility mode flag.

Java.addToClasspath(location)

• adds the specified location (file name or path name, as String) to Java’s classpath

Polyglot #

The functions of the Polyglot object allow to interact with values from other polyglot languages.

The Polyglot object is available by default, unless deactivated by setting the js.polyglot-builtin option to false.

Polyglot.export(key, value)

• exports the JavaScript value under the name key (a string) to the polyglot bindings:

  function helloWorld() { print("Hello, JavaScript world"); }
  Polyglot.export("helloJSWorld", helloWorld);
  

If the polyglot bindings already had a value identified by key, it is overwritten with the new value. The value may be any valid Polyglot value.

• throws a TypeError if key is not a String or is missing

Polyglot.import(key)

• imports the value identified by key (a string) from the polyglot bindings and returns it:

  var rubyHelloWorld = Polyglot.import("helloRubyWorld");
  rubyHelloWorld();
  

If no language has exported a value identified by key, undefined is returned.

• throws a TypeError if key is not a string or missing

Polyglot.eval(languageId, sourceCode)

• parses and evaluates the sourceCode with the interpreter identified by languageId

The value of sourceCode is expected to be a String (or convertable to one).

• returns the evaluation result, depending on the sourceCode and/or the semantics of the language evaluated:

  var rArray = Polyglot.eval('R', 'runif(1000)');
  

Exceptions can occur when an invalid languageId is passed, when the sourceCode cannot be evaluated by the language, or when the executed program throws one.

Polyglot.evalFile(languageId, sourceFileName)

• parses the file sourceFileName with the interpreter identified by languageId

The value of sourceFileName is expected to be a String (or convertable to one), representing a file reachable by the current path.

• returns an executable object, typically a function:

  var rFunc = Polyglot.evalFile('R', 'myExample.r');
  var result = rFunc();
  

Exceptions can occur when an invalid languageId is passed, when the file identified by sourceFileName cannot be found, or when the language throws an exception during parsing (parse time errors, e.g. syntax errors). Exceptions thrown by the evaluated program are only thrown once the resulting function is evaluated.

The Polyglot.evalFile function is available by default when the Polyglot builtin is available, unless deactivated by setting the js.polyglot-evalfile option to false. It is also available when js.debug-builtin is activated.

Debug #

• requires starting the engine with the js.debug-builtin flag

Debug is a GraalVM JavaScript specific function object that provides functionality for debugging JavaScript code and the GraalVM JavaScript compiler. This API might change without notice. Do not use for production purposes.

Global Functions #

printErr(...arg)

• behaves identical to print

The only difference is that the error stream is used to print to, instead of the default output stream.

loadWithNewGlobal(source, arguments)

• behaves similarly to load function

The relevant difference is that the code is evaluated in a new global scope (Realm, as defined by ECMAScript).

Source can be of type:

• java.lang.URL: the URL is queried for the source code to execute.
• a JavaScript object: the object is queried for a name and a script property.
• all other types: the source is converted to a String.

The value of arguments is provided to the loaded code upon execution.