Adding global functions/variables to GraalVM JavaScript engine

user_203633 0
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How do I add custom global functions to GraalVM's JavaScript 'this' object without using Nashorn compatibility mode via the Object.bindProperties() method (see this: Defining a default/global Java object to Nashorn script engine?) ? I could not find a way using the Context methods and reflection of the Context object does not show a similar bindProperties() method I can use.

Example:

I have these methods:

class GlobalMethods
{
public void a() {...}
public void b() {...}
public void c() {...}
}

and I want a(), b() and c() to be callable in JavaScript without scope/qualification:

context.eval("js", "a()");

Thanks in advance!

graalvm javascript java
2 Answers

For js functions, you can define a file, say globals.js, where globals are initialized and added to the JavaScript global object :

globalThis.someFunction = function() {...}

and set the option js.commonjs-global-properties to load this file at context creation time :

Context context = Context.newBuilder("js")
                         .option("js.commonjs-global-properties", "./globals.js")
                         .build();

In order to bind java options to JS, you need to enable polyglot.js.allowHostAccess via Bindings :

Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
bindings.put("polyglot.js.allowHostAccess", true);
bindings.put("polyglot.js.allowHostClassLookup", (Predicate<String>) s -> true);
bindings.put("javaObj", new Object());

or with Context :

Context context = Context.newBuilder("js")
                         .allowHostAccess(true)
                         .build();

context.getBindings("js").putMember("a", GlobalMethods.a);

Or you can use the @HostAccess.Export decorator in the class definition :

  @HostAccess.Export
  public void a() {...}

Though I'm not sure you can access a static method like GlobalMethods.a() without referencing the classname at least one time, maybe passing the function output directly is sufficient to suit your needs, or passing the class definition, followed by an eval globalThis.a = GlobalMethods.a (not tested).

user_25299540

Eric's answer that uses a global.js works if the methods are not overloaded. In addition, the complete setup for the Context object looks like this with a few additional required options:

context = Context.newBuilder("js")
          .allowHostAccess(true)
          .allowExperimentalOptions(true)
          .allowAllAccess(true)
          .allowIO(true)                                            // required
          .option("js.nashorn-compat", "false")                     // Must be false
          .option("js.commonjs-require", "true")                    // required
          .option("js.commonjs-require-cwd", "/path/to/globals.js") // required
          .option("js.commonjs-global-properties", "./globals.js")
          .build();
user_2036330
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